A stock 261 piston sits over .100 in the hole. So there is more clearance then you think. On a 235 with a 848 head and flat tops, I have run cams as much as .480 lift 280 degrees duration without clearance problems. That was a fairly aggressive street cam. So that same cam would not create issues with a 261. But when you are hot rodding one of these motors, you really do need to check the clearance with the intake valve. Isky cams use to sell checking springs so you could degree your cam and at the same time graph the clearance at so much cam timing. The other old school way to do it is to clay up the pistons, install the head and gasket, rockers, adjust the valves and rotate the motor twice. Then pull it all apart and section the clay to find the thinnest part and measure.
Keep in mind, the 848 head has a combustion chamber of about 79.1 cc while the earlier 5913 head is about 86 cc. So the 848 head results in slightly more compression. That head entered service in 56 and the 235 motors were rated at 8.0 compression ratio. In 58 or 59, the 235 was rated at 8.5 compression ratio. I have never been able to sort out the difference as the motor specifications did not change. One of these days I will measure the combustion chambers of a 57 head and a later head to see if there is any difference.
In terms of flow, i have measured the intake and exhaust posts with calipers and was never able to see any difference in the port casting. So the flow is equally poor for either head. On the intake side, the port flows right to a dead end and bounces off, made worse but the short side radius design. On the exhaust side, where do I start, lol. Not great. But that is to be expected. These motors were work horses. So that was the design.
Last edited by Dragsix; 03/26/2020 2:03 AM.
