what is the difference between the 915-2 heads and the 915-3, from what i see they are both numbered 2780915 -

I think the size of exhaust valve. Any machine shop can open it up to install bigger valve.

it might be that whenever Ma changed the sand casting mold that the -number on the end was bumped up to the next number. The heads are identical & afaik the only difference is some 915's had the 1.60 ex valve (maybe the 375 horse HP versions got the 1.75 valve)

mine are -3 and have the bigger ex valve but not sure if they were added or not, was just curious thanks

The big block 915 was the same basic casting, just machined 2 different ways, the 350hp got the 1.60 exh, the 375hp got the 1.74.

The 915 small block head was also the same casting, just machined 3 different ways, 1.88 int, 2.02 int, 2.02 int with relocated push rod holes.

Don't worry about the dash #, block castings also had them. There was some thought the lower dash # had less core shift but I believe that has been disproved.

The 1.74 exhaust valve heads usally came stamped "HP". Either a large stamp on the alternator mounting flange or a small stamp under the intake runners.

To repeat.....Casting numbers have NO BEARING on how the casting was machined.

R.