C-4 C4 Forward Drum Assembly. This assembly weighs 2.82 lbs. and is 45% lighter than the OEM cast drum and input shaft. The input shaft is located by a shoulder stop and snap ring. The spline counthas been increased to 35 from the OEM count of 24 and the spline diameter has been increased by 25%. Drum is made from 7075 T6 billet aluminum. Input shaft is made from 300M.