[b-man]

The bore and stroke determine an engine’s displacement.

You’re swapping a 455 crank into a 421 or 428 block, creating a 455ish engine. The crank stroke increased from 4.00” to 4.21” when the 428 was upsized to become the 455, the slightly bigger 4.151” bore of the new 455 wasn’t the biggest factor in the displacement difference between the two engines it was the added stroke.

The 428 has a 4.120” bore and the 421 is 4.094”, both use the 4.00” stroke crank.

The piston pin location in relation to the piston top must be changed by half the additional stroke you’re adding. In the case of swapping in the 4.210” 455 crank you need pistons with the pin .105” closer to the piston top, because the rod length (6.625”} isn’t changing.

If you tried using a 428 piston with the 455 crank it would end up .105” further above the block deck than a 455 piston with the correct wrist pin location that matches the 455 crank.

The parts don’t care which block they’re residing in, a 428 block bored +.031” to standard 4.151” 455 bore size housing a 4.21” stroke 455 crank now becomes a 455 cubic inch engine.