Hi Everyone

I just pulled the port motor because it was low on compression, and had massive blow by. One hole was down to 50psi. I only checked 4 of the easiest holes. Didn't bother withe the rest since the motor was coming out anyway. I have one broken piston and low compression on all cylinders.
This motor was rebuilt by a local builder about 150 hours ago. Stock 415hp 502 with a Merc 502 MPI set up in a 42 Harley that is cruised around 3500 rpm's 99% of the time. Occasional full throttle getting on plane if loaded heavy.
I think it was assembled incorrectly, but I don't know. The starboard engine has over 900 hours on it, and is getting tired, but still runs fine. So I don't think that I am abusing it. I'm sending it out to a different builder this time.

Question: What would cause this?
What questions should I be asking the new builder to ensure that it doesn't happen again?

thanks,

Steve H