[U47]

The 350 H-O 330 Hp. M/T block WV or A/T block XU use # 48 head at 10.5 compression.
The 400 RA3 366 hp M/T block WS use #48 head at 10.75 compression.
The 428 390 hp M/T block WJ use #48 heads at 10.75 compression.
From Pete McCarthy's book. Could this be incorrect?
My 68 & 69 service factory manual shows a picture of all three pistons (350-400-428) together and all three have relief's.

I bought a fresh rebuild/ never fired up 428 from a friends wife who had it for sale after he passed away. A bore scope revealed it had dished pistons. I was certain it was built with dished pistons to lower compression to run on today's gas. Pontiac division had been doing this on all low compression engines from 1955-1963, However reading "Pontiac high performance engine design and blueprint assy" from H-O racing it says " all pistons used by Pontiac have flat tops with valve reliefs". I'm thinking they are talking about high performance engines.


D.