happened to me today what is happening is that actually the metal kinda somehow is trying to turn. This is bad you have to take off the head and carberator and get a rubber malatte and get something that wont damage your piston then bang it down. I have 12 years experience with hotrods and 8 years experience nitro rc. Lisen to me when I say that. But make sure first that is isnt something damaged. PS you might have to break in the engine again if you screw up. BTW if that dont work then you have to open the back of the engine up because believe it or not some 2 stroke engines could get a airpocket that wont release and will deffinately be strong enough to hold your engine.

One last thing if your exaust is open (which if it isent then dont do this) then just take alittle nitro and put it in the block when you have the heatsink off then put it back on and ignite it. That broke my engine free in an instant though there are risks to this method.