REASSEMBLING A TWO-STROKE ENGINE

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

REASSEMBLING A TWO STROKE ENGINE
STEP 1: INSTALL THE CRANKSHAFT BEARING IN THE CRANKCASE
STEP 2: INSTALL THE CRANKSHAFT
STEP 3: ATTACH THE PISTON TO THE CONNECTING ROD
STEP 4: SET THE PISTON RING END GAP
STEP 5: INSTALL THE RING OR RINGS ON THE PISTON
STEP 6: INSTALL THE PISTON AND CONNECTING ROD ASSEMBLY
STEP 7: BOLT THE CYLINDER POT TO THE CRANKCASE
STEP 8: BOLT THE BLOWER HOUSING BRACKET TO THE CRANKCASE
STEP 9: INSTALL THE FLYWHEEL
STEP 10: INSTALL THE IGNITION MODULE
STEP 11: INSTALL THE MOTOR’S BACK PLATE AND FUEL TANK
STEP 12: INSTALL THE CARBURETOR AND REED VALVE ASSEMBLY
STEP 13: INSTALL THE AIR CLEANER
STEP 14: INSTALL THE MUFFLER
STEP 15: INSTALL THE RECOIL START AND BLOWER HOUSING
STEP 16: INSTALL THE SPARK PLUG
STEP 17: ATTACH THE SPARK PLUG WIRE
PARTS CLEANING AND ORGANIZATION

REASSEMBLING A TWO STROKE ENGINE

Two stroke engines generally simpler than four stroke, with fewer parts. Can usually be reassembled much quicker. Exact steps used to reassemble will vary depending on engine make and model. Whenever reassembling, refer to service manual for exact procedures to follow. In most cases, basic assembly steps will be the same. Here is a step by step procedure you can use to reassemble typical two stroke engine:

1)Install crankshaft bearing into crankcase.
2)Install crankshaft.
3)Connect piston to connecting rod.
4)Set piston ring end gap.
5)Install piston ring or rings on piston.
6)Install piston and connecting rod assembly.
7)Bolt cylinder pot to crankcase.
8)Bolt bracket for blower housing to crankcase.
9)Install flywheel.
10)Install ignition module.
11)Install motor’s back plate and fuel tank.
12)Install carburetor and reed valve assembly.
13)Install air cleaner.
14)Install muffler.
15)Install pull start mechanism and blower housing.
16)Install spark plug.
17)Attach spark plug wire.

STEP 1: INSTALL THE CRANKSHAFT BEARING IN THE CRANKCASE

Begin reassembly by working on crankcase. In most two stroke, crankshaft supported only on one end by bearing. If removed crankshaft’s support bearing from crankcase, need to put back before can reinstall crankshaft. In most cases, bearing consists of ball bearing with seal built into it.

To install bearing, place into hole in crankcase. Bearing will usually fit tightly into hole. To fully seat bearing, usually have to tap into hole. A seal driver can be used to tap bearing into place. Deep socket of correct size will work also. When tapping on bearing, always tap on outer race area. Never tap on inner race; tapping inner race can damage bearing.

Often engine will have metal retaining ring that fits into groove right above bearing. Even though bearing may fit tightly in hole, retaining ring may still be necessary. Aluminum crankcase will expand when gets hot. If no retaining ring, bearing could work free. So remember to insert retaining ring once bearing in place. Can use pair snap ring pliers to pull ends ring together when inserting into groove. Once ring in groove, release.

STEP 2: INSTALL THE CRANKSHAFT

Now that bearing for crankshaft in place, can install crankshaft. Can now check clearance connecting rod bearing. To determine clearance, would measure outside diameter of post on crankshaft and inside diameter connecting rod. Won’t be able to check clearance by installing connecting rod and using Plastigage because most two stroke connecting rods one piece. Gauging plastic would smear when pushed one piece connecting rod onto post. Therefore must measure inside and outside diameters to determine clearance.

After confirmed proper clearance for rod bearing, insert other end crankshaft through its bearing in crankcase. Push in crankshaft until all the way against bearing.

STEP 3: ATTACH THE PISTON TO THE CONNECTING ROD

After checked clearance of rod bearing and installed crankshaft, can turn to piston and connecting rod assembly. First stage in putting assembly back together is to attach piston to top connecting rod. First insert end of connecting rod into piston so holes for wrist pin line up. With holes aligned, slide in wrist pin to fasten connecting rod to piston. Wire retainers used to prevent wrist pin from sliding out of piston. Wire retainer fits into a groove on each hole on either side piston, just past end of wrist pin. Can install wire retainers with needlenose pliers. If don’t install retainers, pin could easily slide out piston and into cylinder wall, causing severe damage to cylinder.

STEP 4: SET THE PISTON RING END GAP

Must check and adjust end gap on each piston ring so can install rings on piston. Procedure identical to one used on four stroke. Place ring into bare cylinder (without piston) and measure end gap with feeler gauge. Adjust end gap as needed by filing ring down with either special ring filing tool or standard fine file.

