A 261 has a compression ratio of 7.17:1 for 54 thru 55.2 and 7.8:1 for 56 and later versions, which I'm assuming had the 848 head. [nope the 848 head was only used on 235's - only one PN used on 261 heads, so don't know why the difference in compression ratio] This data is from the Chevy Truck Vehicle Info Kits. With the bore and stroke, you should be able to calculate the combustion chamber volume from those numbers. Displacement + cc volume /cc volume = compression ratio. Just a little algebra needed.
[on edit] Looks like Paul weighed in while I was digging with more exact info than I could find.
Last edited by klhansen; 01/04/2026 5:27 AM. Reason: added more info
