Is the Ford V8 351 a small block or big block?

Answer 1

It depends. There is a 351 Windsor, a small block, a 351 Cleveland, a small block, and a 351 Modified, which is a strange mix of big and small block features.

These are referred to as 351W, 351C, and 351M, respectively. The 351W looks just like a 302, only it's a couple inches wider. The 351C has a smaller block than a 351W, but much wider cylinder heads than the 351W. The 351C was offered in both 2 barrel and 4 barrel versions. The 2 b heads had ports a bit bigger than the 302/351W, but the 4b heads had HUGE ports. The Cleveland, although it's quite wide, is still very much a small block. (the block isn't much bigger than a 302's block)

And then the 351M.. I refer to these as 351"Mutant", even though the M really means "Modified". This strange 351 has a hellhousing pattern and engine mounts that are interchangeable with 429/460 big blocks, but heads very similar to 351C 2barrel. The block itself is roughly the same dimensions as a 351W. It's more small block than big block, but a lot of guys refer to it as a medium block. Whatever. I just call them Mutants.

And there you have it.

A real quick way to check is to look at the number of bolts holding down the valve cover. Eight indicates 351C or 351M, Six indicates 351W. Also, 351W valve covers are flat and have a rectangular profile, while 351C and 351M usually have a trapezoidal profile. They never offered the 351C in a truck in the US, only the 351M/(and 400 - a stroked version of the 351M)