FOUR-STROKE CYCLE ENGINE SERVICE AND OVERHAUL

MEASURING DEVICES

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TELESCOPING GAUGE

The telescoping gauge gets its name from the way the "legs" of the device slide together. To ustr the telescoping gauge, first loosen the thumb nut and slide the legs together. When the legs are together, tighten the thumb nut. The gauge is then placed inside the hole or space to be measured. The thumb nut is then re-teased. This permits the legs to spring outward against the sides of the hole. With the handle parallel to the sides of the hole, tighten the thumb nut. The gauge is then removed and the distance across the legs is measured with an outside micrometer.

FEELER GAUGE

The feeler or thickness gauges are used to measure small distances or clearances. These tools measure in thousandths of an inch. The flat feeler gauge contains blades (thin strips of metal) of different thicknesses. The thickness is stamped on each blade. By placing different blades together many different measurements can be made.

SPARK PLUG GAPPING GAUGE

The spark plug gapping gauge normally is made from small wires. The diameter of each wire (thickness) is marked on the gauge. The hook on this gauge is used to bend (adjust) the ground electrode of the spark plug.

DIAL INDICATOR

The dial indicator measures distances in thousandths of an inch. This device is fastened securely in place by special holders and clamps. The pad is pressed against the shaft or unit being tested for movement such that the needle gives a reading. The outer ring of the gauge is rotated until the needle is on 0. The shaft is then moved or turned. The movement of the needle indicates the variation in thousandths of an inch.

PLASTIGAUGE

Plastigauge is used to measure the clearance between a plain bearing and the journal. The clearance is determined by placing a piece of the Plastigauge (stringlike plastic) across the bearing and then assemblying the bearing cap. The cap is tightened to specifications. Without moving the shaft, the cap retaining nuts or bolts are loosened and the cap is removed.

The flattened Plastigauge will be on the shaft or the bearing. The width of the flattened Plastigauge is compared to the scale on the Plastigauge package to determine the bearing clearance. Plastigauge is available in several different sizes. Clearances up to nine thousandths can be measured by this method.

THE MICROMETER

The micrometer is used to measure distances in thousandths of an inch. Micrometers come in different sizes. For example, a 0-1" "mike" will measure from 0 to ! inches in thousandths of an inch: a 2 3" mike will measure from 2 to 3 inches m thousandths of an inch.

One complete turn of the thimble changes the distance between the anvil and the spindle 25 thousandths (.025). Each number on the hub is read hundred thousandths. Each mark on the thimble is one thousandth.

To read a mike first note the size of the micrometer — I", 2", etc. Second, read the longest numbered tine that is visible. Third, count .025 for each of the sJiort marks on the hub to the right of the numbered line. Fourth, to these readings add the reading on the thimble.

Example A.	1. 2. 3. 4.	0.000 .200 .050 .005	(0 1" mike) largest numbered line two short marks beyond numbered line 5 on the thimble
		.255 =	- distance between anvil and spindle
Example B.	t. 2. 3. 4.	2,000 .300 .075 .017	(2-3" mike) largest numbered line three short marks beyond numbered lin 17 on the thimble
		2.392 =	= distance between anvil and spindle
Example C.	1, 2. 3. 4.	4,000 .400 .025 .000	-tr*.
		4.425 =	distance between anvii and spindle
Example D,	1. 2. 3. 4.	.000 .000 .000 .002
		.002 =	distance between anvil and spindle

DISASSEMBLE AND REASSEMBLE INSTRUCTIONS AND PRECAUTIONS

The engine should be dismantled ONL Y if there is good reason. Thorough testing can determine whether or not complete teardown is necessary. (See the Compression Testing section.)

REASONS FOR DISASSEMBLY

1. Poor compression because of internal wear or leak at head gasket.

2. High oil consumption because of poor rings.

3. Excessive noise or knocks coming from inside the crankcase.

4. Crankshaft won't turn because of internal problem.

5. Excessive oil leaks around the crankshaft seals or crankcase gasket.

6. Failure of lubrication system.

PRECAUTIONS

Take your time. Do not be in a hurry when disassembling the engine. Observe how the unit is assembled before taking any component apart. If it is a first-time experience, it is wise to make notes and sketches of how parts go together.

Work in a dean work area. Lay parts in order on the work bench as they come apart. Place small parts and bolts in a container. In some instances it may be helpful to thread bolts into their respective engine parts rather than place them in a container.

NEEDED TOOLS

Disassembly of a small engine does not require extensive special tools and equipment. The following common hand tools are recommended:

• 3/8" square drive socket set with reversible ratchet and extensions
• (sizes 1/4" through 7/8" by sixteenths)
• 3/8" square drive torque wrench (150 inch-pound capacity)
• Combination wrench set (5/16" through 3/4" by sixteenths)
• Ball peen hammer (12 oz)
• Combination st:p joint pliers
• Needle nose pliers
• Assorted flat blade screwdrivers (3)
• Phillips screwdriver
• Carbon scraper or putty knife
• Flat feeler gauge

BASIC COMPONENTS

ENGINE BLOCK

Mf it all internal-combustion engines have similar basic components. The engine block is the major component of all engines. This unit contains all the moving parts which are necessary to convert heat energy (burning gases) to mechanical energy (rotating crankshaft). AH engines have a smooth, round cylinder which permits the piston to move up and down. The crankshaft turns on main bearings, which are also located in the engine block.

In most piston-type engines the piston is connected to the connecting rod by a piston pin. The other end of the connecting rod is connected to the crankshaft at the connecting rod journal.

Piston rings are fitted on the piston to prevent the loss of compression and power xxxxxxxxxxxx sealing the cylinder, the rings also clean the cylinder wall of xxxxxxxxxxxxxx and exhaust deposits and transfer some of the heat from the cylinder wall to the oil.

All the moving parts inside the engine must fit properly to their mating parts or the engine will not perform properly.

