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View Full Version : Loose 1/4" and 3/8" NPT Threaded Holes



Silverback
08-10-2015, 09:29 PM
I've run across 2 NPT threaded holes that are loose around the male threads that thread into them. Well, not loose but they don't go all the way snug before the male threads bottom against their hex (one is a 1/4NPT to AN-6 adapter, the second is 3/8" NPT a sensor). Neither side is easily replaceable. One in particular (the 1/4"NPT) is in a tight spot so it would be quite difficult to drill out and even more difficult to use a 3/8-1/4"NPT bushing. They are threaded into a coolant passage.

In both cases I just need them to seal, and I've been debating what I could use on the threads to seal them, and still be able to pull them out in the future (preferably not easily, but something short of drilling them out also). Something like epoxy is a last ditch answer, I don't want to fight them that much to pull them out if I have to. I've contacted both Permatex and Loctite for suggestions and neither has responded.

I've considered things like Permatex #1, their various aviation sealants, Indian Head Shellac adhesive, Anaerobic gasket maker http://www.permatex.com/products-2/product-categories/gasketing/anaerobic-gasket-makers-flange-sealants/permatex-anaerobic-gasket-maker-detail , the assorted thread sealants rated for straight threads into coolant passages (this is the stuff that i've been using recently http://www.henkelna.com/product-search-1554.htm?nodeid=8797903224833).

Any suggestions what will work?

EddyCurr
08-10-2015, 09:41 PM
Are new male fittings replacing ones installed previously?

NPT is tapered. If this is a replacement situation, then
perhaps the replacements have a longer tapered section
and thus bottoming where their shorter predecessors
could tighten without bottoming.

Regardless, can the outer ends of the present male fittings
be shortened/relieved such that full mechanical thread
engagement occurs ?

A wrap of pipe tape or daub of pipe sealant still counts on
mechanical engagement for sealing under pressure. My vote
is that in the absence of that mechanical thread engagement,
sealing will be illusionary.

Edit: Oops. On rereading, I see that bottoming occurs between
the hex shoulder of the male fitting against the surface of the
female threaded hole. Not at the male tip against an interior wall.

Sorry.

Still do not think a sealant is your answer for excessive gap-o-sis.

.

EddyCurr
08-10-2015, 10:00 PM
No chance that this is an instance of mixing BSPT
(British Standard Pipe Thread) and NPT fittings ?

.

Robin R
08-10-2015, 10:02 PM
Perhaps you could remove some material from the shoulder, so the threads can engage deeper.

doctordoctor
08-10-2015, 10:10 PM
JB weld the loose fittings together

RichR
08-10-2015, 10:42 PM
Before gooping up the threads, I 'd try 2 or 3 wraps of Teflon tape and see how that works out.

oldtiffie
08-10-2015, 11:19 PM
Before gooping up the threads, I 'd try 2 or 3 wraps of Teflon tape and see how that works out.

+1.

Teflon tape would be my first choice/try as well.

wierdscience
08-11-2015, 12:11 AM
Is it possible that you're dealing with Dryseal threads instead of NPT?If all else fails Loctite 567 thread sealant would be my pick.

rdfeil
08-11-2015, 12:59 AM
I would try the teflon tape first and if that did not work I would move up to a product called Xpando http://www.xpando.com/. This is a black powder that is mixed with water to form a paste. The paste is applied to the threads and then assembled. The paste will dry out and expand slightly as it drys and will both bond and seal. I used this product when I worked as a pipefitter for a refrigeration company. We used this on black iron pipe thread fittings in ammonia refrigeration systems. Never a leak. It is possible to unscrew the fittings after it drys, but it is really tough.

