Original 261 head (casting 3836850) compression ratio: ~7.2:1 to 7.5:1 stock (varies slightly by year; 1954–1962 truck engines).
848 head (235 high-compression, casting 3836848): ~8.0:1 to 8.25:1 on 235s, but when swapped to 261 without milling: ~8.0:1+ (higher than stock 261 due to smaller chamber).
Combustion chamber volumes (approximate, from forums/inliners):
Stock 261 head (850): ~60–62 cc (larger chamber, lower compression)
848 head: ~52–55 cc (smaller chamber)
To match 848 compression on a 261: mill stock 261 head ~0.125" (or use 848 head milled ~0.030").
CC your heads exactly (with valves/seals installed) for precise mill amount. Use a compression calculator (e.g., Wallace Racing) with your bore, stroke, gasket thickness, deck height, and measured cc.
Stock 261 was low-compression truck engine; 848 swap/milling is common for better performance.
