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Although the 401 and the 425 nail head is almost idential I have read that you cannot bore a 401 to a 425 . The casting numbers are different for the 401 and the 425 with one exception : number 1364705 for 1964 through 1966 is designated as 401/425 . Does any one have any information on this engine block casting ? Why it was made , were it was used etc.
I have read that you cannot bore a 401 to a 425 .
depends on core shift. the 1/8" or .125" bore difference is a huge overbore for a thinwall casting block but the Nailheads are either before thinwall casting or the first iteration of thinwall that Buick did. they are a bit more robust than the later BBB and SBB designs.
you would still have to have any 401 block sonochecked for cylinder wall thicknesses before you ever even considered doing something like that though.
Buick was one of the 1st. to start using the thin wall casting method. This started in 1957. No Way, No How can you safely bore a 401 to a 425 bore. You MUST HAVE THE BLOCK SONIC TESTED!!!!!!! Even then it's kinda iffy.
Tom T.