FOUR-STROKE CYCLE ENGINE

Four-stroke engines require components which permit operation of poppet valves to control the movement of intake and exhaust gases. The camshaft is driven by a gear on the crankshaft at one-half the crankshaft speed. This gear ratio enables each camshaft lobe to move its lifter and thereby open its valve one time while the crankshaft completes two revolutions. The valve spring maintains constant tension on the valve. When the valve is opened, it is moved against the spring tension. The spring causes the valve to return to its seat. The valve will seal as long as the sealing surfaces of the valve and seat are in good condition.

A constant supply of oil is delivered to the points of friction by a pump and splash system on some engines and entirely by a splash system on others. Seals on the crankshaft and gaskets on the bolt on components prevent leakage of the oil. Gaskets and seals should not be reused. Always install new gaskets and seals when assembling an engine.

Power of the four-stroke engine is wasted when wear occurs on the internal parts. Most wear occurs on the piston rings, piston, cylinder, and valves. Improper maintenance greatly increases wear. If the engine is run low on oil, it wears very rapidly and can be ruined in a short time.

Low compression and knocking inside the engine usually are signals that an overhaul is needed. When overhauling the engine, check the clearances and either replace or recondition worn components. Piston rings should always be replaced if the piston is removed.

HEAD GASKET REPLACEMENT

1. Remove the air shroud and the parts that cover the cylinder head. Be sure to mark any wires which are disconnected. If the fuel tank is to be removed, shut off the fuel at the tank or plug the line. Clean dust from fins on the block and cylinder head.

2. Remove the spark plug.

3. Remove the bolts that secure the head to the engine. Caution: Some engines have bolts of different lengths. No-; the location of the longer bolts.

4. Remove the old head gasket and clean the carbon from the cylinder head.

5. With a putty knife or carbon scraper, remove all carbon from the piston head and block surfaces.

6. Check for warpage of the block and/or head by placing the head on the engine without using a gasket. With a feeler gauge check the maximum space between the head and the block. If the gap is more than .015 inch, the head should be checked on a surface plate. If the warpage is in the head, the head should be resurfaced or replaced. If the warpage is in the block, the block should be resurfaced. To resurface the head or the block, place a sheet of medium grit emery cloth over a surface plate or other flat steel surface. (The table of a power saw will work.) Move the head or block over the emery cloth until it shows a true gasket surface. The head gasket will permit a slight amount of variation between the cylinder head and the block mating surfaces.

7. Make certain that the block and head are clean and place a new head gasket on the block. Carefully check the gasket to be sure that it aligns properly with the bolt holes and the block surface. Place a small amount of graphite grease on the threads of the head bolts and install the head on the engine. (If graphite grease is not available, scrape a soft lead pencil over the threaded portion of the bolts.) Make certain that all bolts are installed in the proper locations. (If the bolts were accidentally mixed, check with a small rod or nail to detennine where the longer bolts are needed to ensure that the bolts adequately thread into the engine.)

8. Snug all head bolts by hand. Torque the bolts to one-third torque specifications following the tightening sequence shown for your bolt arrangement. Refer to the manufacturer's manual for the torque specifications and tightening sequence. If these data are not available, use a criss-cross pattern as shown in the head bolt tightening sequence. Retighten all bolts to two-thirds torque specifications. Tighten to the torque specifications and go over them one more time to be sure that all are at the proper torque.

9. Clean, gap, and replace the spark plug. (It is best to install a new gasket under the spark plug.) Torque the plug to 20 foot-pounds. If a torque wrench is not available, thread the spark plug in until it is snug against the new plug gasket. Tighten the plug one-third turn, and it will be at the approximate torque recommendation.

10. Replace the air shroud and other components that were removed or disconnected.

ENGINE OVERHAUL PRELIMINARY STEPS

The engine should be overhauled if there is poor compression because of worn cylinder components which reduce compression or cause high oil consumption. Complete disassembly also will be necessary to correct problems such as a "locked engine"' or one which "knocks." Diagnosing these problems is explained in the Compression Testing section.

On most engines it is best to do a complete overhaul even though the diagnosis shows only problems with the valves.

Cleanliness. Clean the engine before starting disassembly. Clean all parts so that they can be inspected and accurately measured. Keep the work area clean and orderly.

Ordered Procedure. Keep track of how the engine comes apart—where each unit attaches and the mating of moving components. Draw sketches and make notes on the wiring connections and carburetor linkage. Working to Specifications. All components must be checked carefully for wear and failure. During reassembly all parts must fit properly and all bolts must be tightened to the proper tension. Lubrication. When any engine is reassembled, all internal parts must be well oiled or the engine will be damaged the instant that it is started.

The following special tools are needed when completing an overhaul:

• Dial indicator
• 0—1" micrometer
• 1—2" micrometer
• 2—3" micrometer
• Telescoping gauges (0-1", 1-2", 2-3")
• Ring ridge remover
• Piston ring compressor
• Valve spring compressor
• Valve reseating tool

RING RIDGE REMOVAL AND BASIC DISASSEMBLY

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EXTERNAL CRANKSHAFT CHECKS AND VALVE REMOVAL

EXTERNAL CRANKSHAFT CHECKS AND VALVE REMOVAL

11. Remove the ring ridge from the top of the cylinder. If a ring ridge remover is not available, carefully remove the ridge by hand with medium grit emery cloth. Failure to remove the ridge may damage the piston when it is reassembled with new piston rings. The sharp corner on the new rings wi! catch on the ridge and break the piston ring land beneath the first ring groove.

12. Remove the valve cover. Some covers contain a filter unit and serve as a crankcase breather. Be careful to observe the assembly order of units which contain filter and breather valve components.

13. Remove the valves by compressing the valve spring and removing the retainer with pliers. If a valve spring compressor is not available, two screwdrivers can be used to compress the springs.

Keep the valve springs with their respective valves. On some engines the exhaust valve has a spring which is heavier than the intake valve spring.

