The actor says he and Tom used to box when they were teenagers filming "The Outsiders" way back in the day ... and Tom got the better of him in one memorable sparring session.

As Rob recently explained on "The Rich Eisen Show" ... he was 17 years old and Tom was 18 when they filmed the movie together. He says they would work out, jog and box together while they were holed up in a hotel on location ... and the testosterone was flowing.

Rob says the sparring matches were intense and competitive ... complete with headgear and mouthpieces ... and Rob says Tom once knocked him out in their hotel's hallway.

While Rob says he had "pipe cleaner" arms back then, he recalls Tom being absolutely jacked and a "beast." He says he landed a pretty good punch and rang Tom's bell during one sparring session and then, the next thing he realized, he was waking up on the ground.

Rob says after he landed the first punch, Tom's eyes went black ... and TC knocked him out.