Howdy all. I'm curious to get schooled on opinions of using 425 pistons in a '65 400 block. Back in the '80s when we rebuilt my '65's engine, the days before the internet and easy search tools, we chose to bore out the block (which was already bored .030 we found when we opened it up) to .090 over to use 350 pistons. I'm needing to get into this motor again and am facing the choice of sourcing an original block to start from scratch with or using what I've got. The thought had crossed my mind about just boring out a bit more to use the 4.125 425 pistons. From memory, I remember the blocks are the same size, I'm not sure what other differences there were between the two. Years past I did read an article about this subject and whether it was doable or crazy. I think the article leaned towards crazy as it was felt the walls would get too thin and too much heat. Has anyone done it, heard of it, read any more about it?

 

Get it Sonic checked. There is a 4.065" and 4.100" piston from Mahle.

 

Years ago when I did my 66, I was told the water jackets were the same as a 425, but I chose to go .057 over & use a standard 350 piston.
Sounds like you used the .030 over 350 piston (4.057" std), which should have been .087 over

I would inspect the bore first as it is not worth it for 6 or 7 more cubes.

I found an old article which sonic checked an 390925 block & stated it would have .191" minimum cylinder thickness at the std 425 bore.
Obviously this could vary engine to engine, but it at least proves it can be done in some cases safely.

 

The other issue you will have with the 1965 400 block is the lifter angle is 45 degree requiring a custom ground camshaft. If you want those extra cubic inches, start with a 1967 425 block or a 455 block, they will have the 39 degree lifter angle. Further, the 1966 and 1967 Toronado 425 blocks and the same year 442 400 blocks although having the 39 degree lifter angle used the .921 diameter lifter which is pricey when you can find them.