In order to set the timing properly, the camshafts must be held at TDC, especially after setting the inner timing chain. Both camshafts on each head must be held to time either head in turn. The left camshafts need to be held in place while timing the camshafts on the right head. During this procedure, the left head camshafts are under spring pressure and can cause the cams to turn, leading to miss timing the engine.
On the left head, AST-48383-1 is installed to hold the cams. Timing is set per published service information. When the engine is at the second timing position to set the right head chain, install the AST-48383-2 to hold the left head cams. AST-48383-3 is installed on the right head cams. Both AST-48383-1 and AST-48383-2 are slotted to allow them to be installed over the SIDI (Spark Ignition Direct Injection) drive portion of the camshaft in future applications.
Failure to use AST-48383 can result in incorrect timing, leading to engine damage.