1/4" NPT. Your local LWS should have it on the shelf.

 

Thank you. 12V71.

 

1/4 NPT. And being that it’s on the high pressure side of what I’m assuming is oxygen, be VERY careful installing it. A simple fingerprint can end up explosively badly.

 

It's actually 1/4 G thread. There should have been a seal around the 1/8" nipple.

 

Let's see the hole that it came out of.

 

That's right it's a 1/4G thread, parallel not taper. The replacement Harris gauge comes with a teflon seal ring in the bag. You can't buy the seal separate. I tried just recently end ended up cutting a seal ring out of aluminium for my acet reg. Don't know how it would go for an oxy reg.