NPT fitting doesn't quite fit in NPT tap!

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DIYorBust

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So this was a surprise. I have some radiators with four tappings. Normally two would be for flow and return and one would be used for an air vent, the fourth would be capped, but other configurations are possible. The manufacturer confirmed that the tappings are 1/2" NPT, and such NPT fitting thread in. However, I can never quite get them tight. I wonder if there are enough threads on the tapping. Some fittings with longer threaded sections do fit in, but some don't, like street elbows and close nipples. Any idea what's going on here, am I buying the wrong fittings?

Thanks,

DIY
 

Fitter30

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Factory fittings and nipples should screw in 2 turns by hand same as field cut. But a lot of fitting and pipe is coming out of china and they vary. If the radiators have tight threads might want to pick up a tap. My go to pipe dope blue monster telfon tape and anaerobic thread sealant any brand.
 

DIYorBust

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Factory fittings and nipples should screw in 2 turns by hand same as field cut. But a lot of fitting and pipe is coming out of china and they vary. If the radiators have tight threads might want to pick up a tap. My go to pipe dope blue monster telfon tape and anaerobic thread sealant any brand.
So I'm using blue monster tape, and I added megaloc to see if that would help. The problem is it never really gets hand tight, it just keeps threading in. But longer fittings work.
 

Fitter30

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So I'm using blue monster tape, and I added megaloc to see if that would help. The problem is it never really gets hand tight, it just keeps threading in. But longer fittings work.
Threads have a taper? Or what your screwing it in is slighty oversized.
 

Reach4

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Consider buying a 1/2 inch NPT die to clean and maybe even increase the thread length a bit. Not going to work on your close nipples probably, but other stuff could improve.

You can mount the die in a vice and turn the fitting.

Also, consider using more good tape rather than the cheapest.
 
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Clutchcargo

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What type of radiators are these and who is the manufacturer? It could be that the manufacturer over tapped the NPT threads.
 

Reach4

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It's hudson reed. It could be, I think the tappings don't have enough threads. The question is, what might be a workaround?
Also, consider using more really good tape with more PTFE in the tape. Pure PTFE has a specifig gravity over 2 so I would prefer have my good tape with a specific gravity (g/cm3) over 1. Oatey pink tape complies with military spec A-A 58092 . It has a specific gravity of 1.35 or more. As many wraps as it takes, but only put tape at the threads-- not off of the end.​

You could also consider one of the yellow tapes usually used for gas if you find the yellow cosmetically acceptable.

Also use good anaerobic pipe compound as fitter30 suggests.
 
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DIYorBust

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Thanks, again folks.. Since the fittings are chrome, it might mess up the finish to cut more threads, but if that's why I have to do it is. However the way most npt street elbows are made, there isn't really a way to cut more threads. Do you think maybe they overtapped the radiators? There isn't really room to tap it to the next size and reduce it. I could probably use a long threaded nipple, and coupling to get out of the radiator, but that doesn't look great on finish work, and takes up space in the room especially when it's just a bleeder vent.
 

Reach4

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Thanks, again folks.. Since the fittings are chrome, it might mess up the finish to cut more threads, but if that's why I have to do it is. However the way most npt street elbows are made, there isn't really a way to cut more threads. Do you think maybe they overtapped the radiators? There isn't really room to tap it to the next size and reduce it. I could probably use a long threaded nipple, and coupling to get out of the radiator, but that doesn't look great on finish work, and takes up space in the room especially when it's just a bleeder vent.
You could consider chromed plastic snap-around pipe covers such as Plumb-Pak Radsnap Radiator Chrome Effect Pipe Sleeves.
Cut to the length you need.
 

DIYorBust

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What brand of radiator? Is it European?
It's European made, but sold in the US market, not Grey market, and the manufacturer says its tapped NPT. If you bought a caleffi product that said it was npt you'd assume that was true even though the company is Italian right? But maybe not. But my understanding is an npt male threaded into a bpt tapping would jam after 1 or two threads, not keep going in without getting tight.
 

John Gayewski

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I would assume something got mixed up. Not only have I not had this happen I've not heard of it. The only time I've heard of it the threads were made differently than was thought. British threads leak. I would guess because they are smaller.
 

Fitter30

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Could use x- pando pipe sealant. Its a powered compound mixed with water applied to the threads takes 24 hours for setup. It doesn't matter how bad the treads are or how loose the nipple is screwed in this is tough stuff. Mix it in throw away paper/ plastic cup to a paste spray threads with a degreaser screw in doesn't have to be super tight. Just don't mix a lot up at once 10 minutes working time.
 

DIYorBust

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That might be the ticket, but I'd probably want to make sure whatever I xpando in never needs to come out! Here's a photo of something I did. I tried threading the npt street elbow into the the radiator. No luck keeps threading in. But with this npt x npt check valve it work perfectly. The npt check valve seems to have a longer threaded section.

20220116_231611.jpg
 
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Fitter30

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That might be the ticket, but I'd probably want to make sure whatever I xpando in never needs to come out! Here's a photo of something I did. I tried threading the npt street elbow into the the radiator. No luck keeps threading in. But with this npt x npt check valve it work perfectly. The npt check valve seems to have a longer threaded section.
So that brass fitting is a check valve? Never used a auto air vent on a radiator just a coin vent just in case the vent would start leaking possibly ruining everything under it.
 

Jeff H Young

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Threads are threads . Only thing I can think of is poor manufacturing or even possibility of that bushing being 1/2 NPST (straight thread I think my abbreviation is correct) It would be a rare type fitting but sure sounds like a straight thread .
 
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