Can someone please help me figure this out? I've never used a tap before and I have need to tap an already existing hole in the intake. I went out and bought a tap set and a brass fitting to put in the hole. Attached is a picture of the fitting and the harbor freight tap. The fitting says 3/8" so I used the 3/8" tap. On one hole I used a tap that says 3/8"-16NC. The 3/8" brass fitting I have will NOT go in there. I was able to get a 1/8" fitting to fit and so I put that in with Teflon tape. On the other hole I am using a tap labeled 3/8" NF 24. It made the threads and looks good to my eye, but the fitting still will not screw in to it. Can anyone tell me what tap I need to use in order to get this stupid fitting to thread in? Thanks for any help....