TWO-STROKE CYCLE ENGINE SERVICE AND OVERHAUL

BASIC COMPONENTS

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The two-stroke cycle engine delivers one power impulse each time the crankshaft completes one revolution. Intake and exhaust gases move in and out of the cylinder through ports (holes) in the sides of the lower part of the cylinder. This arrangement takes the place of the valve train found on four-stroke cycle engines.

Oil is mixed with the fuel for the two-stroke cycle engine. Therefore, there is no need for a volume of oil in the crankcase. The oil suspended in the fuel vapor sticks to the surfaces of all the moving parts. This keeps all the parts coated with a film of oil, regardless of how much the engine is tiited.

Reed valves (thin strips of metal) are located in the side of the crankcase. The reed valves permit the intake gases to enter the crankcase. The gases are transferred to the cylinder through the intake port in the lower part of the cylinder.

There is no exhaust valve used with the typical two-stroke cycle engine. The exhaust gases leave the cylinder through the exhaust port when the piston nears the end of the downward stroke.

The exhaust port tends to build up with carbon as a result of burning some of the oil in the fuel mixture. The port must be kept clean and free of this carbon.

The engine has main bearings and rod bearings which are similar to the four-stroke cycle engine. On some two-stroke engines anti-friction (roller) bearings are used. When servicing an engine with anti-friction bearings, it is important that the rollers (needles) are not lost. Extreme care must be taken when disassembling the engine to make certain that alt the bearing rollers are kept together.

The crankcase gaskets and crankshaft seats on the two-stroke cycle engine must be in good condition. Leakage past the seals or gaskets can prevent the engine from running properly. If the crankcase is not sealed completely, the intake gases will be diluted by the air leaked to the crankcase.

MINOR ENGINE SERVICE

CLEANING CARBON FROM THE EXHAUST PORTS

CHECKING FOR AIR LEAKS AT CRANKCASE GASKET

Two-stroke cycle engines require some basic services quite different from the four-stroke cycle engine. There is a tendency for carbon to build up around the ports of this engine because of the oil in the air-fuel mixture. If the carbon buildup is excessive, the engine will lose power.

EXHAUST PORT CLEANING

1. Remove the shields and other components necessary to loosen the muffler or exhaust passage bolts.

2. Remove the muffler (or exhaust system).

3. Remove the spark plug and rotate the crankshaft until the piston blocks the exhaust port.

4. Use a wooden dowel rod to scrape carbon from the ports. Be very careful that the cylinder walls and piston are not damaged.

5. WITH THE SPARK PLUG WIRE DISCONNECTED and connected to ground, crank the engine over to blow any carbon particles from the piston head and port area.

6. Clean the muffler and cover plate

7. Serape the threads of the muffler bolts with a lead pencil. This will prevent the threads from seizing in the block casting.

8. Reassemble the muffler and shields.

9. Check the spark plug gap and replace the spark plug and spark plug wire.

CRANKCASE AIR LEAKS

The two-cycle engine depends on a sealed crankcase for efficient intake of gases into the crankcase and movement of the air-fuel mixture to the cylinder. Any defect that permits air to enter the crankcase on intake will also permit loss of the compressed air-fue! mixture in the crankcase at the time it is moved to the cylinder.

CHECKING FOR CRANKCASE AIR LEAKS

1. Check for loose screws and bolts which would allow crankcase leaks.

2. Tighten any loose components,

3. Apply a film of motor oil on the gasket sealing area of any portions that were loose.

4. Crank the engine and observe whether or not the oil is drawn into the crankcase through a defect in the gasket. Start the engine and continue to check suspicious areas fbr leaks. If any leakage is observed, the gasket should be replaced.

REED VALVE INSPECTION AND SERVICE

The reed valve(s) employed in the air-fuel intake system must function properly or else the two-stroke cycte engine will not run. A speck of dirt or a piece of grass in the reed valve will cause improper pressure buildup in the crankcase of the engine.

SERVICE PROCEDURE

1. Remove the carburetor assembly from the engine.

2. Carefully remove the reed valve assembly.

3. Inspect the unit for foreign material between the reed and its seat. Also inspect the reeds for warpage or other damage.

4. Remove the reed and rinse the valve components in clean solvent. AVOID USING COMPRESSED AIR because the blast of air can distort or bend the reeds. If the reeds are damaged or badly worn, replace them with new reeds.

5. Carefully reassemble the reed valve assembly. Check the clearance between the reed valve with a feeler gauge. Compare the clearance to specifications. If the clearance is improper, check to see that the unit is assembled properly.

