EMPLOYEE WAS ON THE LOWERED DRILL MAST TO REPLACE AN 0-RING ON A BOSS FITTING. HE WAS ATTEMPTING TO LOOSEN THE FITTING WITH THE USE OF A CRESCENT WRENCH AND 1 1/8""COMBO WRENCH. HE COULD NOT GE T THE FITTING TO BREAK LOOSE AND HE ATTEMPTED TO STRIKE THE CRESENT WRENCH WITH THE COMBO WRENCH. THE COMBO WRENCH STRUCK THE LOWER PORTION OF HIS LEFT THUMB CAUSING LACERATION.SUTURES RECVD.