Tap and drill bit size?

Not a pressure washer problem. Just a dumb guy problem. I can’t find online what the heck size this bushing actually is. My calipers are telling me it’s 83/128. It’s not actually 3/8" as my caveman brain thought it should be.

Anyway, I’ve got a compressor with a 1/2 hole in the bottom, actually a 1/2", I drilled it out that way. And now I need to get this drain in the right way. Anyone with any clue what the proper drill bit size and tap size is for this b*****d?

Probably a 1/4 inch calipers don’t work as pipe goes by inside diameter seeing how it has pipe threads on it that would be my guess…


Pipe tap! Bingo! Not a regular one. Duh! Thanks!