EMPLOYEE WAS REPLACING BOTTOM ROLLER ON TRACTOR.A ROCK THAT WAS CAUGHT BTWN PUSH ARM & FRAME FELL & HIT EMPLOYEE ON BACK OF HEAD. HIS HEAD STRUCK CONCRETE FLOOR PUSHING HIS HARD HAT INTO HIS F OREHEAD CUTTING HIM. ROCK WAS 18-24"" DIAM. THE TRACTOR WAS @ MMI REPAIR SHOP & HAD JUST BEEN MOVED TO CONCRETE APRON AROUND SHOP. EMPLOYEE SAID HE NORMALLY CHECKS FOR ROCKS ON THE FRAMES.