Robin

oldtiffie
08-11-2015, 01:25 AM
NPT details

https://www.google.com.au/search?q=google&rls=com.microsoft:en-AU:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7IRFC_enAU360&gfe_rd=cr&ei=8IbJVcmrKbLu8wea4Z-YBg&gws_rd=ssl#q=npt+thread+chart

https://www.google.com.au/search?q=npt+thread+chart&rls=com.microsoft:en-AU:IE-SearchBox&rlz=1I7IRFC_enAU360&biw=1536&bih=683&source=lnms&tbm=isch&sa=X&ved=0CAYQ_AUoAWoVChMIjIaAj6WgxwIVR56mCh0HRgHX

Silverback
08-11-2015, 02:08 AM
Are new male fittings replacing ones installed previously?

new


Still do not think a sealant is your answer for excessive gap-o-sis.


I agree, but I don't really have the option of replacing the parts that are "overthreaded," and they'd be pretty tough to weld up and re-cut, at least a whole ton of work.


No chance that this is an instance of mixing BSPT
(British Standard Pipe Thread) and NPT fittings ?

Nope, recognize both... anyway, I don't know of any standard carparts that use BSPT... Honestly I think I've only seen BSPT once and it wasn't on a car


JB weld the loose fittings together

Last option... I know that as soon as I epoxy them together the fitting will get munged and need to be replaced or the sensor will die, and then I have a bigger problem to deal with.


+1.

Teflon tape would be my first choice/try as well.

RichR and oldtiffie, I was under the impression that thread tape (and teflon pipe dope) were both mostly there to lubricate the tapered threads so you can thread them together tighter, that they don't really do much sealing. That was the major reason why I discounted them from the get go. The only reason I considered still using a thread sealant intended for use with head bolts that thread into a coolant passage was that in that case it's something designed to seal a thread that will never tighten against itself (straight cut bolt threads), so there is some chance of that working... maybe I'm wrong here. It's probably the easiest thing to try, but I _really_ don't want to take this thing back apart and fix it if it doesn't work.

macona
08-11-2015, 04:41 AM
Could they actually be NPT straight and you use one of the o-ring style fittings?

EddyCurr
08-11-2015, 07:44 AM
No chance that this is an instance of mixing BSPT
(British Standard Pipe Thread) and NPT fittings ?
Nope, recognize both... anyway, I don't know of any standard
carparts that use BSPT... Honestly I think I've only seen BSPT
once and it wasn't on a carBit of a tease, aren't you. Oh, well. I'll play along ...

So these are cooling system fittings where the female parts
belong to a non-North American part, because they are not
NPT. Not British, either - that would be too obvious. Not
German, those would be DIN.

Could we perhaps be dealing with something of Japanese mfr ?


JIS Tapered Pipe Thread

The Japanese tapered pipe thread connector is identical to
and interchangeable with the BSPT (tapered) connector. (http://www.ryco.com.au/index.php?id=224)

Japanese Komatsu equipment uses the Komatsu fitting
(with metric threads). Other Japanese equipment most likely
will be JIS (Japanese Industrial Standard), or in some cases,
BSP, straight or tapered threads. (http://www.gates.com/products/industrial/industrial-hydraulics/hydraulic-couplings/international-couplings)

I struggle with the notion that your female cooling system
fittings are NPT items that have worn to the extent that
they now allow a new male NPT fitting to pass through so
far that the male's hex bottoms against the outside lip of
the female boss.

.

EddyCurr
08-11-2015, 08:53 AM
Cursory research shows that numerous Japanese auto
mfrs employ JIS / ISO / BSPT imperial-dimensioned
tapered threads. Including:

Datsun
Isuzu
Lexus
Nissan
Subaru
Toyota

Any number of topics can be found on websites where
owners are grappling with swaps, gauge installations
and so on.

Point of note about TPI differences between BSPT & NPT

1/8": BSPT = 28 TPI / NPT = 27 TPI
1/4": BSPT = 19 TPI / NPT = 18 TPI
3/8": BSPT = 19 TPI / NPT = 18 TPI

.

