It might be easier if you posted what engine you have and what if any modification have been done to it. Some one with the same engine might have the info for you. To be able to calculate your CR you will need to measure the cylinder heard volume at Top Dead Center (CC's). Then your max vol will be your 123.5cc + this head vol. Your min volume will be this head vol. The CR = max vol/min vol. The effective CR is measured just as the exhaust port closes. That is, your effective max vol is (the vol that is left of your stroke at exhaust port closing + min vol)/min vol. Hope this helps.