11 tips for rebuilding a two-stroke top end
from MCB Performance (edited for paramotors by Had Robinson)
1. Before you disassemble your engine, clean it as best you can with air and rags soaked with mineral spirits. 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. It is so easy to drop things into the crankcase. Also, the rag will help keep the connecting rod from moving up and down while working on the engine.
2. The cylinder and head use alignment pins to hold them straight in position. 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 (on some engines). 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 0.012" to 0.024".
5. Always install the circlips with the opening facing straight up or down, that way inertia will hold it tight in the clip groove. Place one clip in the groove before installing the
piston on the connecting rod. It's easier to install a clip with the piston in your hand rather than on the rod. There is also less chance that you'll drop the circlip in the crankcase. If they do fall in the crankcase, a magnet-type tool will easily get them out.
6. Always install the rings on the piston with the markings (if there are any) facing up. Coat the rings with 2 stroke oil so they can slide in the groove when trying to install the piston in the cylinder.
7. Always install the piston on the connecting rod with the arrow on the piston crown facing towards the exhaust port.
9. For steel head gaskets: place the round side of the "bump" facing up. For O rings NEVER use RTV to hold them in place, use an aerosol spray adhesive e.g. head-liner/molding adhesive, instead. A good alternative is to gently stretch the O ring by looping with your fingers. Do it just enough so that it stays in place.
Two common myths for proper engine break-in are:
- Set the engine at a fast idle.
- Add extra pre-mix oil to the fuel.
When you add extra oil to the fuel you are effectively leaning the carb jetting. This can make the engine run hotter and seize. However, there are some paramotor manufacturers that specify adding extra oil. If the engine manual says to add extra oil, do so. In general, always follow the instructions from the manufacturer, if any.