GX-31 piston in a GX-35 is 12.5 -12.8:1
On methanol its ok, on pump gasoline it may detonate.
With a given CR you can back plot the combustion chamber volume. I have it in my notes but can't access them right now.
don't use water! to measure volume use ATF, no rust problems with ATF
Well, I do use water because ATF will dissolve the grease I use to seal the cylinder and ATF will take the numbers off the syringe.
So, a bit of grease on the sealing surface of the head:
Then plank it down on a flat surface, such as this aluminum plate. Glass works very good.
Now fill up the combustion space with water, measuring what you put in, up to the bottom spark plug thread:
Easy peasy! This gives a good CC comparison between various heads but isn't very good for calculating compression ratio with a domed piston.
In this case, grease the outer edge of the piston top and measure the water to fill.
Combustion volume is this volume plus the bore area x squish distance.
Compression ratio is (displacement + combustion chamber volume + squish volume) / (combustion chamber volume + squish volume)
Squish distance is the closest distance from your piston at TDC to your head. Typically measured by crushing a lead wire.