14. Check the end play of the crankshaft by clamping a dial indicator to the crankshaft with the pad resting against the crankcase. Move the crankshaft in and out. The indicator will show the end play in thousandths of an inch.

EXTERNAL CRANKSHAFT CHECKS AND VALVE REMOVAL

EXTERNAL CRANKSHAFT CHECKS AND VALVE REMOVAL

If a dial indicator is not available, the end play can be checked with a feeler gauge. On some engines the feeler gauge can be placed between the drive pulley and the crankcase. Measure the gap when the crankshaft is pulled "out" and then measure the gap when it is pushed "in." The difference between the two measurements is the crankshaft end play.

Another method of checking the end play is to clamp a square on the crankshaft with a C-clamp. Take a measurement with the feeler gauge when the crankshaft is pulled "out" and another measurement when it is pushed "in." The difference between the measurements is the crankshaft end play.

Record the crankshaft end play in the Data Block below. If the end play is not correct, it can be adjusted when the engine is being reassembled.

If the engine is from a rotary lawnmower, check for a bent crankshaft. To make this check, mount a dial indicator on the engine with the pad against the crankshaft. Slowly rotate the crankshaft and observe the movement of the indicator needle.

An alternate method is to clamp a bar or square on the engine so that it is very close to the end of the crankshaft. Rotate the crankshaft and measure the difference in the gap between the bar and the crankshaft with a feeler gauge. A difference in the gap indicates a bent crankshaft.

A crankshaft with more than .005 run out should be replaced.

OPENING THE CRANKCASE AND PISTON REMOVAL

OPENING THE CRANKCASE AND PISTON REMOVAL

15. Position the engine so that it is easy to remove the removable crank-case side or base.

16. Remove the bolts which attach the crankcase side or base cover. Carefully remove the cover. If a gasket sealer was used, it will be necessary to tap the cover with a hammer to "break it loose."

Don't discard the old crankcase side cover gasket because it must be measured when selecting the gasket to be used in reassembly.

On engines that have a side cover, check for timing marks on the camshaft and crankshaft gears. Some engines have a mark on the camshaft gear which aligns with a mark on the crankshaft counterweight. Other engines have a mark on the camshaft gear which is aligned with the crankshaft gear key or a mark on the crankshaft gear.

Rotate the crankshaft until the two timing marks align. If no marks are visible, wipe the oil from the matching teeth on the two gears and paint a mark on each gear.

17. Remove the camshaft oil distributor or slinger if one is used, Carefully remove the camshaft and valve lifters. Mark the lifters with

masking tape so that they can be reinstalled in the same location.

Note: The above step Can be omitted on an engine that has a removable base.

18. Study the connecting rod and cap to identify marks or reference tabs. If no locator marks or tabs are visible, make a small punch mark on the rod and the rod cap on the camshaft side of the engine. Note these data at the end of this chart.

With a punch and hammer or pliers straighten the sheet metal locks on the connecting rod nuts. (Some engines use self-locking nuts instead of a sheet metal lock.)

Remove the connecting rod nuts or bolts with a socket wrench or box-end wrench. Carefully push the piston-rod assembly out the top of the cylinder.

Caution: DO NOT permit the rod bolts to "bang" against the crankshaft.

DO NOT place any tools against the bearing portion of the connecting rod.

19. Once the piston assembly is removed from the engine, reinstall the connecting rod cap on the connecting rod. Remove the crankshaft from the engine block. Make certain that the bearing surfaces of the crankshaft aren't nicked by careless handling. The main bearing retainer or side of the block must be unbolted on an engine that has a removable base.

OPENING THE CRANKCASE AND PISTON REMOVAL

20. Clean all parts, including the inside of the block, in parts cleaning solvent or in kerosene. DO NOT use gasoline to clean parts because gasoline leaves a chalky deposit when it evaporates and it is a fire hazard.

21. Inspect the head of the piston for its size. If the engine cylinder has not been resized (bored), the top of the piston will have no marking or STD stamped upon it. STD means that the cylinder is standard sized. A cylinder that has been resized will have the amount of diameter-over-standard stamped on the top. Oversizes are usually .010, .020, or .030 greater than the manufacturer's standard bore for the engine.

CYLINDER MEASUREMENTS AND CHECKS

CYLINDER MEASUREMENTS AND CHECKS

CYLINDER CHECKS

22. The cylinder must be checked for wear and scores or scratches. There are three different kinds of wear which can occur in the cylinder-oversize, out-of-round, and taper.

The three types of wear are checked by taking precision measurements at two locations in the cylinder—three-quarters of an inch from the top and three-quarters of an inch from the bottom.

The measurements can best be taken with a telescoping gauge and outside micrometer, an inside micrometer, or a cylinder dial gauge.

First, take two measurements three-quarters of an inch from the top of the cylinder and at 90° to each other (measurements A and B in the first illustration) tion). The difference between the smaller of the two measurements and the original bore of the engine (2 inches, 2-1/4 inches, 2-5/16 inches, etc.) represents cylinder wear. Record the cylinder wear in the Data Block.

The difference between the two measurements taken three-quarters of an inch from the top of the cylinder (measurements A and B in the first illustration) represents the out-of-round of the cylinder at the top. Record the out-of-round in the Data Block.

The difference between the measurements taken at right angles to the crankshaft (measurements A and C in the first illustration) is the cylinder taper. Record the cylinder taper in the Data Block.

If precision measuring instruments are not available, the cylinder taper can be measured with a new or used piston ring and a feeler gauge (third illustration). To measure taper in this manner, carefully remove a piston ring from the piston and place it in the cylinder. With the head or skirt of the piston push the ring down until it h three-quarters of an inch from the top of the block, Make sure that the ring is square with the cylinder.

Measure the gap between the ends of the ring with a feeler gauge.

Move the ring to three-quarters of an inch from the bottom of the cylinder and measure the gap between the ends of the ring with the feeler gauge. Subtract the reading taken at the bottom from the reading taken at the top and then divide by 3. This will give the taper in thousandths of an inch.

