You are correct, Leathermang and those pistons are likely up to temperature within a minute of exposure to combustion heat.

With the old TRW 'Powerforged' Racing Pistons, you had to run them really loose, about .007" clearance cold and they would 'slap' until they warmed up. It was a very hollow, distinct sound.

The pistons picked up enough heat to quiet down after 30-45 seconds of running...Bert