401/425 Buick Nailheads

Are there any visual differences? Are most parts interchangeable?
Thanks for any help guys.

 

Look identical on the outside. Both share the same crank with a 3.640 stroke. Blocks are different. 401 has 4.1875 bore, 425 has 4.3125 bore. Intake and exhaust manifolds, water pump, timing cover, dist, etc will interchange.

 

Are the block castings actually different, or is it simply the bore difference? In other words, if a '66 401 was bored to 4 5/16, would it then be exactly the same as a factory-made '66 425? - Rick

 

http://www.teambuick.com/forums/view.php?pg=ident_nailhead_interchange

Here is a link that will show you what will interchange!

 

If it could be done that would be true but there's not enough meat in the block to bore it that far.

 

Same casting for the 61-66 401 (1185405 or 1185485) 63-66 425 (1349046, 1354704, 1364704, 1364705)... not enough of a difference to chance it, there are supposedly a few 401's that have enough meat to bore. I don't bother even trying. You'd have to have the block checked really well, but a 401 makes great torque.