RichR
08-11-2015, 09:50 AM
RichR and oldtiffie, I was under the impression that thread tape (and teflon pipe dope) were both mostly there to lubricate the tapered threads so you can thread them together tighter, that they don't really do much sealing. That was the major reason why I discounted them from the get go. The only reason I considered still using a thread sealant intended for use with head bolts that thread into a coolant passage was that in that case it's something designed to seal a thread that will never tighten against itself (straight cut bolt threads), so there is some chance of that working... maybe I'm wrong here. It's probably the easiest thing to try, but I _really_ don't want to take this thing back apart and fix it if it doesn't work.

A taped part will be much easier to remove if it doesn't work (both now and in the future, think repairs) as opposed to a sealant that sets up.
If there is enough material, cutting back the shoulders on the two items so they can thread deeper may be an option. You mentioned one of
the items is a sensor, so a fixture might be in order. Chuck up a piece of steel in the lathe and drill and tap it so your device can screw in without
bottoming out. Use a parting tool to shave 10 to 20 thousands at a clip off the shoulder to remove however much you think you need to. Then
see if the part snugs up without bottoming out.

ironmonger
08-11-2015, 09:57 AM
<<snip>>


RichR and oldtiffie, I was under the impression that thread tape (and teflon pipe dope) were both mostly there to lubricate the tapered threads so you can thread them together tighter, that they don't really do much sealing. That was the major reason why I discounted them from the get go. The only reason I considered still using a thread sealant intended for use with head bolts that thread into a coolant passage was that in that case it's something designed to seal a thread that will never tighten against itself (straight cut bolt threads), so there is some chance of that working... maybe I'm wrong here. It's probably the easiest thing to try, but I _really_ don't want to take this thing back apart and fix it if it doesn't work.

Teflon tape is misused quite often, and as you surmised is not meant to be built up in thick layers to make a 'gasket'.

One added drawback is the insulating properties of PTFE tape. It can be applied in such a manner that it will insulate the two parts electrically... not too good is your sensor requires a ground path...

paul

ecortech
08-11-2015, 10:29 AM
I have occasionally run into this problem a number of times.
There seems to be a bit of discrepancy on the depth of tapered pipe thread from different manufacturers.
Some tap the holes a little deeper than others, some fittings are made a bit on the smaller side.
Solution#1 is to try a fitting from a different source, many times they will be slightly bigger.
Solution#2 use a thread sealant loctite or permatex both make a thread sealant that will do the job.

http://www.permatex.com/products-2/product-categories/thread-compounds/thread-sealants/permatex-high-performance-thread-sealant-detail

Or

http://www.henkelna.com/industrial/vehicle-repair-maintenance-thread-sealants-22184.htm
Use 592 or 595

I have used both the Permatex and Loctite products many times and I don't think they have ever failed to seal up the problem.
Both are pretty impressive products.

Ed

Toolguy
08-11-2015, 10:32 AM
A pipe tap should only be run in about 3/4 of the teeth on the tap. It sounds to me like the parts were just threaded too deep. There's nothing that says new parts can't be made wrong.

duckman
08-11-2015, 11:27 AM
Just clean your parts and soft solder the threads, screw them in and done, it's a quick cheat and they will be easy to get out if you have to at a later date.

Silverback
08-11-2015, 02:26 PM
I would try the teflon tape first and if that did not work I would move up to a product called Xpando http://www.xpando.com/.

I've heard about the stuff before, but I've never seen anywhere to buy it that you don't need to invest in a bulk package of it like from their web site.


Bit of a tease, aren't you. Oh, well. I'll play along ...

Wait, sorry, no game. Not sure what gave you that impression. Just didn't list more info to prevent things from getting too long/drawn out, I tend to be wordy...

Really, it's just a female threaded part and a male threaded part, both NPT, good old american SBC parts, the female parts are aftermarket (an intake manifold and an intake elbow) and the 1/4npt fitting is an aftermarket NPT to AN adapter and the sensor is an OEM, GM temp sensor.



