I have a tripower 389 that keeps breaking the number 5 piston. All others are ok. So far I have replaced that piston three times and no one can come up with a reason why and how to stop it. Anyone out there ever heard of this?

I'd like a little more information about the build up of your motor.

Have you modified it from stock specs? More aggressive profile cam?

Breaking a piston is usually a sign one of two things: (A)  too much compression - well more than the piston was designed to take or (B) a valve coming in contact with the piston when it clearly shouldn't.

unless something is not lining up right in the block bore or crank journal?

Yea, a LIL more info on the piston. Are probs due to valve hitting piston, breaking ring lands or burning holes?