Compare the cylinder wear, out-of-round, and taper to the manufacturer's specifications. If any wear exceeds the maximum recommended by the manufacturer, the cylinder should be resized by a machine shop.

Cylinders are usually resized to .010, ,020, or .030 over the standard bore.

If manufacturer's data are not available, the following data can be used as a guide to determine whether or not the cylinder should be resized:

• Maximum oversize diameter .003
• Maximum out-of-round .005
• Maximum taper .004

Inspect the cylinder for deep scores or scratches. A deep scratch will cause compression loss and oil burning even though the cylinder wear is not excessive.

A badly scored or scratched cylinder should be resized.

Resizing is accomplished by increasing the cylinder size with special cylinder hones, a boring bar, or a metal cutting lathe. Resizing procedures recommended by the resizing equipment manufacturer should be followed carefully.

PISTON CHECKS

23. If the cylinder is resized (rebored), a new piston must be installed.

If the engine is not to be rebored and the piston is to be reused, the old rings must be removed. To remove the rings, use a ring expander as shown in the chart. If a ring expander is not available, carefully expand the ring by hand and move it up over the top of the piston. AVOID SCRATCHING the ring lands with the ends of the ring. Remove the other rings in the same manner.

Clean the carbon from the top of the piston with a carbon scraper or putty knife. DO NOT use a wire brush to clean the piston because it will wear away part of the ring lands. Stuck pistor rings, broken rings, severe piston burning, and top groove wear can be the result of improper combustion or detonation. Careful service of the ignition system and the use of proper fuel will prevent such problems from occurring.

A ring groove cleaning tool is used to remove carbon from the bottom of each ring groove. Adjust the cutter head so that the proper sized cutter will be in line with the ring groove when the tool is placed on the piston. Place the cutter in the groove to be cleaned and adjust the tool so that spring tension keeps the tool under tension. Rotate the tool by hand until all the carbon is removed from the groove. This procedure is followed for each ring groove.

If a ring groove cleaning tool is not available, a broken piston ring can be used to scrape the carbon from the ring grooves. If a broken ring is to be used, the scraping end should be square so that it scrapes evenly along the bottom of the groove. If the carbon is not cleaned from the grooves, it can cause the new rings to bind in the cylinder when the engine is reassembled.

Wash the piston in solvent and carefully inspect it for burned areas on the piston lands or deep scores, scratches, or cracks on the skirt.

If the piston has any of the above defects, it should be replaced. Scoring or scuffing usually results from inadequate lubrication or a dirty cooling system. If scores are present, carefully examine the cooling fins and passages on the block.

Dirt-clogged passages can cause the engine to overheat. Poor lubrication can also cause cylinder scoring. Inspect the lubrication system for broken or damaged components.

Check the ring groove for wear by placing a new ring in the groove and checking the remaining space with a feeler gauge. If a .007 or larger feeler gauge can be inserted between the ring and the piston, the piston should be replaced. Excessive wear in the ring groove can cause the rings to "pump" oil to the combustion chamber. Such wear can also cause the piston rings to break.

24. Inspect the old piston rings. Careful inspection may indicate the causes of wear or damage.

Fine vertical scratches on the ring faces show that dirt has entered the engine along with the intake air. Failure to correct the problem will cause the new rings to wear out quickly. The usual source of such dirt is the air cleaner. The air cleaner must be serviced periodically. (See Air Cleaner Service section.) It must be connected tightly to the carburetor to prevent entry of dirt.

Deep scratches or scoring on the rings are normally the result of an engine which has overheated. Overheating can be caused by insufficient cylinder lubrication, clogged cooling fins, or air passages on the block, incorrect combustion, or insufficient ring or piston clearance.

PISTON PIN AND CONNECTING ROD CHECKS

PISTON PIN CHECKS

25. Piston pin wear can be cheeked by holding the piston firmly in one hand while attempting to rock the rod with the other hand. The rod must be rocked in line with the piston pin. It is normal for the rod to slide freely across the piston pin. Don't confuse this with piston pin wear.

If looseness is detected, the piston pin should be removed. Before removing the pin, note identifying marks or mark the piston and rod with a punch mark so that the two can be reassembled properly. Remove the piston pin retainer with small nose pliers and remove the piston pin. Measure the piston pin with a micrometer. Compare the measurement with the standard size recommended by the manufacturer. If the pin is worn over .0005 out-of-round or below the minimum specified by the manufacturer, the pin should be replaced.

Check the pin bore in the piston for wear with a telescoping gauge and a micrometer. If the bore is over .0005 out-of-round or oversize, the bore should be machined for an oversize pin or the piston should be replaced.

Oversize piston pins are available for some engines. To install an oversize pin, the piston and connecting rod bore must be reamed to fit the new pin. This machining operation should be performed by a machine shop equipped to complete the precision reaming required.

Some manufacturers include new piston pins with new pistons. The fit between the rod and the new pin should be checked. If the rod is loose on the new pin, the rod should be replaced.

CONNECTING ROD CHECKS

26. Inspect the connecting rod for wear. If there was loosen'ess when the piston-rod assembly was checked for wear, the pin bore in the rod should be measured with a telescoping gauge and micrometer. If the pin bore in the rod , is over .0007 out-of-round or L scored, the rod should be replaced.

The crankpin bearing in the connecting rod should be inspected carefully for pits and scratches. If the bearing surface is pitted or scored, the connecting rod should be replaced. If there are wear spots on diagonally opposite points of the road and the rod cap, the connecting rod is bent and should be replaced. It is NOT recommended that the connecting rod cap be filed to compensate for wear in the rod bearing or crankpin journal.

If the connecting rod was "loose" on the crankshaft when the engine was disassembled, the crankpin beating in the connecting rod should.be measured carefully with a telescoping gauge and micrometer. The bore should not be over .0005 out-of-round. If the bore is over .0005 over the manufacturer's standard size, the rod should be replaced.