Note: Used reeds may appear to be good but not give the proper clearance, In such cases the reed should be replaced.

6. Using new gaskets, replace the reed valve assembly and carburetor on the engine.

ENGINE OVERHAUL
(TWO-STROKE CYCLE ENGINE)

The engine should be disassembled completely for overhaul only if there is good reason. Thorough testing can determine whether or not an overhaul is necessary.

REASONS FOR OVERHAUL

1. Poor compression because of internal wear {compression testing is explained in the Compression Testing section of this text).

2. Excessive noise or knocks coming from inside the engine.

3. Crankshaft will not turn because of internal problem.

OVERHAUL PROCEDURE

1. Remove or disconnect the control cables and linkage. Disconnect the spark plug wire and attach it to the engine ground.

2. Disconnect the power drive belt, blade, or other power mechanisms. On shaft-driven, self-propelled units, mark the gears so that they can be reassembled in the same relative position. Check Point.......................____.........

3. Remove the engine from the lawnmower, generator, or equipment.

4. Clean the outside of the engine. Hot, soapy water can be used to scrub the engine. Rinse with water.

On engines which are very dirty and greasy, apply engine cleaning solvent and rinse with water. DO NOT USE GASOLINE TO CLEAN THE ENGINE.

5. Dry the engine with cloths or compressed air.

6. Remove the shroud and the fuel tank. Note the types of screws used to mount these components. They should be replaced in the same positions.

7. Clean all rust and dirt from the drive end of the crankshaft as shown in the illustration. Any rust or roughness on the shaft will damage the drive-end main bearing when the crankshaft is removed from the crankcase.

8. Check for a bent crankshaft. (See the Crankshaft area of the Four-Stroke Cycle Engine section for methods of checking the crankshaft.)

9. Remove the flywheel. On some models a filter screen must be removed to gain access to the flywheel nut. (See the Ignition section for the details on flywheel removal.)

10. Remove the governor assembly and the armature plate. The armature plate contains a main bearing. On some engines the bearing is a loose-needle type. Be careful that none of the needles is lost.

11. Remove the carburetor and the reed plate.

12. Loosen the connecting rod bolts or nuts but DO NOT remove them from the rod.

CYLINDER. PISTON, AND CRANKSHAFT REMOVAL

REMOVING THE CRANKSHAFT

REMOVING CONNECTING ROD CAP

CYLINDER, PISTON, AND CRANKSHAFT REMOVAL

13. Remove the bolts or screws which attach the cylinder 0ug) or the cylinder head. Note: There is no provision for removing the cylinder head

or cyltnder on some two-cycle engines. On these engines the piston is removed through the bottom of the cylinder.

14. Tap the cylinder with a soft hammer to break it loose from the crankease.

15. Remove the cylinder by pulling it away from the piston quickly. Note the match marks on the connecting rod and the connecting rod cap. Remove the connecting rod bolts or nuts and remove the piston assembly from the engine. Be careful to retain all the loose-needle bearings of the connecting rod bearing. Note: On some two-cycle engines the rod cannot be removed from the crankshaft. On such engines check carefully for roughness by revolving the connecting rod on the crankshaft. If there is roughness or if the side play is excessive, the crankshaft-rod assembly should be replaced.

16. Remove the crankshaft from the crankcase.

CHECKING WEAR OF ENGINE COMPONENTS

CHECKING WEAR OF ENGINE COMPONENTS

17. Wipe crankshaft bearing journals with a clean cloth. Carefully inspect the journals which employ needle-type bearings. Wear can be noted by feeling the journal. Slight roughness or wavmess indicates wear and the crankshaft should be replied.

When the crankshaft journal is a plain bearing, it should be measured with a micrometer. Rtfer to the Crankshaft Measurement section in Four-Stroke Cyde Overhaul for information on measuring crankshaft journals. Compare the measurements to the manufacturer's specifications. If they exceed the maximum acceptable wear, the crankshaft should be replaced. If the manufacturer's data are not available. .001 can be considered the maximum out-of-round which is acceptable. Record the crankshaft out-of-round and taper in the Data Block.

18. Inspect the cylinder for wear and scoring. Some two-cycle engines utilize a chrome-plated cylinder If the chrome has "worn through," the cylinder should be replaced. If the cylinder is scored, it should be replaced.

Check the cylinder for wear and taper by taking measurements with a m telescoping gauge and micrometer. For additional information on cylinder
measurements, refer to the Cylinder Measurement section in Four-Stroke Cycle Overhaul. Compare the measurements to determine out-of-round and ta per. Check the manufacturer's specifications for the maximum allowable taper and wear. Record the information in the Data Block.

