What happend?
On April 15, 2001, I was drafting behind Craig's watercooled Aprilia RS50 downhill going roughly 60 MPH (according to the RS's speedo). I was told twostroke motors love to rev and designed to do so. I had it pinned for about 2 min when all of a sudden I heard large pop before the engine just shut off. I was able to push start it and I tried nursing it home when it just died.
I could tell there was no compression when I tried push starting it and the wheels would spin freely. Thinking back I probably should have kept my stock gearing. I was running my track gearing of 46 x 12 when I should have been running 42 x 12 for the street. The piston might have survived. Live and learn.
Luckily, it was a partial seizure because the piston did not fuse to the cylinder. If it had then it might have killed the crank (bottom end). Thus, a major engine rebuild and many YSR $$$ later ($400-$900 from what I've been told). My little incident only took $125 out of my pocket which included parts and labor. Thank god cause this hobby is getting costly.
Important: If you have a stock bore i.e. (40, 40.25 (1st over) or 40.5 (2nd over) then use stock Yamaha pistons. These are more bullet proof than Weisco pistons like the one I had. From what I've been told, racers have to change their Weisco pistons every 4-5 races...or else you get the piston above and heartache. Unfortunatly, if you go larger than 2nd over bore, then you are forced to go Weisco. The largest bore a stock cylinder can take is 44 which makes it a 60cc monster!!!! Weisco makes a piston kit for this...