CRANKSHAFT CHECKS

27. The crankshaft of the single cylinder engine incorporates three bearing surfaces. Each surface is referred to as a journal. On some engines the surface of the journal serves as the bearing surface. This bearing is referred to as a plain bearing. On some engines an anti-friction bearing (taper roller or ball type) is pressed on to the crankshaft.

If the engine has plain type bearings on the "mains" and the rod, each bearing surface must be checked carefully. Each surface should be inspected for roughness and scoring. If any of the journals is very rough, the crankshaft should be replaced. In a few cases the shaft can be polished with emery cloth and reused.

Each bearing journal should be measured carefully with a micrometer, The measurements should be taken as illustrated in the chart.

The drive-end main journal should be checked for out-of-round by comparing the measurement H-I to measurement J-K. The difference between the measurement is out-of-round. Record the out-of-round measurement in the Data Block. If the manufacturer's data are not available, the out-of-round normally should not exceed .001.

The flywheel or magneto main bearing journal should be checked for out-of-roundness in the same manner. Record the out-of-round measurements in the Data Block. Check the connecting rod journal for out-of-round by comparing measurement C-D to measurement E-F. Record the out-of-round in the Data Block.

The taper of the bearing journals can also be checked. The taper on any plain bearing journal is measured by taking measurements at each end of the journal as shown in the chart. The taper on journals of small engine crankshafts normally should not exceed .001. If the journal is not scored and is not out-of-round, it seldom will have "taper."

Crankshafts that exceed the wear limits or ones that are badly scored should be replaced. Some manufacturers have replacement main bearings and rods for use on undersized crankshafts. If these are available, the crankshaft can be reconditioned by a machine shop. Be certain that undersized bearings are available before having the crankshaft reground.

On engines with anti-friction (taper roller or ball type) main bearings, the bearings should be carefully inspected for nicks on the rollers or balls and Taces. Defective bearings can also be identified by rotating the bearing by hand. Roughness in the bearing can be felt as the bearing is turned slowly. Note: The bearing must be clean when it is checked for roughness.

If roughness or wear is detected, the bearing should be replaced. Some of these bearings are a press fit on the crankshaft. To remove these, an arbor press or special bearing puller is required.

Replacement procedures for this type of bearing vary. The manufacturer's recommendations should be followed when installing the new bearing. Note: When installing these bearings NEVER use a torch to heat the bearing.

As explained earlier in the text, most manufacturers recommend that a bent crankshaft be replaced. Straightening a bent crankshaft weakens the shaft. Such a shaft could break later while in operation.

Inspect the timing gear on the crankshaft for wear and damaged teeth. Normally these gears will last the life of the engine. If there is much wear or if there are damaged gear teeth, the gear should be replaced. On a smaller engine the timing gear is made as a part of the crankshaft. It will be necessary to replace the crankshaft if the timing gear is faulty.

Data Block

123

Drive-end main journal out-of-round	Actual	Specifications
Magneto-end main journal out-of-round	_________	_________
Connecting rod journal out-of-round	_________	_________
Drive-end main journal taper Magneto-end main journal taper Connecting rod journal taper	_________	_________
	_________	_________
	_________	_________

MAIN BEARING AND CAMSHAFT CHECKS

ENGINE WITH REMOVABLE BASE AND CAMSHAFT HELD IN THE BLOCK BY A PIVOT SHAFT

MAIN BEARING CHECKS

28. On engines that have plain type main bearings it is important to check the bearing bores for wear.

Carefully check each bearing (one on the block and one in the crank-case cover) for scoring. If there is much scoring or pitting, the bearing should be replaced. Wear of the main bearing can be checked further with a telescoping gauge and micrometer. On some engines the bearing bore can be reamed with a reamer, and a replacement sleeve can be installed. Such sleeves are not available from some manufacturers. If the bearing is not serviceable and a replacement sleeve is not available, the bearing can be reamed out and a sleeve can be custom-made by a machine shop. In such cases the costs of the repair must be weighed against the costs of replacing the engine or short block.

A short block includes the block, crankshaft, rod and piston assembly, valve train, and side cover. Installing a short block involves relatively little work and provides a literally new engine.

CAMSHAFT CHECKS

29. The camshaft opens the valves by operating valve lifters which push the valves off their seats. The camshaft will normally last the life of the engine if the engine oil is properly maintained.

On some engines the camshaft is easily removed once the crankcase side or base is removed. On other engines the camshaft is on a pivot shaft which runs completely across the block. To remove this camshaft requires that the pivot shaft be forced out of the block. It is not normally necessary that the camshaft be amoved as a part of the overhaul.

Check the camshaft for wear by carefully inspecting each lobe for a scpred or scratched surface. If there is no scoring, normally the camshaft is satisfactory. The lobes can be checked further by comparing the lobe measurements with the manufacturer's specifications.

The camshaft gear or assembly should be replaced if the gear teeth are worn, broken, or chipped. Camshaft bearing wear can be checked by comparing the shaft measurements with the standards recommended by the manufacturer.

On engines in which the camshaft is held in the block on a pivot shaft, bearing wear can be checked by feeling for looseness. To make this check, grasp the camshaft and attempt to move it toward the valve area of the engine. Then try to move it in the opposite direction. If there is much movement of the camshaft on its bearings, the camshaft should be removed and further checks made with a micrometer.

On engines that have governor units fitted to the camshaft, the governor should be checked for wear. Refer to the manufacturer's manual for data on a particular governor. Some manufacturers mount a centrifugal mechanism on the camshaft to provide compression release. On these units check for binding of the weights and free operation of the springs.

VALVE LIFTER AND VALVE GUIDE CHECKS

CHECKING VALVE GUIDE WEAR WITH DIAL INDICATOR

30. Examine the valve lifters for wear or scoring on the lifter face which rides on the cam lobes. If the face is worn or scored, the lifter should be replaced. The face should not be remachined or ground because this will remove the hardened surface of the lifter. Without the hardened face surface, the lifter will wear very rapidly.

