2" aluminum pipe thread, need an answer.

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I'm working on a aluminum hydraulic tank for a dump trailer, (tractor trailer). The originally installed nipple was stripped and leaked not bad but a decent size puddle after a few hours sitting. So I bought a new nipple, threaded from both sides, cut it in half and that's when I discovered this issue, I stuck a pic of black pipe in the vice screwed on the aluminum nipple best I could, as in 1/2, 1-1/2 turns and that was with a pipe wrench toward the end. Definitely not like pitting regular black pipe together. Getting it apart was a nother battle. I'm afraid to weld it to tank figuring the truck mechanics will never get their fittings screwed in, (and it's a 1st for new customer).Is there a difference in the threading, threads or fifferent type thread pattern in aluminum visas steel? Try an post some pics here to.
Reply:There is a chance the coupling you bought is NPS thread, not NPT.  NPS is straight thread-  normally used for rigid electrical conduit.Sent from my XT1254 using Tapatalk
Reply:Oh, a bung someone says?
Reply:That's what I think I got, didn't realize there was a difference
Reply:I use half couplings for repairs like that. http://www.mcmaster.com/#44705k324/=12plxloErnie F.
Reply:I see a couple issues with your post. 1st you talk about "nipples" but all I see are couplings. It helps a lot to be using the same terminology, especially when you start trying to determine if things are threaded the same way. Tapered threads/ straight threads, fine threads vs course threads... it's all in the details.Also some metals tend to gall when screwed together. Stainless is a good example. Plain steel pipe will screw together no problem, but do the same thing with stainless and it very well may gall and not seal, but weld itself together enough you can't get it back apart either. I can't remember about alum, but I think it will gall as well. If that is the case, it's very important to use a good lube like teflon tape or pipe dope so the threads won't gall but will seal..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:There are a vast amount of styles and threads used for hydraulic fittings. I would buy what you think it is and go try it on HIS fitting before welding. He may know what it is or get a picture of the male end and post it. Normally a NPT TAPERED thread will thread in easily 3 to 31/2 turns by hand . Straight threads keep going until none left showing. That coupling doesn't even look like you could make 3 trns into it.
Reply:I would purchase a 2" not weld flange, that way you weld on the outside edge of the flange.  This has saved  lot of problem.  Should be able to get them from McMaster Carr.  Good luck John
Reply:Originally Posted by DSWI see a couple issues with your post. 1st you talk about "nipples" but all I see are couplings. It helps a lot to be using the same terminology, especially when you start trying to determine if things are threaded the same way. Tapered threads/ straight threads, fine threads vs course threads... it's all in the details.Also some metals tend to gall when screwed together. Stainless is a good example. Plain steel pipe will screw together no problem, but do the same thing with stainless and it very well may gall and not seal, but weld itself together enough you can't get it back apart either. I can't remember about alum, but I think it will gall as well. If that is the case, it's very important to use a good lube like teflon tape or pipe dope so the threads won't gall but will seal.
Reply:Originally Posted by metalman21I use half couplings for repairs like that. http://www.mcmaster.com/#44705k324/=12plxlo
Reply:8694T19McMaster carr, is  an aluminum weld on tank fitting.