[b-man]

All 428 pistons used were the same, the combined volume of the dish and 4 valve reliefs was 11ccs. This allowed Pontiac to use the same heads on 428 and 400 engines and maintain the same compression ratios.

The 428 compression ratio came in two different flavors, 10.5:1 and 10.75:1，there were no low compression regular fuel or 2-barrel economy versions of the 428. The difference in combustion chamber volume would be a scant few ccs at best. Either the heads were milled a little to reduce their volume or slightly thinner head gaskets were employed to get to a theoretical 10.75:1 from 10.5:1. The pistons were all the same 11cc dished ones no matter what.

Keep in mind that the actual real world compression ratio on these engines were roughly a half point (or more) lower than the advertised numbers.

The 421 engines did not use a dished piston to maintain the same compression ratio of the 389 engines that they shared cylinder heads with. They had 4 very large valve reliefs to get the added volume needed. Consequently the 389 pistons had 4 very small reliefs.

1964 421 piston on the right, same year 389 piston on the left. There’s no reason other than increasing the piston head volume for the larger 421 valve reliefs. Both engines utilized the same heads (both 1.88”/1.60” and 1.92”/1.66” valves in ‘64 depending on engine options) and same valve lift camshafts.