Before valve guides can be checked for wear, clean carbon from the guide with a valve guide brush. Check for valve guide wear by placing a new valve (a used vaive wiii work providing the stem is not worn over .001) in the guide and measuring the side play with a dial indicator. The maximum allowable clearances are listed below:

Valve Head Diameter	Maximum Side Play
Up to 1-1/4"	intake .005 Exhaust .007
Over 1-1/4"	Intake .006 Exhaust .008

Another means of checking valve guide wear is to measure the valve guide with a small hole gauge and micrometer. Compare the guide diameter one-quarter inch from the top of the guide to the specifications listed by the manufacturer,

If valve guide wear exceeds the manufacturer's recommendations, the guide should be replaced or reconditioned.

The guide can be resized by a knurling process that can be done by most automotive machine shops.

Some manufacturers have valves with oversized stems that can be installed in guides that have been reamed to the proper oversize.

Replacement guides are available for many small engines. Follow the manufacturer's recommendations when replacing the valve guide.

VALVE SEAT SERVICE

51. Clean the carbon from the valve ports and valve seat area.

To ensure a good seal between the valve and the valve face requires that the valve seat be in good condition. If the valve seat is loose in the block, it can be tightened by staking the block adjacent to the seat with a punch and hammer.

Badly burned valve seats should be replaced. Follow the recommendations of the engine manufacturer when replacing valve seats,

Valve seats that are not badly burned can be reconditioned by machining or grinding a new sealing surface. One type of seat reconditioner uses a hand-operated cutter. To use this tool, first place the proper-sized pilot in the clean valve guide. The proper-angled cutter (usually 45° or 30°) is then placed over the pilot. The cutter is turned in a clockwise rotation by the special T-wrench that comes with the reconditioning tool set.

Pressure is exerted on the tool as the cutter is turned. The cutting action is continued until a clean seating surface appears completely around the seat. Caution: The seat should be machined as little as possible. The width of the finished seating area should be from 3/64 inch to 1/16 inch.

If the seating area exceeds 1/16 inch in width, it should be narrowed by removing stock from the top of the machined area. The narrowing operation is performed with a cutter which is of a lesser angle than the valve seat. (A 30° cutter can be used to narrow 45° seats.)

If much narrowing of the seat is required, be sure to check the height of the valve head when it is seated. The lower edge of the valve margin should be above the top of the vaive scat (see illustration). If the valve rides too low in the seat, check to be certain that the valve margin is sufficient.

If the margin is satisfactory, the problem probably is caused by the seat being too large. Before condemning the valve seat, however, place a new valve in the engine to see if it will correct the problem. If it rides properly in the seat, there is no need to install a new seat. If the new valve rides too low, a new valve seat should be installed. Refer to the manufacturer's data for valve seat replacement on a particular engine.

VALVE SERVICE

32. Clean the intake and exhaust valves with a wire brush. Examine each valve for burned faces. If the faces are pitted or badly grooved, the valve should be replaced.

Measure the margin on each valve. If the margin is 1/64 inch or less, the valve should be replaced.

Check the valve stems for scoring and wear. Check for wear by measuring the valve (with a micrometer) one inch above the retainer groove or hole and one-quarter of an inch below the upper point of contact with the valve guide. If these measurements differ by .001 or more, the valve should be replaced.

If the valves are to be reused, the faces should be reconditioned by grinding on a valve refacer. Follow the recommended procedures for the valve re-facer in the refacing operation. Be certain that the machine is set for the proper face angle before starting the refacing operation. Some engines use different angles for the intake and exhaust valves. If a valve refacer is not available, the operation can be performed by most automotive machine shops.

An alternate method of reconditioning the valve and the valve seat is known as lapping. If the valve seat and valve face are in relatively good condition, the seating surfaces can be reconditioned by the lapping process. To do this, first place the valve in its guide in the engine. Put a small amount of tapping compound between the valve face and the valve seat. (Be sure that none of the lapping compound gets on the valve stem!) 

Place the suction cap of the tapping tool over the head of the valve. Rotate the valve back and forth against the seat. Frequently raise the valve off the seat to allow the compound to flow to the point of friction. Continue the lapping operation until a good seating surface is obtained. Clean all the lapping compound from the valve and valve seat.

The final process on. the valve (adjusting the clearance) can only be completed later when reassembling the engine. This operation will be explained as a part of the Reassembly Procedure.

REASSEMBLY PROCEDURE

ALL COMPONENTS OF THE ENGINE SHOULD BE CLEANED AND PLACED ON A CLEAN WORK SURFACE BEFORE BEGINNING THE REASSEMBLY

When reassembling the engine, it is very important that every tiling -parts. work area, your hands, and tools—be clean. Surgical cleanliness is a must if the engine overhaul is to be successful.

1. Rinse all engine parts in solvent and dry them with compressed air. As each part is cleaned, place it on the clean surface of the workbench. Avoid drying parts with rags because lint will collect in the small crevices. If your work on the engine is interrupted for any length of time, cover the engine components with a clean, lint-free cloth.

2. Install new crankshaft seals in the engine block and the cover housing. The old seals can be pried out with a large screwdriver or they can be driven out with a punch and hammer.

When new seals are being installed, be certain that the sharp edge or lip of the seal is toward the inside of the engine. Some manufacturers recommend that a film of sealer (Permatex #2) be applied to the outer part of the seal (between the seal and the casting) to prevent oil leakage between the seal and the housing. Seat the seal in the housing with a seal driver (or block of wood) and a hammer.

3. If the piston, piston pin, or connecting rod is being replaced, proceed with installation of the piston on the connecting rod. First, matcii the old components that you marked prior to disassembly. Position the new replacement components adjacent to the old parts. Assemble the piston and rod so that it is just like the old parts were prior to disassembly.

