Adjusting an NPT fitting

[skippydiesel]

For clearance reasons, I would like to have two NPT fittings, screw in further than they currently do.

 

I cant figure out how to do this, short of obtaining an NPT tap, that would enlarge the female side, to allow the male to screw in further - your thaughts?

Edited July 13, 2023 by skippydiesel

[facthunter]

That's the usual way of doing it, Tapered taps often don't work/cut as well as  "straight " threads.  Conventional taps consist of a set of (3). Taper, Intermediate and Plug. In that case the taper is different being used to start the cutting of a parallel thread. If it's BLIND you need the plug tap to finish the job as far as possible into the blind hole.  If it's an alu fitting use trefolex to stop the alu picking up and clean the tap  flutes frequently  with something like a toothbrush.  Nev

Edited July 13, 2023 by facthunter
more content.

[onetrack]

Buy an NPT tap, it's that simple. The following eBay seller in Wentworthville sells a huge amount of new and used machine tools.

 

I need to do the same to a couple of 1/2" NPT threads, and despite having a large number of taps, I still don't have a 1/2" NPT, so I'm just going to have to buy a 1/2" NPT tap.

 

The alternative is to try and borrow one from a Rec Flying member.

 

https://www.ebay.com.au/itm/193770163604

[facthunter]

I only elaborated to ensure there's no confusion with  the "OTHER"  taper tap. Suttons make good ones. You'll use them occasionally to check and clean up existing threads. It's a common drain plug thread on american machinery.  Nev

[Allen Tromp]

Depending upon how critical the application is you can use 1/2" BSPT to achieve what you seek.  1/2" & 3/4"BSPT is physically less than 0,010" smaller than NPT and for most industrial (non-critical) applications can be interchanged. The TPI is the same for these two sizes only.

Generally where space is limited it is better to recut/resize the male fitting to get it to sit deeper in the port. Over cutting the female thread ( say in a case drain or sump plug) may create problems later should you wish to revert to the original configuration.

Just be careful not to go too deep with the thread recut particularly if the port fitting being used is an angled fitting as the corners of the hex nut of a hose or tube fitting might not clear the casing around the female thread.

Another thought might be to recut the tapered 1/2" NPT to a parallel 1/2" NPSM (or BSPP) and use an adjustable angle fitting with an O'ring seal and locking nut.

[skippydiesel]

This is in a fuel fitting.

I am short on space , currently the male fittings sit proud, quit a few threads showing, so need the male to fully enter the females (😆) so that other stuff can get in.

 

Rang around the area - no luck.

 

 

Have ordered a tap & die 1/8-27 NPTF - probably wont get delivered until next week (another delay!!!) Expensive result for just two fittings - Oh well if its worth doing  do it right!

[facthunter]

It's NOT recommended to change  from npt to bspt as the form is 5 degrees different and  one has flats and the other radiuses & it seems only NPT is 27 tpi. Having 3-4 threads proud is about normal when tightened.  Nev

[skippydiesel]

9 minutes ago, facthunter said:

It's NOT recommended to change  from npt to bspt as the form is 5 degrees different and  one has flats and the other radiuses & it seems only NPT is 27 tpi. Having 3-4 threads proud is about normal when tightened.  Nev

"Having 3-4 threads proud is about normal when tightened."

 

All true but I need the space ie the male  needs to go in about 4-5 mm , so that other fittings can meet up

[facthunter]

Do the male part then, but the die will have to adjustable to do the larger dia part first ideally or you risk a sudden change of diameter somewhere along the thread length. Nev

[skippydiesel]

I have my tap & die.

Used the die on the male fittings - some improvement

The tap, due to depth restrictions, does nothing on the female side.

Am considering modifying/shortening the tap, so that it can better "profile" the female fitting

[Blueadventures]

Why not use a die on the male fittings so they can be shortened and go further into the female; spending some more of your son's inheritance and build his tool collection at the same time:)  Will allow keeping the taps orininal.

Edited July 19, 2023 by Blueadventures

[skippydiesel]

This is what the adapater  fitting looks like.

 

One screws in on either side of a Facet pump cube. The pump has a 1/8 -27 NPTF fitting on each side.

 

The adapter above screws in okay but will not go past a certain point leaving about 3-5 mm of the the thread exposed.

 

This would not be a problem, except that I would like to fit a non return valve to one side of the pump. The fittings that facilitate this, are about 5mm short of completion/joining . If I can get the adapter (above) to screw in just a wee bit more my concept will work.

