Whenever I run a new engine on the test stand I always feel the crankcase underneath the carb are after a minute or so to make sure it's still cool. Any heat there would indicate too close a fit between the crankshaft and crankcase. Try this with a bushed engine and it'll still feel cool which is an indication of how little friction there is with a bushed crank. The crankcase itself is (or better be!) always cool for the reason given by Pe. After shut down there'll be a heat soak down from the cylinder and then it'll warm up some.

Some time ago I was running in a rear induction OS Max-H 80 and just for curiousity I dropped my thermocouple down past the carb into the elbow where it turns towards the drum valve (no chance of the drum valve chopping the thermocouple into bits ) and the temp was about 20F below ambient (which from memory was around 70F).