Hey remby, lookie what I found on the Cox engine Wiki: http://en.wikipedia.org/wiki/Cox_model_engine

"The Venom was Cox's last attempt at making a really fast 049 mouse racing engine. Again taken from ideas learned from years of competition, this engine put all those ideas into an off the shelf product. It used the Killer Bee crank shaft loosely fitted into the crankcase, and a cylinder with porting very similar Tee Dee cylinder and tapered like the Tee Dee and with a lightened piston like the Tee Dee. The rest of it was like a Black Widow. The problem was that the production engine was not the same as the prototype. A mistake had been made in manufacturing and the piston was lightened too much. This made the engine fast but the piston weak and they would blow the top off the piston after a few runs at high speed."

Kinda sounds like what you experienced, huh? [X(] (Wouldn't surprise me with so many interchangeable parts lying around at the factory...)