Installing a Metrakit 65cc on a ZA50. Everything has went well and when running piston to tdc all looks good. When running to bdc the piston does not quite clear the exhaust port. Obviously can shim cylinder but any experience on how far to shim/spacer? Don't want to lose too much compression but also do not want to burn piston or rings in path of exhaust. Picture is through exhaust port at bdc. Thanks.