So take 8cc of each calculation for flat-top pistons: a 12G202 has, I think, a 26.1cc chamber ... but check head as it may have been skimmed!
Total uncompressed volume = 249.66 + 26.1 + 8 + 1.4 + 0.6 = ~277.8cc
Total compressed volume = 26.1 + 8 + 1.4 + 0.6 = 28.1
Compression ratio = uncompressed volume / compressed volume
C/R = 277.8 / 28.1 = 9.88
These are approximate figures and you will need to check the cc volume of the combustion chamber and replace the 26.1 in the above calculations with your combustion chamber volumes.
Edited by gazza82, 03 February 2015 - 08:26 AM.