If the wear or taper exceeds the maximum, the cylinder should be bored or honed oversize. The cylinder resizing should be done by a machine shop equipped to perform this machining. Check the availability of n oversize pis ton and rings before reconditioning the cylinder. In some instances it may be more economical to replace the cylinder rather than have it reconditioned.

Data Block

	Actual	Specifications
Drive-end main journal out-of'-round	____________	____________
Magneto-end main journal out-of-round	____________	____________
Connecting rod journal out-of-round		____________
Drive-end main journal taper
Magneto-end journal taper Connecting rod taper
Cylinder wear		____________
Cylinder out-of-round
Cylinder taper

PISTON PIN AND CONNECTING ROD CHECKS

19. Piston pin wear can be checked 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. Do not confuse this with piston pin wear.

Iflooseness 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 they can be reassembled properly. Remove the piston pin retainer with small nose pliers and remove the piston pin. Be careful that none of the rollers is lost on units which utilize a needle-type bearing.

Examine the piston pin and connecting rod pin bore for scoring or roughness. Any component that is scored should be replaced.

Measure the piston pin and the connecting rod bore with a micrometer.

On a model that utilizes a plain-type bearing at the pin, the clearance between the piston pin and the connecting rod pin bore should not exceed .002 on most engines.

Oversize pins are available from some manufacturers. When an oversize pin is being installed, the rod and piston must be reamed or honed to achieve the proper fit.

20. Inspect the connecting rod bearing for scoring and wear. The bore can be measured with a telescoping gauge and micrometer to find its size. Compare the measurement to the manufacturer's specification to determine wear. If over .0005 wear is present, the rod should be replaced.

On a model that utilizes a needle bearing in the rod, carefully examine the rod or the insert for roughness. If roughness is present, replace the component. Inspect the rod bearing needles for pits and flat spots. If either is present, the complete set of needles should be replaced.

19. Piston pin wear can be checked by holding the piston firmly in one hand while attempting to ""rock" the rod with the other hand. The rod must he "rocked" in line with the piston pin. It is normal for the rod to slide freely across the piston pin. Do not 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 they can be reassembled properly. Remove the piston pin retainer with small nose pliers and remove the piston pin. Be careful that none ol' the rollers is tost on units which utilize a needle-type bearing.

Examine the piston pin and connecting rod pin bore for scoring or roughness. Any component that is scored should be replaced.

Measure the piston pin and the connecting rod bore with a micrometer.

On a model that utilizes a plain-type bearing at the pin, the clearance between the piston pin and the connecting rod pin bore should not exceed .002 on most engines.

Oversize pins are available from some manufacturers. When an oversize pin is being installed, the rod and piston must be reamed or honed to achieve the proper fit.

20. Inspect the connecting rod bearing for scoring and wear. The bore cart be measured with a telescoping gauge and micrometer to find its size. Compare the measurement to the manufacturer's specification lo determine wear. '.If over .0005 wear is present, the rod should be replaced.

On a model that utilizes a needle bearing in the rod, carefully examine the rod or the insert for roughness. If roughness is present, replace the component. Inspect the rod bearing needles for pits and flat spots. If either is present, the complete set of needles should be replaced.

PISTON CHECKS

21. Carefully examine the piston for scoring and wear. If a two-cycle engine is operated on fuel with no oil added, the piston will score very quickly. If too much oil or a low-grade oil is added to the fuel, carbon will build up quickly in the exhaust port. This excessive carbon can cause scoring of the piston.

A badly scored piston should be replaced. To verify the wear of a piston, measure it with a micrometer and compare the measurements to the manufacturer's specifications.

22. If the piston is not scored or worn, it can be reused. Remove the piston rings with a piston ring expander. The piston ring grooves must be cleaned thoroughly. Many manufacturers use "pins" or wires to prevent the piston rings from turning on the piston. Either remove the pins or wires or use extreme caution when cleaning the ring grooves. Clean all carbon from the grooves with a piston ring groove cleaner or a broken piston ring. If a broken ring is used, grind the end of the ring square so that the carbon in the base of the groove can be removed.

23. Check the ring groove wear by placing a new ring in the groove and measuring the side clearance with a feeler gauge. The side clearance measurement should be checkedI against the manufacturer's specifications. If the manufacturer's data are not available, .006 is considered maximum side clearance on many engines. If the ring side clearance is excessive, the piston should be replaced.

24. Install the piston on the connecting rod. (Match the punch or identifying marks on the rod and the piston.) Be certain that the rod and piston are assembled so that when installed in the engine the long slope of the piston is on the exhaust port side of the cylinder. When installing the piston pin retainers, make certain that the retainer ring opening is toward the top of the piston.

