WORKER WAS TORQUEING THE INSIDE WHEEL NUTS ON A CAT 77 USING A 1"" AIR WRENCH. THE WRENCH WAS NOT ""SEATED"" PROPERLY ON THE WHEEL NUT WHEN THE AIR WAS APPLIED, RESULTING IN IT TURNING & PINCHING HIS RIGHT THUMB AGAINST THE VALVE STEM GUARD. THE WORKER REQUIRED TWO STITCHES IN ORDER TO REPAIR THE CUT.