The RPi3B runs pretty hot and may need radiators or a fan in some situations, especially java intensive operations. Some of the radiators for sale (like the copper ones on Amazon) have a thick, black, tarry, double-sided adhesive that is a very good thermal insulator. Remove (preferably before installing) with "Goof Off" or similar solvent and use Arctic Alumina epoxy in a very thin coating. You'll never need to remove it so permanence is not an issue.
The RPi2 is not nearly so hot as the 3. The console command "vcgencmd measure_temp" is your best bet for checking CPU temp. The RPi auto-throttles the CPU speed at around 85C but my 3B never gets above 62C with the copper/Arctic Alumina combination (overheated often without and black goo only reduced my temp by 2C). No flames if you don't light it up with a high voltage:slight_smile: