So the US 261 truck motors were installed with a head that utilized a larger combustion chamber. There were two casting numbers, one larger then the other, I think its like 86 something vs 92 something, in that range any how. The 848 head's chamber is at 79.1 so a little tighter and worth a little more compression. The rub, no 848 head was ever installed, that I know of, on any US built 261 motors. So to take advantage of the slightly higher compression using the 848 head, you generally mimic the steam holes on the 261 head by drilling them on the 848 head. The trick is the holes were drilled at about a 12 degree angle to clear some of internal casting. Drilling straight in, two of the holes, one on each pair, never hits water, just drills into the casting.
Now the exception to the 848 on a 261 motor is Pontiac of Canada. Pontiac of Canada used the 261 as its entry level motor. These motors did not get the full flow option, but did get the 848 head factory installed on the 261. So if you are lucky enough to come across an 848 head from a Canadian Pontiac 261 motor, its already drilled. If anyone is so interested, the valve cover for these motors is the same as the US motors (55-57) but the "Chevrolet" logo has been removed, a plain valve cover.
Last edited by Dragsix; 01/14/2024 4:35 PM.
