# plumbers/pipefitters- identifying thread size **URL:** \ **Category:** Factual Questions\ **Created:** [January 3, 2018, 3:07pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799 "2018-01-03T15:07:21Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 3, 2018, 3:07pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/1 "2018-01-03T15:07:21Z")
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: [![](https://i.imgur.com/8wgqbM0.jpg) ](https://i.imgur.com/8wgqbM0.jpg) it’s roughly 24-25 tpi. any ideas what it is? ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [January 3, 2018, 6:13pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/2 "2018-01-03T18:13:30Z")
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](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) ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [January 3, 2018, 7:11pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/3 "2018-01-03T19:11:12Z")
> [@jz78817](#): > > 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: > > [Imgur: The magic of the Internet](https://i.imgur.com/8wgqbM0.jpg) > > it’s roughly 24-25 tpi. > > any ideas what it is? 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. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 3, 2018, 7:16pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/4 "2018-01-03T19:16:45Z")
> [@MonkeyMensch](#): > > 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. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [January 3, 2018, 7:23pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/5 "2018-01-03T19:23:41Z")
12 mm ? ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 3, 2018, 7:47pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/6 "2018-01-03T19:47:31Z")
> [@watchwolf49](#): > > 12 mm ? pitch is wrong; M12 is closer to 17 tpi. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [January 3, 2018, 7:54pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/7 "2018-01-03T19:54:00Z")
Well I’ll be… I guess I need to get my guessing equipment re-calibrated. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [January 3, 2018, 8:11pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/8 "2018-01-03T20:11:50Z")
> [@jz78817](#): > > pitch is wrong; M12 is closer to 17 tpi. Thanks … ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [January 3, 2018, 8:32pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/9 "2018-01-03T20:32:56Z")
Flared fittings and tubing threads are different than pipe, kind of sounds like a tubing thread size. ---
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [January 3, 2018, 10:43pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/10 "2018-01-03T22:43:14Z")
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? ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 4, 2018, 12:29am UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/11 "2018-01-04T00:29:10Z")
> [@ZurBob](#): > > 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. [![](https://i.imgur.com/removed.png) ](https://i.imgur.com/removed.png) [![](https://i.imgur.com/removed.png) ](https://i.imgur.com/removed.png) 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. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [January 4, 2018, 12:31am UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/12 "2018-01-04T00:31:50Z")
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? ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 4, 2018, 12:41am UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/13 "2018-01-04T00:41:24Z")
> [@HoneyBadgerDC](#): > > 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. ---
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [January 4, 2018, 2:32am UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/14 "2018-01-04T02:32:03Z")
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. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 4, 2018, 2:37am UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/15 "2018-01-04T02:37:46Z")
> [@ZurBob](#): > > 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.” 😉 ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [January 4, 2018, 1:51pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/16 "2018-01-04T13:51:55Z")
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. ---
**Author:** ![Chopsticks](https://avatars.discourse-cdn.com/v4/letter/c/cab0a1/32.png) [@Chopsticks](https://boards.straightdope.com/u/Chopsticks)\ **Post date:** [January 4, 2018, 5:07pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/17 "2018-01-04T17:07:44Z")
You said it was for air horns, which makes me think special high pressure fitting. So I went googling… [Scuba adapter](https://www.amazon.com/Scuba-Choice-Adaptor-AS568-011-UNF-2A/dp/B007X8G9SY) 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. [Welding gas 1/2-25](https://www.amazon.com/Goss-801-s-Adaptors-2-25-Thread/dp/B004RID8UU) ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 4, 2018, 5:32pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/18 "2018-01-04T17:32:28Z")
> [@Chopsticks](#): > > You said it was for air horns, which makes me think special high pressure fitting. So I went googling… > > [Scuba adapter](https://www.amazon.com/Scuba-Choice-Adaptor-AS568-011-UNF-2A/dp/B007X8G9SY) > 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. > > [Welding gas 1/2-25](https://www.amazon.com/Goss-801-s-Adaptors-2-25-Thread/dp/B004RID8UU) I think you may be on to something. I googled “high pressure pipe fitting” and found [this](http://ph.parker.com/us/17554/en/10000-psi-pipe-fittings) which might be going in the right direction. ---
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [January 5, 2018, 11:39pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/19 "2018-01-05T23:39:14Z")
> [@jz78817](#): > > I think you may be on to something. I googled “high pressure pipe fitting” and found [this](http://ph.parker.com/us/17554/en/10000-psi-pipe-fittings) 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](https://www.marineparts.com/partspages/Accessories/ACC7.HTM) 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. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [January 7, 2018, 11:06pm UTC](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799/20 "2018-01-07T23:06:51Z")
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. [Next page](https://boards.straightdope.com/t/plumbers-pipefitters-identifying-thread-size/805799.md?page=2)