Ok, I’ve got this tee fitting which is pipe threaded on both outlets, but I haven’t been able to find out what the “official” thread size is. It isn’t 1/4" NPT, the threads are too fine. here’s an image:
it’s roughly 24-25 tpi.
any ideas what it is?
This thread gauge should help. Or take the part to a plumbing supply store. They should have a gauge and can measure it.
https://m.lowes.com/pd/IRWIN-Standard-SAE-Thread-Gauge/50146290?cm_mmc=SCE_PLA-_-ToolsAndHardware-_-SpecialtyHandTools-_-50146290:IRWIN&CAWELAID=&kpid=50146290&CAGPSPN=pla&store_code=3331&k_clickID=fbd86a8f-6127-426a-aa19-089c4240f5ae&gclid=CjwKCAiAm7LSBRBBEiwAvL1-L4j84J0BDS4Sk2QlemgOX3DAtGF0F0mb_oHnMc-1AUF1zL_7pg2UmhoCwvgQAvD_BwE
Hamsters ate my first reply.
I’d have guessed around 18 tpi. Which pegs it nicely for 0.25"/18 NPT.
But if it’s important at all, yeah, get thee to a gauge.
found one of the pipefitters at work; it’s def. 24 tpi with a ~.490" major diameter. he was stumped, it’s either something really obsolete or just an oddball, non-standard size.
12 mm ?
pitch is wrong; M12 is closer to 17 tpi.
Well I’ll be… I guess I need to get my guessing equipment re-calibrated.
Thanks …
Flared fittings and tubing threads are different than pipe, kind of sounds like a tubing thread size.
Any more pics? From the one you provide it certainly looks like 1/2" NPT. However, that thing is no garden variety “T”… Looks like some sort of an eductor or maybe a natural gas lamp/burner fitting?
The leg on the right appears to be a support flange?
it’s for a set of Buell marine air horns.
I don’t know how old these things are; given how they’re made I’d say 1940s? Or maybe '50s? this piece is both a pneumatic manifold/tee but also the mounting flange as you correctly deduced. I was hoping to replace this tee with a pair of right-angle nipples and supply them via high-pressure air hose, but I’ve stumped everyone I’ve asked as to what these threads are.
edit: note that the short piece of copper tubing feeding into the tee/mount is not the same thread; it appears to be a standard (older) fine pipe thread.
That looks like pipe threads to me in the picture and it doesn’t look like 25 TPI, did you try a pipe fitting on it to see if it fits?
it is tapered pipe threads, but it doesn’t match anything standard. The facilities guy I talked to at work got out his thread gauges and confirmed it’s 24 tpi. but for the life of us we can’t think of what male pipe thread had a .490" major diameter but 24 tpi pitch. Standard 1/4" NPT is too big and too coarse. I’ve tried 1/8" compression, too small.
I mean, this isn’t life-or-death or anything, but damn if my curiosity isn’t piqued now.
Closest thing I can come up with is 1/4" BSP (Brit. Standard Pipe) straight thread @ 19 threads/inch with a 0.518 major. Sort-of, kind-of, close to your numbers.
BSP threads are used in lots of metric applications. I know, crazy but true.
as a wise man once told me, “close only counts in horseshoes, hand grenades, and thermonuclear warfare.” 
Could it be a proprietary thread?
My sodastream CO2 bottles had a wacky proprietary thread to force the user into using the company’s CO2 bottles. Luckily, someone designed an adapter to facilitate the use of paintball bottles.
You said it was for air horns, which makes me think special high pressure fitting. So I went googling…
Scuba adapter
This one is 3/8-24. That size might also be what some stove natural gas regulators use, I get hits but can’t find thread specs.
I think you may be on to something. I googled “high pressure pipe fitting” and found this which might be going in the right direction.
I googled: Buell marine air horns and got your exact horns @ Buell marine air horns Strombo and Classic Sets
They are designed to operate at 80-200 psi. Not exactly high pressure: but who knows what kind of squirrely adaptations they may have utilized in their fittings.
The trouble with the 3/8 x 24 fitting referred to: It has a major diameter of .3750" . Your wise man may not find this acceptable.
Suggest you contact these folks and find out what the heck kind of pipe fitting you are dealing with.
A 5/16 tubing size compression fitting uses a thread size of 1/2" 24 threads per inch. You mostly see these used around light duty water applications like ice makers and coffee makers but air horns and a few other air fitting use them also. I am sure you have a 5/16 compression fitting.