 

 

[skippydiesel]

2 hours ago, Blueadventures said:

Why not use a die on the male fittings so they can be shortened and go further into the female; spending some more of your son's inheritance and build his tool collection at the same time:)  Will allow keeping the taps orininal.

I have done this BUT these are tapered fittings. If you imagine a cone that must fit into another cone you will see that both sides (male/female)  need to have exactly the same taper to fit snuggly any deviaton will see either a loose fitting or a "proud" fitting.

[Blueadventures]

13 minutes ago, skippydiesel said:

I have done this BUT these are tapered fittings. If you imagine a cone that must fit into another cone you will see that both sides (male/female)  need to have exactly the same taper to fit snuggly any deviaton will see either a loose fitting or a "proud" fitting.

It sounded like you were retapping so perhaps a tapered die and cut or grind the top (wider tapper section) away to allow having the narrow taper made closer to the top end of your fitting.  Grind with care to retain the securing indent for locking into the die handle tool.  Just a thought.

[facthunter]

An adjustable die would do that. Usually only "dienuts" aren't adjustable. Nev

[Allen Tromp]

Hi Skippy,

My apologies for my suggestion about using BSP in an NPT port (13 July). I made the BSPT suggestion as I had assumed you were trying to do the same as "Onetrack" with a 1/2" fitting.  Your posting about the 1/8"  NPT showed on screen AFTER my suggestion posting, hence my confusion.

Can you tell me the size of the JIC end of the adaptor (either the dash size or physical thread dimension).

 

 

[Area-51]

What about a straight NPSM tap and converting to a banjo fitting; more choices when it comes to dealing with tight spaces.

[skippydiesel]

3 hours ago, Area-51 said:

What about a straight NPSM tap and converting to a banjo fitting; more choices when it comes to dealing with tight spaces.

Check out the specification's (above) on the pump. To make such a significant change, would involve dismantling a brand new, completely sealed, unit with the possibility that reassembly  may not result in a reliably functional pump.

[skippydiesel]

7 hours ago, Allen Tromp said:

Hi Skippy,

My apologies for my suggestion about using BSP in an NPT port (13 July). I made the BSPT suggestion as I had assumed you were trying to do the same as "Onetrack" with a 1/2" fitting.  Your posting about the 1/8"  NPT showed on screen AFTER my suggestion posting, hence my confusion.

Can you tell me the size of the JIC end of the adaptor (either the dash size or physical thread dimension).

 

 

No apology needed - we are all friends on the Forum.

 

I think the part number of the adapter fitting may be: AF816-06-02

 

If it be the correct part it is 1/8-27 NPT

[Allen Tromp]

OK, so your fitting thread detail is male 1/8" NPT x Dash 6 JIC (9/16" thread) male adaptor.

To recap, you wish effectively reduce the length (ht) of the adapter once it is tightened into the fuel pump port. If this is correct then it is relatively easy to achieve if you have access to a lathe. You will also need a female dash 6 (JIC) blank cap to hold the fitting in the lathe, and an NPT die/die nut.

In brief the concept is to shorten the NPT by about 2 to 3 thread pitches and recut the NPT thread. You may need to machine off a bit of the underside of the adapter hex to clear the die nut. Clean up the lead thread, mark the thread end with a felt tip, then do a trial fit in the port. The felt tip will indicate bottoming in the port. You may need to repeat this as it is really a trial and error "jobby".

Now if you still need more clearance then you can take a small shaving off the JIC 37' seat taking care to check fit the female nut does not engage the adapter hex and correctly seats on the JIC sealing cone. Trim either or both as required. There is always a little bit of wriggle room in the original design that needs imagination to find.  

Avoid recutting the female NPT port for obvious reasons.

 

[skippydiesel]

Thanks Allen - your well informed  (experience) advice has come a little to late , sorry!

 

I have bitten the bullet and modified the female pump fittings.

 

I started by using the die on the male NTP fitting - some small improvement but no way near enough.

 

I found that tap was too long to work effectively, on the pump female thread fittings, so I measured carefully and cut/reduced the length of the tap accordingly.

 

I took my time, used some cutting grease (which also helped contain shavings) cleaned the tap frequently and test fit the NPT fittings often.

 

Result: 

 

The NTP fittings are nice and snug/pump and will have thread sealing paste applied ,when system installed permanently. (Note: Pump will be installed on cockpit side of firewall)

 

My thanks to all who advised.