Hi everyone
Trying to get a handle on what compression ratio I currently have. I recently bought a used rebuilt engine and have the paperwork of the parts that were used.
the pistons are
Speed pro forged
18cc dish
specs
rod length 6.735
stoke 4.250
comp hgt 1.735
My problem is when i do the math i get a compression height of 1.765 based on deck height of 10.625
what am i missing
thanks

 

The compression height is a function of the piston and is independent of the deck height. Your math indicates that the piston is 1.765-1.735 = 0.030" down in the hole at TDC, assuming the block hasn't been decked.

 

^^^^^^^^
And based on an 82cc chamber, your comp ratio with that piston will be down around 9-9.25:1.

 

Thanks now I just have to find the head gasket thickness

 

C heads 80 cc

 

If they’ve had a valve job they’ll be every bit of 82.

 

80cc is the blueprint number for the purposes of NHRA stock class racers. Nearly every Oldsmobile head came from the factory with larger chambers than the blueprint spec. 82-ish CC is about right.