I've had a number of failures of Cox engines. In one the ball socket became so loose that it pounded a hole in the top of the piston. In another on a Tee Dee pylon racer a long screaming dive was terminated by the engine going silent and the prop windmilling in the slipstream. I had broken athe crank pin off the crank. I thought that was quite amazing since I rarely used any more than 20% nitro and usually used sport props.
The reset tool is a good investment for anyone planning on using Cox engines because they will all require resetting at some point in their life.
Peter