When Nitro Engines go BANG!

Nitromethane is a powerful fuel and when you run it in an engine, even a tiny engine like you find in RC Planes, then there's a huge potential for stuff to break. Here's a little look through my box of engines that have gone bang and a postmortem to find out why they failed.
The engines I"m looking at today are:
- The Russian MDS 61 FIRE
- The Thunder Tiger GP61
- The Chinese SK90
All these engines were flown in the same airframe, an AT40 balsa trainer originally designed for a .40-sized engine (oops!).