Oil Pressure fitting thread size [calibrate gauge]

[Atrus]

I had to disassemble my oil pressure/water temp gauge to install some new lighting in there. As such, I ended up having to remove the needles and gauge face.

I'd like to calibrate the oil pressure off of my compressor. I have a brass fitting that I think I can tap to fit the gauge, but I am measuring it at a 3/8-28. It's definitely finer than a 3/8-24. Can anyone confirm this? If I am correct, anyone know where I can get a 3/8-28 tap?

If anyone has an alternate easy and fairly accurate method, I am all ears.

Thanks!

 

[Billm]

I would bet that if you check into 1/16NPT you would have it, it specs out to be .312 OD x 27 TPI.
Bill

 

[Dug]

Kevin,
3/8-28 is a good size. I just checked one on my bench (late model). As far a finding a tap,
There are many places to get a tap, MSC, Enco, and McMaster Carr. Here is but one
https://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1735796&PMT4NO=63557565

You are asking a really good question about using air to calibrate your gauge. Air compresses differently than oil / fluid. Does this matter in this case? Good question to ponder for accuracy.
Hope this helps.
Dug

 

[Atrus]

Bill - I'll check out the 1/16NPT...When I measured the major diameter, I was coming out at about .370. Using a pirch gauge, it seemed to be a 28TPI

 

[aeronca65t]

Standard thread on most of these type of fittings is <span style="text-decoration: underline">1/16-NPT</span>.

This thread size has 27 threads per inch and has about a 0.313" outer diameter.


The next size up in common, small-size pipe threads is <span style="text-decoration: underline">1/8-NPT</span>

This next thread size also has 27 threads per inch and has about a 0.405" outer diameter.

 

[Atrus]

cool, thanks guys. There's a new 1/16-NPT tap on ebay for $7 shipped. I'll give it a shot.

Dug - I think it'd work OK to use air to calibrate. In my mind, PSI is PSI no matter the medium. It should at least get me in the ballpark (I'd hope).

 

[dklawson]

I don't wish to contradict those who have posted above, but the threads on the gauge are more likely to be "G" (a.k.a. BSPP) parallel pipe threads. The 1/8 BSPP thread is nominally 9.7mm (0.381") by 28 TPI. As with most straight threads, the actual male threaded part will be just a little smaller than its nominal diameter. BSPP thread seals rely on a compression washer (usually under the head of the fitting but) in the case of the Smiths gauges the sealing washer is on the "inside" and rests on the little boss on the back of the gauge.

You would also be quite able to use a 1/8 NPT fitting with a wrap or two of Teflon tape on the male threads of the gauge stem. I've done this several times myself both to calibrate gauges and to fit new copper feed lines to old Smiths gauges where I no longer have the original plumbing.

Dug, pressure is pressure regardless of air or oil. This discussion comes up most frequently when people argue about whether or not it is necessary to bleed the oil out of the oil pressure line to the gauge. The answer is: For general readings, it is not necessary to bleed the air out of the lines. However, the gauge will respond ever so slightly faster if there are no air bubbles in the tubing. When you review your owner's manual or the instructions supplied with a new gauge you will not find any mention of bleeding oil gauge pressure lines.

 

[Sarastro]

Shouldn't be a problem to use air for calibrating the gauge. I can't think of anything that would be affected by the difference.

 

[Atrus]

Doug L - bummer, I JUST ordered the 1/16 NPT. I could see if the seller has a 1/8 NPT tap as well. This 1/8 size definitely works? It won't harm the threads on the gauge?

 

[jlaird]

I just used a peice of soft hose between the guage copper line and the compresser, tapered end of air feed, and used duct tape to seal it up. Worked fine.

 

[Dug]

Doug L,
I was hoping you would see this thread and enter into this, I missed thinking 1/8 BSPP. That would make sense, for size and TPI it’s spot on. The OD is right at 0.370, straight, and with enough threads to get a very good reading from.
In one area of my work we have challenges with compressibility vs. shear and flow when dealing with concocted glues, resins and air over oil cylinders with reaction times related to cushions (smoothness of operation). I thought it a good question about calibrating a gauge designed for oil using air pressure; good hear it’s of no difference.
Dug

 

[jlaird]

Lets not make this rocket science guys. I is not.

 

[Atrus]

Jack, I should have waited for your response! I can't believe I didn't think of using some tubing! I just spent $14 on taps that I probably don't need. Oh well, they'll go in the toolbox and maybe some day in 15 years I'll find use for it.

Looks like I'll be picking up some flex tubing on the way home!

 

[jlaird]

Kevin, get maybe 6 feet of three or four sizes to stick in the bottom drawer of your tool box. Come in handy around the house from time to time and is so very cheep. The clear stuff from the Hardware.

Glad to help.

 

[Atrus]

Good point Jack, will do.

 

[dklawson]

If the seller has a 1/8NPT tap and will let you change (or cancel) your order, I'd do so. A 1/8 NPT tap will have lots of potential uses for other projects around the house. 1/16 NPT is a nearly obsolete standard. We've used them a couple of times on projects at work and it's difficult to find fittings that size. If I remember correctly, it's still a common size for pressurized grease fittings for auto-lubing machinery.

For calibration, Jack's suggestion of soft tubing should work fine. Obviously a loose fit and duck tape isn't going to work with the oil.

Kevin, you asked if the 1/8 NPT would damage the 1/8 BSPP threads. In this application there is not enough engagement between the two threads to cause a problem. If you go to the plumbing section of your local hardware store (or Lowes or Home Depot) you will find a handful of compression fittings with 1/8 NPT female threads sized for 1/4" plastic tubing. Any of those (brass or plastic) would be good choices for calibrating your gauge.

Dug, glad to hear things are still going well at work. Since January our staff is down 20% in response to sales.