There are lots of different materials used for the piston and liner, and the conrod may or may not have bushings etc. Each engine has a different tightness to it and the fuel is perhaps even more important, as some oils will simply evaporate when they get too hot.

Your engine gets too hot when it is run on the lean side of max rpm and/or when it cannot hold the rpm and sags with time. These are the things to avoid rather than a specific target temperature that you will struggle to measure correctly.