Here is an actual example of engine failure. Mostly due to an engine modification known as "a knife edge rod". It's suposed to lower the internal drag, but what it is prone to do is concentrate loads on a notch and propagate a crack. I didn't buy any more engines from that tuner. This was on an engine that ran at around 26-27K in the air. The 6.5 indicates that it is a .40 sized racing engine. This is the only engine that I've had grenade in the air in over 40++ years.