I believe it was mentioned here, but just go ahead and tap that nipple for 1/8" NPT and put a real pipe plug in it. Then use the dorman cap just for the aesthetics of a finished edge. I ran that on my 318 for at least 7 years.
Well I thought that I would take your advice and do just this. The problem is that I pulled out an old water pump to do a test fit with the 1/8" NPT tap. It drops right in.
Did you mean to use a 1/4" NPT tap?
Mine was actually an 1/8" NPT, perhaps just a different casting aftermarket pump than what you have.
