NPTF Gage On NPT?

[drw7162]

Hi guys,
I looked gut couldn't fined the answer in the forum, is it acceptable to
inspect a 2" female npt with an NPTF L1 plug gage. I know the basic differnce between an NPT thread and the dryseal thread is only the roots/crest and not the lenght and pitch so is our current gage ok or do we need to replace it with an NPT L1 plug gage. Thanks!

 

[MartinShane]

According to these guys National Pipe Thread Vs National Pipe Thread Fuel you need a new gauge.

"Specifically, the L1 gages for sizes below and including 3/8" are identical. The sizes over and including 1/2" are not identical and thus not interchangeable. The difference between NPT-L1 and NPTF-L1 is at the Plug Gage Major Diameter and the Ring Gage Minor Diameter."

 

[drw7162]

Thanks, yes I actually had come across that website, but I am also curious why a new gage is needed, does the difference in the thread major and minor diameters cause the nptf gage to stop ahead or after that of the NPT gage? Im looking to have a good explanation for my inspection department other than this page on the internet say's so(although good information). I have read the asme spec and unfortunately it mentions L1 and L3 inspection but doesn't say anything about if you can use an NPTF L1 on NPT, I know vice versa is unacceptable.

 

[MartinShane]

Curiosity lead me to do some more looking and I found some numbers:

major diameter for a NPT 2" plug gauge at L1 2.35418,
for an NPTF plug gauge (2", @ L1) 2.35593

So with the larger major diameter of the plug you would presumably have the gauge stop too soon I suppose.
Always better to have the right tool for the job right?

source: Judge Tool

 

[drw7162]

I wrote our thread gage supplier an email as to if I could use the nptf on an npt and this was the response:

By virtue of the standard these gages are not interchangeable. However the only difference in the design of the gages is their major diameters.
The design of the gage is such that the root of the thread clears the crest of the products.
The major diameter of the gage thread is designed to assure the major of the product is within specification.
Although the difference of .00175 is small the effect on stand-off at a 16:1 ratio could be .028 so this would be your call.

Where I would see a concern the risk runs using and NPTF (major diameter larger the NPT) if one would inspect it with an NPT there would a difference gage stand-off.

2”-11 ½ NPT L1 Plug
Gage Plane OD 2.35418 -.001
Small end OD 2.32693

2”-11 ½ NPTF L1 Plug
Gage Plane OD 2.35593 -.0015
Small end OD 2.32868


...So looks like it barely makes a difference although technically I agree, you would want the exact tool for the job.