The high-pressure fuel pump mounts to the cylinder head at an upward angle, and the cam lobe must be at its lowest point to prevent damaging the HP fuel pump plunger. GM-51365 is placed into the fuel pump mounting bore to verify that the camshaft is in the correct position for installing the fuel pump. GM-51365 indicates the correct position when it is fully seated against the mounting surface of the cylinder head. Installing the fuel pump without using GM-51365 may cause damage to the fuel pump.