# NPT 1/8 x 27:  dies vary in diameter



## visenfile (Dec 30, 2017)

A common need in auto work is to deal with sensors having 1/8 x 27 NPT male thread such as OEM pressure switches and aftermarket gauge kits.  Just solved a problem with a supplied sensor which would not start into the engine block female because the new sensor measured .020"  larger along the taper than the one removed.  After stewing around and double checking the threads between the two sensors I turned to the two 1/8 - 27 dies I had.  One an older Craftsman with split design allowing some adjustment on "fit," and another solid one  in a metric set without that feature.  It appeared that the metric die might reduce the diameter problem so I carefully lubed and ran that die on the "oversized" gauge male.  That reduced the diameter of the thread  and allowed it to enter the block female.

Why is there variation die to die?  Being tapered it seems the die manufacturers ( in this case at least) chose two different points along the taper to form their dies??? I know there are classes of fit for straight threading, but this experience reminds me of how threading is done on a lathe with the final being checked with a die???


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## 4GSR (Dec 30, 2017)

Your Metric die set probably had a BSPT thread die and is a little smaller in diameter.  It's also 28 threads per inch and not 27 tpi.  Did you check for that?


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## markba633csi (Dec 30, 2017)

NPT sizes do vary, I've seen that too especially with the 1/8-27 size.  Fittings and pipe often don't match well, taps also vary
Mark


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## middle.road (Dec 30, 2017)

27TPI & 28TPI? THAT explains my difficulty this past summer fixing the Kid's car...
I did not know that and/or it had not registered.


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## Bob Korves (Dec 30, 2017)

I have my 1/8-28 BSPT in a box containing only metric taps.  It is wrapped with masking tape that I wrote on "1/8"-(28TPI !!!) BSPT!!".  Almost as much of a gotcha as 1/2-12 tpi Whitworth stuff...  Two nations, separated by a common language, sometimes barely understood by each other, with separate anachronisms passed down over the centuries and for some silly reason (convention) treasured by both as necessary and proper...


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## Downwindtracker2 (Dec 30, 2017)

When ISO set the standards in '71, they should have used the tapered NPT, it's a much better pipe thread than then the straight British pipe thread.


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## visenfile (Dec 30, 2017)

Thanks for the replies.  The block is 1ZZ Corolla and others.  The sensor taken off has a hex flat with a little sideways "V" next to "27" I took for 27 tpi. The two sensor male threads meshed perfectly as they did with both thread gauges "27" .  That gave me the confidence to use the solid (Japan sourced) die.  The aftermarket sensor has a label "1/8 - 27 NPT (also Japan sourced) so it looked like apples/ apples and it did hand start and tighten after I "reduced" it.  Both sensors and  dies are tapered.  I closed up the Craftsman split type as much as possible with the adjusting screws, but could not reduce the thread like the other die did.  I imagine this problem has vexed many home enthusiasts.


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## visenfile (Dec 30, 2017)

Edit: This problem had me looking at oil cooler "sandwich" adapters which typically have 2 ports threaded 1/8 - 27 NPT  (wonder if they would thread?)  Luckily, the 3/4 x 16 threaded tube which fastens the adapter to the existing oil filter surface is too short to use for this engine, forcing me back to the die fix - nice $/hassle savings!!!!


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## kd4gij (Dec 30, 2017)

Downwindtracker2 said:


> When ISO set the standards in '71, they should have used the tapered NPT, it's a much better pipe thread than then the straight British pipe thread.




 BSPP is used in hydraulics where the pressure is much higher. NPT won't take the pressures.


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## markba633csi (Dec 30, 2017)

The British have both a tapered and a straight:  BSPT and  BSPP
M


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## visenfile (Dec 31, 2017)

Well this thing thickens.  I belong to another forum and have been told there that the block thread is BPT.  An owner gave me a source which sells a T brass fitting with male 1/8 BPT to block and two females, one 27 and one 28.   These guys should know. Still dangling evidence:  The sensor removed has "27" stamped on it.  My comparisons of thread mesh  to one another and to two different thread gauges could be off with tired old eyes. Why would the mighty Japanese auto industry adapt a BPT? The adjusting die is from a Japan sourced set and says "28." The die adjustment seems good .


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## Ebel440 (Dec 31, 2017)

I don't know if it would apply to your situation at all but I know in old Volkswagens they use a metric thread for sensors very close to 1/8npt. It's close enough that lots of people just put the npt into the metric hole without knowing its not the same thread. But I can't remember the size of the metric one off hand so its not much help.


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## Bob Korves (Jan 1, 2018)

There are ISO pipe thread specs, but they almost completely adopt BSPP and BSPT standards.  It is not really a metric system at all except by a conversion from Imperial measurements in the old systems they adopted.  The ISO did so because the system works well, and the wheel did not need to be reinvented.
https://en.wikipedia.org/wiki/British_Standard_Pipe


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## visenfile (Jan 3, 2018)

Here's the latest.  The auto forum guys (familiar with this problem) sent me a source which sells a "T" fitting.  The block takes 1/8 BPT male with two 1/8 NPT females provided for the common aftermarket sensors.  ( Why did Toyota select a Brit thread?).   I "resized" the new male sensor from its identified 1/8 NPT to "something" smaller which "worked."  I say "something" because the reduction in diameter is around .020" and it hand started and felt good tightening to what felt "typical" pipe thread torque.  What still is a little mysterious is the smaller diameter of the Japan sourced, solid die stamped 1/8 NPT.  Perhaps it is really BPT.  The Japan die set contains both a 27 and 28 gauge.  Perhaps I have (so far) gotten by with a smaller 28 thread entering a 27 pitch female and the wedging did a handshake.  Thanks for the comments and hope this helps someone.


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## 4GSR (Jan 3, 2018)

I ran across the same thing a few months ago with some Bijur type oiling system parts that came from Asia.  Wow, what a mess, I wound up buying a set of taps and dies for BSP threads, and they are smaller in diameter.  Tried the dies and taps and yep, they fit to a tee.  That reminds me, I need to get back to that project.


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## LucknowKen (Jan 3, 2018)

I wondered:
Is BSP thread same as NPT?

*BSPT* (*British Standard Pipe Thread*) is similar to *NPT* except for one important difference. The angle across the flanks of *threads* (if you sliced the fitting in half long-ways and measured the angle from root to crest to root) is 55 degrees instead of 60 degrees as it is for *NPT*.



I thought the 1/8 NPT would be smaller overall than the 1/8 BPT?

..the new sensor measured .020"  larger along the taper
..The aftermarket sensor has a label "1/8 - 27 NPT
..have been told there that the block thread is BPT

Question: Why would the mighty Japanese auto industry adapt a BPT? Answer: European market.
Question: Why is there variation die to die? Answer:  Being tapered.

What still is a little mysterious is the smaller diameter of the Japan sourced, solid die stamped 1/8 NPT.
Would that not be dictated by the small end of the die?


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## 4GSR (Jan 3, 2018)

LucknowKen said:


> I wondered:
> Is BSP thread same as NPT?   (No)
> 
> Question: Why would the mighty Japanese auto industry adapt a BPT? Answer: European market.
> ...



See my remarks above in red.


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## turnitupper (Jan 3, 2018)

LucknowKen said:


> Would that not be dictated by the small end of the die?


If you meant tap instead of die,  then yes, exactly. I have had to trim some of the taper off BSPT taps to get enough thread engagement using standard fittings in blind holes The angle grinder is your friend in these situations.
John.


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