One added drawback is the insulating properties of PTFE tape. It can be applied in such a manner that it will insulate the two parts electrically... not too good is your sensor requires a ground path...

Typically that would be a problem, but in this case it's not, the sensor has a 2 pin connector, one is a ground.


A pipe tap should only be run in about 3/4 of the teeth on the tap. It sounds to me like the parts were just threaded too deep. There's nothing that says new parts can't be made wrong.

One of the holes I ran a tap into myself, ran it down about 2/3-3/4 of the length of the cutting part of the tap. I've never had this problem before. The fitting screws in and seems to seat correctly dry, keeps going down to the shoulder if you put something on the threads. I cleaned it off last night and tried it again. It seats with maybe 3-4 threads exposed dry (using a wrench on it), not as much as I would have liked but still OK, I put some Permatex #1 on it (the hardening goop) and it threads all the way in without much effort by hand. I even tried running a different tap into it to make sure that the threads weren't galled or something. The second tap stops and starts cutting around 2/3 of the way in. Tried a new adapter fitting in the hole and IT DOES THE SAME THING?!?


Just clean your parts and soft solder the threads, screw them in and done, it's a quick cheat and they will be easy to get out if you have to at a later date.

Both female ends are aluminum, the fitting is anodized aluminum, the sensor is brass or something similar, i don't think those combinations could be soldered.

Silverback
08-11-2015, 02:33 PM
I have occasionally run into this problem a number of times.
There seems to be a bit of discrepancy on the depth of tapered pipe thread from different manufacturers.
Some tap the holes a little deeper than others, some fittings are made a bit on the smaller side.
Solution#1 is to try a fitting from a different source, many times they will be slightly bigger.
Solution#2 use a thread sealant loctite or permatex both make a thread sealant that will do the job.

http://www.permatex.com/products-2/product-categories/thread-compounds/thread-sealants/permatex-high-performance-thread-sealant-detail

Or

http://www.henkelna.com/industrial/vehicle-repair-maintenance-thread-sealants-22184.htm
Use 592 or 595

I have used both the Permatex and Loctite products many times and I don't think they have ever failed to seal up the problem.
Both are pretty impressive products.

Ed

Huh, I have both sitting around...

One of them, don't remember which was on the fitting when it first threaded all the way in and bottomed on the hex.

A few people have suggested cutting down the hex so it can thread in deeper... the hex on npt to an adpaters is pretty small, maybe around 3/16" thick... not much to cut down. this fitting is in tight place too, I don't think it could stand to be more than maybe 1/8" taller before the fitting/hose that screws to it would hit a fuel injector, so drilling/tapping it for an adapter bushing is out (not to mention the engine would have to come apart to make room to do it).

Cuttings
08-11-2015, 06:31 PM
One thing to keep in mind, whatever you decide to do, if the sensor uses the block for the ground part of the circuit you must provide a mechanical connection between the sensor and the block.
To much tape or sealer will insulate the connection and the sensor will not work.
I would check carefully, as mentioned before, that you are not mixing BSP and NPT. They will fit but not well.

PStechPaul
08-11-2015, 07:52 PM
A few possibly crazy ideas:

1. Electroplate the brass part of the sensor - maybe nickel or tin.

2. Apply solder to the brass male threads and then follow up lightly with a pipe die.

3. Wrap the threads with copper wire or solder before assembly

4. Use a helicoil insert: https://www.grainger.com/category/thread-repair-inserts/thread-insert/fasteners/ecatalog/N-8o9?bc=y#nav=%2Fcategory%2Fthread-repair-inserts%2Fthread-insert%2Ffasteners%2Fecatalog%2FN-8o9Z1z0kopxZ1z0kon8

https://static.grainger.com/rp/s/is/image/Grainger/5UU55_AS01?$lgthumb$

yf
08-11-2015, 09:06 PM
You need to figure out if the female threads are too large, or if the male is too small .

Once you know that, you can decide what to do.

