To stroke (or destroke) a motor you primarily change the crankshaft, and hence the stroke length of each piston (from BDC to TDC), as opposed to boring out the each cylinder as a way to increase the displacement. The calculation for compression ratio is as follows:
Cr = (((3.141/4)*b^2)*s + Vc)/Vc,
Where Cr = compression ratio, b= bore diameter s = stroke length, Vc = clearance volume i.e. volume in chamber incl head gasket at TDC.
You can add a thinner or thicker head gasket as a way to change the compression ratio, but unless you change the crankshaft you are not changing the stroke of the motor.
I'll confirm with Mike at VAC tomorrow and report my findings (I've been wrong before)
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