I found an old cap on a parts truck. Die cast, square 'nut' on top.. Ground off the rivet, sure enough packed with gray powder. I have drilled a 5/16" hole, but need to have the pipe tap to finalize the finished hole size. The vent is a cross drilled hole through the square. Very small like 1/32". I'd like to drill and tap for a tiny pipe plug. I'm sure if I just used a set screw, which is easier to come, by there'd be something to seal it. Edit: Research shows Permatex suggests their #59214 High Temp Thread Sealer for threaded fittings in hydraulic, oil and brake lines.