STEP 5: INSTALL THE RING OR RINGS ON THE PISTON

Once adjusted end gap for each ring, place rings on piston. Since two stroke pistons and rings quite small, can probably spread ring by hand as place on piston. If not, use expander tool.

Since oil and fuel normally mixed in two stroke, crankcase usually won’t have oil reservoir. So won’t usually find oil control ring on piston two stroke engine. Since piston usually small, may only find one compression ring. Single compression ring often all that’s needed to maintain effective seal. Once compression ring in place, ready to install piston rod assembly in cylinder pot.

STEP 6: INSTALL THE PISTON AND THE CONNECTING ROD ASSEMBLY

With four stroke, piston typically installed from top cylinder. In two stroke, must be installed from bottom because cylinder head part of cylinder bore and cannot be separated.

Installing piston in two stroke very simple: piston pushed up into place from bottom cylinder. In two stroke, bottom of cylinder usually tapered. Taper makes installing piston easier. With four stroke, need tool to compress rings. With two stroke, taper acts as built in ring compressor. Taper means cylinder will be wide enough to accept both piston and rings. As you push piston upward, decreasing taper will squeeze rings to fit cylinder. To ensure piston will slide easily into cylinder, should first coat outside of piston with oil.

STEP 7: BOLT THE CYLINDER POT TO THE CRANKCASE

Next bolt entire cylinder assembly to crankcase. First set cylinder gasket into place on top of crankcase. Once gasket in place, insert end connecting rod through top crankcase, and slide over post on crankshaft. Be aware that piston and rod must be positioned a certain way in cylinder when attach connecting rod to crankshaft. With typical two stroke, skirt of piston cut away more on one side than other. Skirt cut to allow clearance for balance weight on crankshaft. Need to rotate piston in cylinder so cutaway portion between crankshaft and inner side of connecting rod. If don’t orient piston correctly, attach wrong side rod to crankshaft, balance weight will strike piston skirt as crankshaft rotates.

Once connecting rod properly attached to crankshaft, cylinder pot can be bolted to crankcase. As attach cylinder pot, make sure holes in gasket and pot stay lined up with mounting holes on crankcase. Once everything in place, insert retaining bolts into holes and tighten to proper torque specifications.

STEP 8: BOLT THE BLOWER HOUSING BRACKET TO THE CRANKCASE

Majority internal engine components assembled. Turn to external components. Begin with bracket for blower housing. On typical two stroke, blower housing mounted on bracket that attached to crankcase. Normally bracket attached to crankcase underneath flywheel. If bracket mounted under flywheel, must install prior to flywheel. Remember to tighten bolts to proper torque specifications.

STEP 9: INSTALL THE FLYWHEEL

Now can install the flywheel. Flywheel key usually used to prevent flywheel from turning on end of crankshaft. To ensure flywheel properly installed, must place flywheel key in slot on end of crankshaft. Using small hammer, lightly tap key into slot to make certain key is properly seated. Can line up key with slot in inner side flywheel. Finally slide flywheel over crankshaft into place.

Once flywheel in place, can install flywheel retaining nut. Can use ordinary wrench to snug down retaining nut, then tighten to proper torque specifications. In many cases, need to use flywheel holder to keep flywheel from turning as finish tightening retaining nut.

STEP 10: INSTALL THE IGNITION MODULE

Once installed flywheel, can attach ignition system. Many engines will use electronic ignition system mounted near outer edge of flywheel. To install, would simply bolt ignition module into place. On some engines, no adjustments need to be made to position module. Are some engines where will need to set air gap during installation. Would set air gap using ordinary paper or plastic air gap gauge.

STEP 11: INSTALL THE MOTOR’S BACK PLATE AND FUEL TANK

Next step in reassembly to install motor’s back plate. On most small engines, back plate used to seal side of crankcase. Back plate usually made of plastic or aluminum. Since crankshaft in two stroke typically supported only on one end, back plate won’t usually contain crankshaft bearing.

In some engines, back plate has additional function. Used to hold fuel tank in place. Fuel tank placed into position on top of engine and back plate then installed. Once plate’s four bolts are tightened, plate holds fuel tank in place.

STEP 12: INSTALL THE CARBURETOR AND REED VALVE ASSEMBLY

Another function of back plate in some engines to hold reed valve. Valve acts as check in intake port of engine. On sample engine, reed valve attached to inner side of back plate. On other engines, reed valve may be mounted directly under carburetor.

As in most two stroke, carburetor in sample engine attached directly to back plate of engine. When attaching carburetor, don’t forget to install gasket usually located between carburetor and back plate. If engine has reed valve directly under carburetor, make sure valve is in place before installing carburetor. Once have installed carburetor, attach fuel line that runs from fuel tank through back plate. Fuel line will carry fuel to carburetor.

