Sylvester Stallone came within a hair's breadth of tanking "Rocky II" because of a gruesome injury, and you won't believe how he powered through to shoot the flick!

Sly and Arnold Schwarzenegger sat down with Harvey, and for the first time ever together they talked about their unbelievable rivalry and how it blossomed into a close friendship.

Arnold's trainer and compadre, bodybuilding legend Franco Columbu, trained Sly for "Rocky II," and when Franco challenged Sly to a bench press competition, the actor was game ... until he ripped his pec off the bone.

Here's the problem. The injury occurred a month-and-a-half before Sly was set to star in and direct the 'Rocky' sequel -- so, you gotta see him explain how he pulled it outta the fire!

As 'Rocky' fans know, Balboa's normally a southpaw (lefthanded), but in "Rocky II" he learns to fight righthanded. That wasn't just a random plot twist -- turns out it was the only way Sly could film with his injury.

As Sly put it to us ... pushing through the pain is just what driven folks like himself and Arnold did to make it to the top.