The 848 head is specified at 79.1 cc. That is GM's engineering specification that Tom Langdon obtained for me way back in the 80s when he was still working for GM. The 261 heads are 86.3 or something along those lines. I have to go back and look at my notes as there may be one casting that is even larger. You can mill the 261 head down to get the combustion chamber in that range, or maybe even machine the deck of the block to up compression and improve quench, but you run head long into additional machine work. If you say mill the head, you generally have to sink the intake valve in the head the same amount. Not terrible in terms of machining but just more work to be done. You then have to shim the rocker stands the same amount, and add a lash cap of the same thickness to the tip of the exhaust valve, or order longer small block chevy valves (if you can find them in the right size). Again, not overly complicated but it seems to me using the 848 head and drilling a couple of holes is the less complicated path.
Btw, the 848 head was introduced for the 1956 pass car model year and was advertised to boost the compression to 8.0. In 58 or 59, I forget now, the compression specification jumped to 8.5. I have cc'd a few heads and I was never able to discern a difference in the 56-57 head to the 58 and up head to explain the jump in compression. The stroke is the same, the compression height of the pistons all seem to be the same, all years seem to sit about .115 in the hole, so I cannot say why we go from 8.0 to 8.5 except maybe someone at GM goofed in their calculations or decided to use new math to come up with a more impressive figure.
Am I dating myself when I refer to "new math," lol.
