Two-stroke motors have fewer moving parts than four-strokes. They don't have a complex valve train. In fact, other than simple-to-replace reeds, two-stroke valves have no moving parts at all. They are essentially just ducts. Fast-revving two-strokes damage their top-ends with far greater frequency than four-strokes. A two-stroke rebuild almost always means a "top-end rebuild" and is a relatively simple process. You don't even need to remove the motor from the chassis.
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Things you need
- Motorcycle lift (Optional)
- Shop manual
- Mechanic's tools
- Gasket kit
Use a clean, well-lit work space. Ideally, put your motorcycle in a lift for easy access and safety. If you don't have a lift, make sure the bike is secured so it doesn't tip over as you're working on it.
Remove the seat, fuel line and fuel tank so you have easy access to the motor. Remove the spark plug wire. Loosen the intake manifold clamp. Remove the carburettor and secure the throttle cable so it doesn't get in the way. If the motor is water-cooled, drain the coolant, detach the cooling hoses and secure them. Remove the air box and anything else that might make it difficult to remove the head, such as the kick-starter.
Unscrew the head bolts in a criss-cross pattern: back one bolt off a turn or two, then the bolt cross from it. Continue this process to evenly unbolt the head then remove it.
Remove the bolts at the base of the cylinder in the same gradual manner as the head bolts. Then gently remove the cylinder. If it doesn't slide off easily, spray a little lubricant/solvent on it. Be careful that you don't damage the piston as you slide the cylinder off. Place a shop towel around the piston to protect it until you remove it.
Remove the piston and wrist-pin. Evaluate the damage. Inspect the cylinder wall, head, piston, rings and wrist-clips. Decide which components need to be replaced and which can be repaired. If you're unsure, take the parts to your dealer.
Determine whether your cylinder is plated with Nikasil. If it is, have your dealer or 2-stroke machinist do any cylinder clean up. If it's not, you can hone the cylinder yourself. Have a machinist repair any head damage or replace the head. Purchase a new piston, rings and wrist-pins, (or any combination thereof) if necessary.
Install the new piston, wrist-pin and circlips. Coat the piston with motor oil and install the rings with a ring compression tool. Install a new gasket onto a clean base surface. Coat the piston thoroughly with oil and slide the fresh cylinder over the piston and rings, then release the ring compression tool.
Remove the remnants of the old head gasket with solvent and a gasket scraper. Replace the head gasket onto the cleaned surface. Reattach the fresh head to the cylinder. With a torque wrench, use a criss-cross bolting pattern to torque the head bolts to factory specs.
Reassemble the carburettor, coolant hoses and air box if you removed them. Replace the fuel tank and seat. Refill the coolant.
Replace the spark plug cable and any other parts you removed for access. When you first start the bike, pay special attention to how it runs and sounds. You will need to give the new, rebuilt head time to "break-in."
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