11 tips for rebuilding a two-stroke top end
1. Before you disassemble your engine, power-wash the engine and the rest of the vehicle.
That will reduce the risk of dirt and debris falling into the engine. Once you remove the
cylinder, stuff a clean rag down into the crankcases.
2. The cylinder and head use alignment pins to hold them straight in position from the
crankcases on up. The pins make it difficult to remove the cylinder from the cases and the
head from the cylinder. Sometimes the steel alignment pins corrode into the aluminum
engine components. Try spraying penetrating-oil down the mounting studs before
attempting to remove the cylinder and head. Never use a flat-blade screwdriver, chisel, or
metal hammer to remove the cylinder. Instead use this technique; buy a lead-shot plastic
mallet, swing it at a 45-degree angle upwards against the sides of the cylinder. Alternate
from left to right, hitting the sides of the cylinder to separate it from the cases evenly.
Clean the steel alignment pins with steel wool and penetrating-oil. Examine the pins
closely. If they are deformed in shape, they won't allow the engine parts to bolt together
tightly. This can cause a dangerous air leak or a coolant leak. The pins are cheap at about
$2 each. Replace them if they're rusty or deformed.
3. Never re-use old gaskets. Remove them with a razor blade or gasket scraper. Don't use a
drill-driven steel wool type pad to remove old gaskets because they can remove
aluminum from the cylinder and head. That will cause a gasket to leak.
4. Always check the ring end gap on a new ring by placing it in the cylinder between the
head gasket surface and the exhaust port. The gap should be between .012 to .024 inches.
5. Always install the circlips with the opening facing straight up or down, that way inertia
will hold it tight into the clip groove. Place one clip in the groove before installing the
piston on the connecting rod. Its easier to install a clip with the piston in your hand rather
than on the rod. There also less chance that you'll drop the circlip in the crankcases.
6. Always install the rings on the piston with the markings facing up. Coat the rings with
pre-mix oil so they can slide in the groove when trying to install the piston in the
7. Always install the piston on the connecting rod with the arrow on the piston crown facing
towards the exhaust port.
8. The traditional way to assemble the top end is to install the piston assembly on the
connecting rod, compress the rings, and slide the cylinder over the piston. That can be
difficult with larger bore cylinders, or if you're working by yourself. Try this method instead.
Install one circlip in the piston, install the piston into the cylinder with the pin
hole exposed, install the piston pin through one side of the piston, position the cylinder
over the connecting rod and push the piston pin through until it bottoms against the
circlip, install the other circlip. It only takes two hands to install the top end using this
manor and there is less chance that you'll damage the rings by twisting the cylinder upon
9. On cylinders with reed valves and large oval intake ports, take care when installing the
piston assembly in the cylinder because the rings are likely to squeeze out of the ring
grooves. Use a flat-blade screwdriver to gently push the rings back in the grooves so the
piston assembly can pass by the intake port.
10. For steel head gaskets, place the round side of the "bump" facing up. Don't use liquid
gasket sealer, use aerosol spray adhesive types instead. For hybrid fiber/steel ring head
gaskets, place the wide side of the steel rings facing down.
11. When you initially start the engine after a rebuild, manipulate the choke to keep the
engine rpm relatively low. Once the engine is warm enough to take it off choke, drive the
vehicle around on flat hard ground. Keep it under 2/3 throttle for the first 30 minutes.
Two common myths for proper engine break-in are:
1. Set the engine at a fast idle, stationary on a stand.
2. Add extra pre-mix oil to the fuel. When the engine is on a stand it doesn't have
any air passing through the radiator and it is in danger of running too hot. When
you add extra oil to the fuel you are effectively leaning the carb jetting. This can
make the engine run hotter and seize.