PISTON RING CHECKS AND INSTALLATION

Cleanliness is most important when reassembling an engine. Make certain that the work area, tools, component parts, and your hands are clean before beginning the reassembly of the engine,

25. Clean all engine parts in solvent. Blow the excess solvent from the parts with compressed air and lay each part on a clean work surface.

26. Check the ring end gap of the new piston rings in the cylinder. ALWAYS INSTALL NEW PISTON RINGS. Position the ring squarely in the cylinder and measure the gap between the ring ends with a feeler gauge. Check the manufacturer's data for acceptable end gap. 

If these data are not available, .005 is considered the minimum gap. If the gap is less than the minimum or ,005, check to be certain that the rings are the proper ones for the engine. On some models the end gap can be filed with a file to a-ehieve proper clearance. Generally the gap should not exceed .020.

27. Carefully instaii the rings on the piston. Use a piston ring expander for this operation. If no ring expander is available, carefully place the rings on the piston. Install the lower piston ring first. If the piston has pin or wire retainers, be careful to position the ring properly over the retainer. The pin retainers or wire prevent the ring from rotating to a position where the end of the dug would catch in the cylinder port.

SEAL REPLACEMENT AND CRANKSHAFT INSTALLATION

28. Remove the old seals from the cnmkcase and/or crankcase covers with u large screwdriver.

29. Install new crankshaft seals with a seal installation tool or a block of wood. Be certain that the tip of the seal is toward the inside of the engine. Note: On some engines which employ needle-type main bearings, the seal is installed after the bearings are in place in the crankcase and armature plate.

30. Wipe the main bearing of the block and lubricate it with SAE 20, SE oil. Wipe the crankshaft clean, oil with clean motor oil, and install it in the crankcase. BE CAREFUL that the oil seal is not damaged as the shaft is installed. On models with loose-needle bearings, coat the needles with Vaseline or other tow-melting temperature- grease to keep the rollers in place while the crankshaft is being installed.

31. Position the piston assembly in place. Be certain that the piston is positioned identical to its position before disassembly. If a rod "liner" is used, position the liner in the connecting rod. Place one-half of the loose needles in the connecting rod. Keep the needles in place with grease and pull the rod into place on the crankshaft. Position the remaining needle bearings in the cap or on the crankshaft and install the connecting rod cap,

Make certain that the match marks on the rod and rod cap are aligned. Carefully tighten the rod bolts until they are snug. DO NOT TIGHTEN THE CONNECTING ROD BOLTS AT THIS TIME.

Note: On engines with a plain-type rod bearing, disregard the above needle bearing instructions.

CYLINDER AND CRANKCASE REASSEMBLY

32. Place the eylinder-crankease gasket on the crankcase. Be certain that the port holes are properly aligned.

33. Apply a liberal coat of oil to the piston and piston rings. Position the rings properly over the retainers on pistons so designed. On engines without pins, stagger the rings so that the end gaps are not in line with each other and are not in alignment with the cylinder ports, install a ring compressor over the rings. Wipe the cylinder clean, apply a liberal coating of oil, and slide the cylinder (jug) over the piston.

34. Tighten the connecting rod bolts to the proper torque specifications with a torque wrench. Bend the lock tabs around the rod bolts or nuts. Rotate the crankshaft to be certain that there is no binding.

35. Install the armature plate or crankcase cover with a new gasket between the cover and the crankcase. Apply grease to the needle bearings to hold them in place if this type bearing is employed. Be careful that the new crankshaft seal is not damaged when sliding the cover into place. A film of wax paper can be wrapped around the crankshaft to prevent the seal from catching on the main bearing shoulder. Tighten the cover retaining bolts.

36. Install the reed valve reassembly and carburetor. Use new gaskets. Be certain that the reeds are toward the engine! Tighten the reed valve and carburetor retaining nuts or bolts,

37. Install the ignition system components and flywheel. See the Ignition Service section for details on the ignition system.

FINAL STEPS OF REASSEMBLY

38. Replace the flywheel shrouds. Reconnect the carburetor linkages and the fuel lines.

39. Reassemble the engine on the lawnmower or powered unit and reconnect all the control cables and mechanisms.

40. Fill the fuel tank with the proper mixture of fuel and oil.

41. Perform the basic carburetor adjustments as outlined in the Carburetor Service section.

42. Start the engine and make final carburetor adjustments.

43. Operate the engine under normal conditions and readjust the carburetor as is needed.

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