I finally got a thermostatic oil cooler bypass. I need to thread in the 1/2" NPT fittings into it. Do I use pipe dope or Teflon tape on the threads? I read somewhere to not use Teflon tape. Nothing on the AN threads, because that is not what makes the seal on that joint.
Thank you.
CG
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As a hairy-chested pipefitter, I'll pipe up (you see what I did there?) on this.
Use a thread sealant, not PTFE tape. There's a lot of schools of thought, but all dope is mostly used as a lubricant-- the taper of the NPT threads actually seals the joint. I used a refrigeration specific "hard setting" joint compound, just to be sure (Leak-Lock) on my joints.
The ANs are just fancy flare fittings (with a different angle than the flares in your home)-- use a bit of oil on the shoulder to keep the nut from galling on the back of the flare. If you have any doubt, dunk the entire end in some oil.
Interesting aside-- what we all think of as "pipe threads" (NPT) is different than
"British Standard Pipe Threads". There is a BSPT similar to NPT, with a tapered thread-- but because the Brits do everything wrong, the taper is different (ours are 60*, theirs are 55*). Nobody uses these-- and with the quality of east Asian pipe fittings, I'm not sure anything is actually cut on the right angle anyhow.
There's also a BSPP (British Standard Parallel Pipe) which has no taper at all, and relies on a seal ring on the shoulder of the male fitting. These fittings are used all the time in hydraulic applications, and your Parker store will have adapters to pretty much anything from BSSP. You don't use a sealant on BSPP joints.
Also note that AN (Army/Navy) fittings are the same thing as JIC (Joint Industries Council) fittings, except that AN fittings are supposed to be made to more exact tolerances ( but again, I'm unsure that the aluminium fittings we all think of as "AN" are really any better than JIC fittings). These also are industrial fittings, and are available at the Parker store (or any other hydraulic hose shop).
There's a reason everybody hates plumbing.
Hey STAN...... Whats your take on Whitworth fittings and wrenches and the castor oil based Castrol Brake fluids of olde????? Lucas electrics????? British lever action shocks???? Yeah, I've played with a few of them from across the pond...... 
Sorry for the HI-Jack..... Covid-19 strikes again....
*breaks through wall a la Kool Aid Man*
Ahem! As a very hairy-chested aficionado of Little British Cars:
No.
To all of that. Just no.
Ask a simple question...
Ask a simple question...
We're bored.
Got to be
I believe there is a straight pipe thread used in electrical such as conduit and in SCUBA Tanks for the valves with an "O" ring. Old SCUBA tanks have 1/2"NPT openings. I always thought they were dangerous because the tapered threads, in order to keep in 2200 PSI, they had to be screwed in really tight. The "wedge" effect of this is trying to stretch the tank in the same direction as the high pressure air inside !
Plumbing is not for easily frustrated or short tempered people !......................Bruce
What Stan said above. I do not use teflon tape. I also do not use or recommend RTV on threads, someone I know very well suggested this to seal an oil pressure switch to an engine case!
I use the liquid teflon pipe sealer. Same stuff you use for boiler fittings. It is impervious to antifreeze, water, oil, gasoline, and natural gas. Blue Monster brand.
Read the label, as all thread sealants are NOT compatible with oil or gasoline.
Jeg's sells a thread oil for use with AN fittings. You don't need more than a drop or two and regular motor oil will suffice, prevents galling of the aluminum threads like Stan says.
Yeah, we're all a little bored these days.
More pipe and thread trivia:
Rigid conduit uses NPT threads. If you're in a pinch, pipe nipples and conduit nipples are interchangeable. As such, EMT (and seal-tite) connectors use the same NPT threads, because electricians are generally sensible people. Not so plumbers and pipefitters.
Copper and iron pipe are both measured in nominal sizes-- 1/2" copper and 1/2" black (or galvanized) pipe do not measure 1/2" anywhere on the pipe. Copper comes in various wall thicknesses-- the O/D stays the same, but the I/D changes as the wall gets thicker. "M" copper is the thinnest wall, "L" is the most common, and "K" was used for direct-bury water services in the 60s and 70s. Pipefitters call dehydrated and plugged 1/2" L copper 5/8" ACR (because the pipe measures 5/8" O/D). 3/4" L is 7/8" ACR, 1" L is 1-1/8" ACR and so forth.
CPVC has the same O/D as copper, PVC as iron pipe. DWV drain pipe is on the iron pipe gauge, so that 4" PVC DWV is the same size as 4" cast DWV. There was a copper DWV used in the 40s and 50s which doesn't really correspond to anything else.
The Whitworth threads my man Leon was referring to are super-weird. The bolt threads are in use on cameras, and the BSPT pipe threads in east Asia. As I alluded, most NPT fittings are now made with pride in China, and have kind of a random taper (depending on which mud-hut factory forged them)-- so I'm not sure there's a ton of difference in the common sizes of pipe thread (between NPT and BSPT). Apparently, MG used bolts with metric threads and Whitworth heads because British mechanics didn't want to buy two sets of wrenches.
Oxygen tanks have standard 750-16 UNF threads. These threads are not compatible with anything else. The all-knowing Google tells me that scuba valves are most commonly 0.750-14 NPS or 0.875-16 UNF.
Fun with fittings!
Is that like Fun With Flags on Big Bang Theory?
Feels an awful lot like the same thing, but with no laughs LOL!
It's like trying to take a sip from a hydrant.
Now tell us about brake line fittings, double flares vs bubble flares, @Stan Galat! Then rifle and hand gun gauges, so I don't screw up my ammo selection when I shoot myself in the head!