STEP 13: INSTALL THE AIR CLEANER

Once carburetor in place, can install air cleaner element and housing. In most cases, air cleaner will mount directly over carburetor.

STEP 14: INSTALL THE MUFFLER

Next can install muffler assembly. Purpose muffler to reduce noise exhaust gases leaving engine. Muffler attached to engine’s exhaust port. In sample engine, muffler held in place by three retaining bolts. Simply place muffler in proper location and line up holes for retaining bolts. Insert bolts and tighten.

STEP 15: INSTALL THE RECOIL START AND BLOWER HOUSING

At this point, reassembly about complete. Next must install blower housing. In most engines, blower housing contains engine’s pull start mechanism. When rewind starter installed, tabs called pulley tangs are bent over assembly to hold it in place. Pulley can thus rotate without getting out of position underneath housing. Pulley has to stay aligned with eyelet or hole in blower housing through which rope must pass. Outside blower housing, end of rope would be knotted into pull starter grip. Spring, like the rope, has one end attached to pulley. Spring attempting to uncoil within housing turns pulley and retracts rope. Grip acts as brake on retracting rope.

After attaching spring onto pulley, can begin winding spring into housing. Use rewind starter tool to wind pulley counterclockwise until spring wound tightly. Careful not to wind in spring too far. If free end slips into housing, spring will uncoil and you’ll have to begin process of winding spring up all over again. When spring wound appropriately, lock free end into tapered retaining slot or separate retainer within blower housing.

Spring, pulley, and inside of housing should have been cleaned thoroughly before started reassembly. Some technicians will straighten spring to allow easier installation and restore tension to spring. Rewind starter tool can be made out of square wooden dowel or piece metal stock.

Once finished winding spring, can install rope. Before installing rope, inspect it. Replace if frayed. If you’re reusing old rope, consider burning pulley end of rope with match to slightly melt end and prevent from unraveling. When do this, melted matter at end rope may run together and cause end to swell. To keep this from happening, wipe end of rope with waste cloth while burned end still hot.

Feeding end of rope through housing can be tricky. Rope must be snaked in through hole in pulley and sometimes must be threaded through guide lug. For use inserting rope, can make inserter tool out piece music wire or spring wire. Wire will be thin enough to pass into guide lug yet stiff enough to push rope along with it. When rope is through, tie it into knot and pull tight. Make sure knot won’t contact pulley tangs.

On almost all two stroke engines, blower housing is mounted over flywheel. Is the case with sample engine. In most cases, housing fastened to crankcase with series retaining screws. To install housing on engine, would place blower housing over flywheel area and insert and tighten screws holding it in place.

STEP 16: INSTALL THE SPARK PLUG

Now that engine completely reassembled, can install spark plug. Because most cylinder heads or pots made of aluminum, threaded areas can be easily damaged. To lessen possibility damage, start spark plug into hole by turning with fingers. Once spark plug threaded into hole few turns, spark plug socket and ratchet can be used to tighten, and torque wrench used to insure reaches torque specifications.

REASSEMBLING A TWO STROKE ENGINE CONTINUED

STEP 17: ATTACH THE SPARK PLUG WIRE

Last item need to attach on engine is spark plug wire. As safety precaution, shouldn’t attach plug wire until have reinstalled engine on piece equipment it powers and are ready to use said equipment.

As can see, the reassembly of a two stroke engine isn’t very different from that of a four stroke engine. In fact, the reassembly steps described above are intended to be general enough so that they can be used on almost any engine. Of course if you ever have a question about how an engine should be reassembled, you should always refer to the manufacturer’s service manual. Refer to the service manual also for torque specifications when tightening the engine’s various fasteners.

PARTS CLEANING AND ORGANIZATION

Before begin to reassemble engine, get all engine parts cleaned and organized. Will make assembly process faster and easier. Dirt one most common causes engine wear. Since parts get dirty sitting around, good idea to clean them just before begin to reassemble. Clean even if parts already cleaned after engine disassembled. Will help ensure no dirt enters reassembled engine.

Most common way to clean parts with solvent and brush. Never use gasoline to clean parts. After have cleaned parts with solvent, rinse with clean water and dry at once. Drying immediately will protect exposed steel surfaces from rust. Can pat parts dry with cloth or paper towels or blow dry with compressed air. Properly dispose any solvent can’t reuse. Never empty liquids into sink or dump on ground. Follow government’s rules and local regulations when disposing liquid wastes.

Once parts clean, time to organize. Sort parts according to order in which will need them. Gather fasteners, gaskets, seals together for each component you’ll work on in each step reassembly. Might be helpful to lay separate parts out on different trays. One tray used for each engine component with all fasteners, gaskets, etc. Cover trays with towels or cloths to protect from dirt until parts needed.

When organizing parts, include new parts that are to be installed. Should compare new parts with old to be sure have correct replacements. Try to think of additional materials may need, such as gasket sealants, specialized tools, lubricants, and service manuals. Should organize these as well since will need them during reassembly and want them to be easily accessible.