4. If the camshaft was removed and was the kind held in the engine block on a pivot shaft, proceed with its replacement. First, clean the block lifter holes and camshaft bearings. Lubricate these surfaces with motor oil (SAE 20 or 30, SE classification). Install the lifters in their respective bores and place the camshaft in the block. Install the camshaft pivot shaft and expansion plugs in the block.

5. Wipe the main bearing surfaces in the engine block and lubricate them with motor oil.

Wipe the main hearing journals on the crankshaft, apply a film of oil. and install the crankshaft in the engine. Carefully guide the crankshaft through the seat on the magneto or flywheel side of the engine. Note: The following procedure is only for an engine whose camshaft is held in the engine on a pivot shaft- If your engine has a removable side crankcuse cover or base containing a main bearing, disregard the remainder of this step.

Align the timing marks on the crankshaft and the camshaft gears. Refer to the Data Block in the disassembly procedure for the crankshaft end play recorded prior to disassembly of the engine. Compare this end play measurement to the manufacturer's data.

If the end play is within specifications, select a new bearing retainer cover gasket of the same thickness as the one that was removed from the engine during disassembly. Install the gasket and bearing retainer. If the end play of the crankshaft was more than recommended by the manufacturer, select a thinner gasket and install the retainer.

Using the dial indicator, cheek the crankshaft end play with the new gasket in place. If the end play is not within specifications, remove the retainer and select a gasket that will provide the proper end play. If a dial indicator is not available, check the end play with a feeler gauge as shown in the disassembly procedure. If manufacturer's data are not available, adjust the end play between .004 and .012.

INSTALLING PISTON RINGS

6. Place one of the new piston rings in the cylinder and push it down to approximately one-half inch from the bottom of the cylinder with the piston. Cheek the gap between the ends of the ring with a feeler gauge. Compare this end gap clearance with the minimum ring end gap clearance recommended by the manufacturer. The end gap must not be less than the minimum clearance specified or the rings will tend to seize in the cylinder. (If the manufacturer's data are not available, allow .003 end gap clearance for each inch of cylinder diameter.)

If the end gap clearance is less than recommended, carefully file the end of the ring to obtain the necessary clearance. If it is necessary to file the end of one ring, it will also probably be- necessary to file the other rings.

7. Install the new piston ring on the piston. ALWAYS install new piston rings in an engine that has been disassembled. Used rings cannot seat in properly and will cause oil consumption and compression loss.

When installing the piston rings, carefully follow the instructions included with the new piston rings. Lubricate the ring grooves on the piston before installing the rings. Use a piston ring expander when installing the new rings. The rings can be "stretched" if they ire installed by hand springing tliem over the piston.

The widest ring is the oil ring that fits in the bottom groove of the piston. Some manufacturers use a thin metal expander behind this ring. The middle ring is referred to as the scraper ring or the center compression ring. It normally has a groove on its outside edge. This groove must be down when the ring is assembled on the piston. The top ring or compression ring normally has a bevel on its inside edge. This b<-vo1 must be up when the ring is installed on the piston.

8. Lubricate the piston and rii s thoroughly with motor oil. Rotate the piston rings so that the end gaps i the rings are not in line. Install the piston ring compressor over the piston and tighten. Continue to tighten the compressor until the bottom edge of the compressor is against the piston.

INSTALLING THE PISTON ASSEMBLY

9. Wipe the cylinder with a clean cloth. Lubricate the cylinder with motor oil and rotate the crankshaft to place the rod journal opposite the cylinder. Place the piston assembly in the bore. Make certain that the marks on the connecting rod are positioned properly in the engine.

Position a hammer handle against the head of the piston. With one hand exert pressure on the handle while bumping the end of the hammer with the other hand. Carefully guide the connecting rod so that it does not catch on the crankshaft.

10. If desired, the clearance between the connecting rod and the crankshaft can be checked at this point.

Wipe the crankshaft and connecting rod bearing surfaces. Position the rod over the crankshaft journal. Lay a piece of Plastigauge on the crankshaft and install the connecting rod cap. Carefully tighten the rod bolts to the specified torque with a torque wrench. DO NOT TURN THE CRANKSHAFT. Remove the rod cap and compare the width of the flattened Plastigauge to the scale on the Plastigauge package. The clearance should be within the specifications recommended by the manufacturer.

If the clearance is less than the minimum, check to make certain that the rod cap was installed properly on the rod and that there is no dirt or foreign material between the connecting rod and the crankshaft. If both items check out, the connecting rod is probably the incorrect rod for the engine.

If the clearance exceeds the maximum, check for dirt or foreign material between the rod and the cap at the point of connection. If no foreign material is present, remeasure the crankpin with a micrometer and compare it with the manufacturer's data. If the crankpin size is correct, either the rod is worn or else it is the incorrect rod for the engine.

If the manufacturer's rod clearance data are not available, tlie following specifications can be used as a guide:

.001 minimum clearance between rod and crankpin .0035 maximum clearance between rod and crankpin

11. Thoroughly lubricate the crankpin, rod bearing, and cap bearing. Install the rod cap (oil dipper, if used), lock plate, and rod bolts or nuts. Tighten the rod bolts evenly to the manufacturer's torque specifications and bend the lock plate tabs to secure the rod bolts or nuts.

Rotate the crankshaft to ensure that there will be no binding of the rod bearing.

CAMSHAFT AND CRANKCASE COVER INSTALLATION

12. On an engine that does not have the camshaft held in the block on a pivot shaft, lubricate the valve lifter bores and install the lifters. Lubricate the camshaft bearings and install the camshaft. MAKE CERTAIN that the timing mark on the camshaft gear and the crankshaft are matched properly.

13. Refer to the Data Block in the disassembly procedure for the crankshaft end play recorded prior to disassembly of the engine. Compare this end play measurement to the manufacturer's data. If the end play is within specifications, select a new crankcase cover gasket of the same thickness as the one that was removed from the engine during disassembly.

If the end play of the crankshaft was more than that recommended by the manufacturer, select a thinner gasket.

