Matl steel sae 4130 or sae 434o; 2.17 in. Min 2.19 in. Max lg, 0.623 in. Min 0.624 in. Max od, 0.50 in. Min 0.52 in. Max id, piston 0.705 in. Min 0.715 in. Max lg; 3 grooves 0.02 in. Min 0.03 in. Max deep; black oxide coat per mil-c-13924; heat treat rockwell c 34-28 per mil-h-6875