To service valve stem seals on-vehicle, compressed air is used to hold the valves closed in the head. On SGE & MGE direct injection engines, the injectors must be removed before removing the cam cover to access the cams and valves, and the injector bores will allow air pressure to escape. Reinstalling the injectors to seal the bore is not recommended because the injector seals will not hold pressure once removed. GM-51511 will attach to the injector rail bolt boss of the cylinder being serviced and seal the injector bore, allowing air pressure to have the valves in place for service. This positive retention of the bolt boss prevents unapproved sealing methods from becoming projectiles due to air pressure.