I understand that the 425 Nailhead is the same block as a 401, but with a larger bore. Seems if that is the case, there may not be a lot of meat left in the block for reboring when the 425 needs a rebuild. Anybody know how far a 425 can be safely bored beyond stock?
Is it truly the same block? Like the later 400 can be bored to 430? Or is it just a cosmetically similar block on the outside, but different casting for the cylinders like the 455 is to the 430?
Same block. Only way to be sure is to sonic test it. Some say the water jackets were slightly different between the two but just sonic test it and you'll know what can be done to that block.
I do not think that the 425 and 401 blocks are the same. I had a 425 bored .040 and it seems to be running cool and fast in a well optioned 65 Skylark GS. I have another 425 bored .040 that will be landing in a 66 Rivvie GS this spring. I suppose if you are going to be running it hard a sonic check is worth it.