I would try to fit a standard nipple into the block, to check the fit and fit the male threads to a coupling or elbow.

I suspect the male threads are not sized properly.

I've seen alot of fittings lately that aren't made properly.

rdfeil
08-12-2015, 03:51 AM
Silverback,

Xpando is available at several places, here are two:

http://kscdirect.com/item/X-PANDO+14OZ/X-PANDO+PRODUCTS+COMPANY_PJC-14+PIPE+JOINT+COMPOUND

and

http://www.mcmaster.com/#x-pando-adhesives/=ygjzwr

RussZHC
08-12-2015, 06:04 AM
what yf said ^^

my thought doesn't make any sense as you have tried a couple of different fittings already (If I am reading your posts correctly) BUT could either of the thread profiles be "wrong" as in too far out of spec?
Your post about fitting it dry and then adding sealant (?) and it fitting deeper is confusing...not disbelief but it does seem odd. Closest thing like this I have had is no sealing but that was, as mentioned, the fitting hitting on an interior wall.

ironmonger
08-12-2015, 06:55 AM
<<snip>>
Your post about fitting it dry and then adding sealant (?) and it fitting deeper is confusing...not disbelief but it does seem odd. Closest thing like this I have had is no sealing but that was, as mentioned, the fitting hitting on an interior wall.

Thread sealants are really more of a lubricant. The sealing is accomplished by the very long spiral path for material flow from the end of the male thread to the outer end of the female thread. If the threads are malformed, damaged or out of round the likely hood of a leak is increased.

Teflon sealants, paste or tapes, have the effect of increasing he wrench size... the effort to thread them in is reduced greatly. We often said it was like doubling the wrench size.

The amount of sealant required is very small. Applying too much tape can result in the female threads being physically stretched out of shape. I have removed Teflon tape from a fully assembled 2" IPS steel threaded joint intact. It does not flow or creep like many plastics in this application. By removing I mean unwound from the male threads after disassembly.

Threaded joints typically should make up around three turns after they are hand tightened regardless of the sealant or lubricant used.

In the case of the 3/8" fitting that does not seal, you might have to machine a new bushing with oversized 3/8" IPS threads and then buy a 1/4" whatever to fit in the oversize bushing.

Xpando is a bad idea on aluminium fittings. It will set up just fine, but if it ever has to be dissembled it will likely destroy the female thread on disassembly. Say hello to your friendly local TIG weldor at that point. You will have to rebuild the hole.

paul

CarlByrns
08-12-2015, 07:39 AM
Thread sealants are really more of a lubricant. The sealing is accomplished by the very long spiral path for material flow from the end of the male thread to the outer end of the female thread. If the threads are malformed, damaged or out of round the likely hood of a leak is increased.

Teflon sealants, paste or tapes, have the effect of increasing he wrench size... the effort to thread them in is reduced greatly. We often said it was like doubling the wrench size.

The amount of sealant required is very small. Applying too much tape can result in the female threads being physically stretched out of shape. I have removed Teflon tape from a fully assembled 2" IPS steel threaded joint intact. It does not flow or creep like many plastics in this application. By removing I mean unwound from the male threads after disassembly.

Threaded joints typically should make up around three turns after they are hand tightened regardless of the sealant or lubricant used.


This should be printed and hung in the plumbing department in every home center.

DR
08-12-2015, 08:43 AM
Hard to say where the problem is.................

Just to add a little to the pipe thread discussion. One of my rentals has old galvanized pipe. During a recent under sink leak I went to Home Depot to get all new fittings rather than work with the old stuff.

What should have been at most a couple hour job turned into an all day job. No matter what I tried I couldn't get one of the threaded connections to stop dripping. Finally it occurred to me the threads might not be correct. I went to a plumbing supply outfit and explained my situation. The clerk told me he'd see this before. Apparently Chinese fittings like HD carries are frequently miss threaded. Got better quality fittings and the leak stopped.