Rebuilding Your Two-Stroke's Top-End

Rebuilding a two-stroke's top-end is a relatively cheap, easy and quick way to give your old two-stroke motorcycle a new lease of life. A worn piston and rings won't form a tight seal in the cylinder, which can scuff the bore and starve your engine of power. Fitting a new piston will protect your engine from expensive damage; if a weakened piston cracks after months of abuse can cause all sorts of strife.

During a top-end rebuild, your motorcycle's most highly stress components will be open to the elements, so ensure you clean your motorcycle thoroughly and completely dry it before taking it apart. Remove the fairing to open up some room so it is easy to get your tools in and then disconnect the fuel line from the carburetor. Remove the fuel tank and now you should have unobstructed access to the top end of the engine.

Remove the coolant if the motorcycle has liquid cooling by the drain bolt and empty the engine and radiators of all coolant. Once all the coolant is out of the radiators disconnect the radiator hoses from the engine and remove the spark plug. Remove the exhaust pipe mounts and remove the exhaust system.
Undo the four cylinder mounting bolts, depending how old your motorcycle is you maybe also have to remove the valve cover and power-valve linkage arm and with the engine mounts removed slowly work the cylinder up off the mounting studs until it is clear of the piston. With the cylinder removed, the bottom-end will be fully exposed so ensure you stuff a clean rag around the conrod so no dirt enters the engine. With the cylinder off take the opportunity to hone the bore, especially if you see any damage.

Remove one of the circlips and use a small socket to push the wrist pin through the piston and out the side. The piston will now come off the conrod. It's a good idea to replace the top-end bearing as it too takes a lot of abuse.

Remove the old base gasket and clean the surface with a rough cloth before carefully fitting the new one. Lightly oil the new top-en bearing and slide it into the conrod. Place the new piston over the conrod and insert the new wrist oin through the piston, ensuring the arrow on the top of the piston points towards the front. Install the new circlips.

Lightly oil the cylinder walls and guide the new piston into the cylinder. Tighten the cylinder base nuts using a ring spanner and, if you can, use a torque wrench to ensure the cylinder is tightened evenly. Install the engine mounts and reconnect the power-valve arm, before sealing it with the cover.

Reconnect all the radiator hoses, making sure the hose clamps are tight around the joins and fill up your radiator with coolant. Install a new spark plug and connect the lead so your motorcycle's got a fresh spark. Throw the fuel tank, shrouds and fairing back on, hold your breath and start the engine...Tag: Two-Stroke Engine Maintenance Repair Mechanical Technical
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