The 24 valve 5.9 and 6.7 engines have 11 steel press in freeze plugs beneath the valve cover. These press in plugs can be blown out due to a combustion leak from a head gasket failure. Engines equipped with a multi layer steel head gasket, can actually leak under high cylinder pressure and reseal themselves once the pressure drops. If the plugs do not suffer this issue, they will simply just rust out over time. As you may know anti-freeze is a poor lubricant, so an extra line of defense in the way of a thick steel screw in plug is good insurance.

These are installed by threading the existing holes, once the press in plugs are removed, then installing the threaded plugs with a sealant. The grooves in the tap can be packed with wheel bearing grease to hold the chips from the threading process, you can also place a magnet in the hole and remove it after threading, before installing the plug.