For an engine with extreme wear, some manufacturers supply a thin washer which is placed between the crankshaft timing gear and the side cover,

14. If the engine has the governor built in to the crankcase, position the governor components so that the crankcase cover can be installed.

J 5. On an engine that has an oil pump, check the pump for wear. Replace worn components and install the pump in the engine. With a pump oil can, apply a liberal amount of oil to the piston pin, crankshaft bearings, and camshaft.

16. Lubricate the camshaft and crankshaft bearings in the crankcase cover.

17. Install the oil slinger or oil pump.

18. Carefully install the cover gaskets and cover. Be sure to work the crankshaft seal into position as the cover is installed. Note: It is not necessary to use a sealer on the side cover gasket. To prevent damage to the crankshaft seat use a special installing cone or wrap wax paper around the crankshaft to guide the seal over the bearing shoulder.

19. Tighten the cover retaining bolts to the specified torque.

20. Check the crankshaft end play with a dial indicator. Compare the end play of the engine to the manufacturer's data. If the end play is not sufficient, remove the cover and install a thicker gasket.

If the end play is excessive, remove the cover and install a thinner gasket.

Always use at least one gasket between the cover and the engine even though the end play slightly exceeds the maximum recommended.

If the manufacturer's data are not available, adjust the crankshaft end play between .004 and .012,

VALVE ADJUSTMENT AND INSTALLATION

CHECKING VALVE OPERATING CLEARANCE

GRINDING END OF VALVE STEM TO ATTAIN PROPER VALVE CLEARANCE

VALVE INSTALLATION

IF VALVE SPRING COMPRESSOR IS NOT AVAILABLE, COMPRESS SPRING IN A BENCH VISE AND SECURE WITH WIRES AS SHOWN. REMOVE WIRES AFTER VALVE IS INSTALLED.

21. On an engine that has a removable crankcase base, place a new gasket on the engine block and replace the base. Tighten the bolts that secure the base. BE CERTAIN to tighten the oil drain plug.

22. Place the intake valve in its valve guide and turn the crankshaft until the valve opens. TURN the crankshaft an additional half turn. This should position the intake valve in the closed position. Hold the valve tightly closed with your thumb and check the clearance between the end of the valve stem and the valve lifter with a feeler gauge.

Cheek the manufacturer's specifications for the proper valve operating clearance for your engine and compare the clearance to the measured clearance of the engine. Since the valve seating surfaces have been machined, it will probably be necessary to remove some material from the end of the valve stem to achieve the right clearance.

The valve stem can be "shortened" by grinding material from the stem on a valve reconditioning machine or on a bench grinder with a V block. Grind a smalt amount; then recheck the clearance. If too much material is ground off, the valve will have to be replaced.

Once the intake valve operating clearance is properly adjusted, place the exhaust valve in the engine. Rotate the crankshaft until the exhaust valve opens; then rotate the crankshaft an additional half turn. Check the clearance specification of the exhaust valve and adjust it in the same manner as the intake valve.

23. Thoroughly lubricate the valve stem and guide. Compress the intake valve spring (the lighter spring) and install the spring and retainer on the intake valve.

In similar manner install the exhaust valve spring (the heavier spring) and retainer. If a valve spring compressor is not available, the springs can be compressed in the bench vise and held with wire. Once the retainers are in position, cut the wire from the spring and remove it from the valve chamber.

Replace the valve cover using a new gasket If the cover contains a crank-case breather, be certain to reassemble the unit in the proper sequence.

HEAD REPLACEMENT AND FINAL REASSEMBLY

24. Make certain that the block and head are clean and place a new head gasket on the block. Carefully check the gasket to be sure that it aligns properly with the bolt holes and the block surface. Place a small amount of graphite grease on the threads of the head bolts and install the head on the engine. Make certain that all bolts are installed in the proper locations. If the bolts were mixed accidentally, check with a small rod or nail to determine where the longer bolts are needed to ensure that the bolts adequately thread into the engine.

25. Snug all head bolts by hand. Torque the bolts to one-third torque specifications following the tightening sequence shown for your bolt arrangement. Refer to the manufacturer's specifications for the torque value. Retight-en all bolts to two-thirds torque specifications. Tighten to the torque specifications and go over them one more time to be certain that all bolts are at the proper torque.

26. Adjust the gap and install a new spark plug. Torque the plug to the specified torque. If specifications are not available, torque the plug to 20 footpounds.

27. Replace the intake manifold and carburetor assembly. Connect the governor linkage before bolting the carburetor to the engine. (DO NOT BEND THE LINKAGE WIRE.)

28. Replace the ignition system and flywheel as outlined in the Ignition section.

29. Replace the muffler. Caution: On a model that has a muffler which threads into the engine block, DO NOT thread the muffler all the way into the block. Thread the muffler into the block approximately one-half inch and tighten the locking nut. If the muffler is threaded into the engine too far, it may "pinch" the exhaust valve guide or the tapered threads may distort the engine block.

30. Replace the air shroud and other components which were removed or disconnected.

31. Fill the crankcase with SAE 30, SE classification, motor oil. Note: Do not overfill the crankcase. Fill only to the full mark or level of the engine. After 5 hours of operation change the engine oil. During break in of a newly overhauled engine the oil becomes contaminated quickly as the new parts "wear in"

32. Replace the engine on the tiller, lawnmower, etc., and connect all controls.

33. Reconnect the drive train or blade.

34. Fill the fuel tank with clean fuel.

35. Start the engine and maintain a fast idle for the first few minutes of running.

36. Adjust the carburetor as outlined in the Carburetor section.

Adapted from:

SMALL GAS ENGINES

James A. Gray and Richard W. Barrow

School of Technology Indiana State Univetsitv

Prentice-Hall, Inc., Englewood Cliffs, New Jersey

© 1976 by Prentice-Hall, Inc. Englewood Cliffs, New Jersey

All rights reserved. No pari of this book may be reproduced in any form or by any means without permission in writing from the publisher.

Drawings by Robert F. MacFarlane