#4 pisten mystery

[nic.wiggins]

My dad and I own a 1929 82A pickup. We replaced the worn out 30A block with a 32B block, winfield head, brinley 3/4 cam, balanced crank, ansen down draft, demon carb and mallory centrifugal advance distributor, mallory coil - professionally built.

Now the mystery - at about 500 miles, driving from a car show the #4 piston fails (rod stripped of babbit, damage to piston skirt). Sounds like a cement mixer.

This has happened now 4 times!

I really need some help.
Any ideas welcome.