Hi, you can calculate totalvolum of stroke (not all ports are "just"closed, allways from BDC to TDC) and bore and measure volume of combustion volume in cylinderhead (use syringe with sewingmachine oil to measure volume with piston at TDC and fill oil until plughole).

The formula are:

Totalvolume of cylinder + combustion volume : combustion volume = Compresion ratio

Jens Eirik