What year and size, both C.I. and H.P., is it? Is it N/A or turbo charged?
Most Lycoming N/A air cooled opposed aircraft engines are below 8.5 to 1 compression ratio, I'm not sure about all of them and or motors like yours.
Those are low RPM motors, usually below 2700 RPM Longer stroke than bore size also, some are rated for continuous full power and some are limited to 5 minutes or less at full power
Bottom line is most of them are not precise engines with tight tolerances on the rotating parts like the turbine motors are