Thread: How do you measure NPT fittings???

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Hello Everyone! , how do I measure NPT fittings? The smaller fitting in the enclosed pic's is the one that I need to re-place so I meaured it & thought it was a 1/2" NPT. Obviously I was wrong {LOL} as the pic of the larger fitting is the one that Jeg's sent me!!! Thanks, Bill

now I'm really confused

I gotta do all my brake and fuel lines soon, I'll have to do some reading.

Sean

Pipe sizing is called out by the nominal I.D.

Few things more frustrating than figuring out pipe and tubing sizes...

Here's some good info on NPT and AN

National Pipe Thread Tapered (NPT) is one of the U.S. standards for tapered threads used to join pipes and fittings. The taper rate for all NPT threads is 1⁄16 (¾” per foot) measured by the change of diameter (of the pipe thread) over distance.

Nominal Pipe Size (NPS) is loosely related to the inside diameter of schedule 40 pipe, i.e. ½” is the ID of the pipe. Because of the pipe wall thickness, the actual diameter of the threads is larger than the NPS, considerably so for small NPS. Other schedules of pipe have different wall thickness but the OD (outer diameter) and thread profile remain the same, so the inside diameter of the pipe is therefore different from the nominal diameter.

The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. However a clearance remains between the crests and roots of the threads, resulting in a leakage around this spiral. This means that NPT fittings must be made leak free with the aid of thread seal tape or a thread sealant compound. (The use of tape or sealant will also help to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.)

The AN thread is a particular type of fitting used to connect flexible hoses and rigid metal tubing that carry fluid. It is a US military-derived specification stemming from a joint standard agreed upon by the Air Force and Navy, hence AN.

AN sizes range from -2 (dash two) to -32 in irregular steps, with each step equating to the OD (outside diameter) of the tubing in 1/16" increments. Therefore, a -8 AN size would be equal to 1/2" OD tube (8 x 1/16 = 1/2). However, this system does not specify the ID (inside diameter) of the tubing because the tube wall can vary in thickness.

Each AN size also uses its own standard thread size. SAE Thread Size for AN:-2=5/16-24, -3=3/8-24, -4=7/16-20, -5=1/2-20, -6=9/16-18, -8=3/4-16, -10=7/8-14, -12=1-1/16-12, -16=1-5/16-12
Regards All,
Glenn

are you shure that fitting is pipe thread it has one lead in thread but does not look tapered I think it may be weatherhead thread I could be wrong maybe just the picture ......ted

denny after looking at it agean I notice the thread is flat at the end I went out to the shop and looked at some fittings all the pipe threads are sharp pointed I dont think its pipe thread but like I said I could be wrong and probly am ....ted

....Thanks!!! ... a couple more pic's. The fitting on the left is the same fitting that I have in my first pic. So by reading my replys I'm thinking that the fitting on the left is an 3/8" NPT with washer {because the threads aren't tappered} & the fitting on the right is an 3/8" NPT w/ out washers because it's tappered.... Oh, fitting on the left is one of the two fittings that comes out of my turbo 400 transmission that goes to the transmission cooler. Fitting on the right is one of two fittings that goes into my transmission cooler.... One more question. I'm assuming the answer is yes but; Are the female threads the same for the NPT with & without washer? Meaning can I use 3/8" NPT tappered fittings in both the transmisson & the trans cooler? I hope I'm making sense! THANKS again!!! Bill

bill thoes two fittings will NOT interchange they will most likley do damage to components if you try and you will be chasing leaks IMHO. .....ted

When you get to the brakes, you will have different fittings altogether to deal with. The back fittings will probably be 3/8" tubing 45 degree flare fitting, and the front probably 1/4" tubing 45 degree flare fittings. I say this based on the cars I have worked on personally, not having checked to see what car you have. IMO if you are replacing the brake lines, the easiest way ( by far ) is to purchase pre-bent lines. Second easiest is to get a good quality hydraulic flare tool, and use regular steel lines and stainless steel fittings. I used PolyArmour steel line. It has a corrosion resistant coating and makes perfect and painless flares.

Thank You!!! ... ok Denny so no washer cuz they don't bottom out. Ah, there's an in/out on the lines? Thank you! I wasn't paying any attention when I took the lines off! ok Ted, I easily threaded both pictured fittings into the transmission so the threads are the same....
Oh, by the way, what I'm doing here is replacing the fittings & lines that hook the trans cooler to the transmission. It was hooked up with a combination of rubber hoses, metal lines & 6 {yes SIX} hose clamps where the rubber hose was conected to the metal lines on one end. It looked liked it leaked & they just keep adding hose clamps to stop the leaking! {ah, but it STILL leaked ..... anyway, I had some -8 an fuel line left over so I just figured I'd use it for the trans cooler lines. Thanks again! Bill

...Denny I'm confused again! I just noticed you said the trans cooler fittings for a turbo 400 should be 1/4" fittings? And the fittings I have measure 1/2". So I subtract 1/4" from the 1/2" & this means my fittings are 1/4"? Correct? {cuz I thought someone said the fitting that I showed in my picture was a 3/8" fitting?}... whew, I hate fittings! Thanks again, Bill

...don't go to all that trouble Denny, If a turbo 400 has 1/4" NPT fittings for the trans cooler in/out then that's what it is!!! Thanks again Bill

the 400 case would not use npt the case would crack if you use tapper . you can get the fitting from russell or others .i think what threads in the trans case is AN you can check it EZ see if you can thread a 3/8 nut on the old fitting with a AN you can use a Oring or a statoseal washer to seal

that looks like a taper thread - to - flair fitting adapter.

thoes are weatherhead to inverted flair....ted