The camshafts must be held at TDC to properly set timing, especially once the inner timing chain is set. Both camshafts on each head must be held in time to allow either head to turn. The camshafts on the left head must be held while timing the camshafts on the right head. During this procedure, the left camshafts are under spring pressure, which can cause the cams to turn, leading to misfiring of the engine. On the left head, GM-48383-1 is installed to hold the cams, and the timing is set according to published service information. When the engine is at the second timing position to set the right head chain, GM-48383-2 is installed to hold the left head cams, and GM-48383-3 is installed on the right head cams. Both GM-48383-1 and GM-48383-2 are slotted to allow for their installation over the SIDI drive portion of the camshaft in future applications. Failure to use GM-48383 can result in incorrect timing, leading to engine damage.