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NPT drill size .0005 difference

M

M Code

Aluminum
Joined
Apr 4, 2021
Hi

Would like to try to drill NPT drill without reamer, so the drill size for 1/4 - 18 is 7/16 = 0.4375 what if i use 0.438 drill so +.0005 .. is there going any problem?

Thnaks...
 
Half a thou oversize on the minor is not a big deal. What you should watch is the # of threads engaged to hand tight or have a plug gage to make sure tap depth is correct.
 
M Code said:
Sir

drill size 0.4375+0.0005= 0.438" instead of 0.4375"

But i think it's not possible since the tolerance in NPT thread very tight, want to make sure
Click to expand...

Errr?

No it's not. NPT is a tapered thread, so even if you could make a "perfect" drill hole, it would only be "perfect" at one depth.

Regards.

Mike
 
You would be fine 5k bigger...you do know the actual gap size of .0005 right? Most household aluminium foil is .93 mils or .00093. So figure half that is 5 tenths. Threading is probably the least hole dimension specific.

Im in a toolroom working on molds and fixtures, along with other projects. I usually drill my holes 1 size over standard, if i want a tight thread i drill one size under. Npt I tend to stay on size just because a size higher is usually way to big. If it was a matter of 5 tenths I would close my eyes and grab either or.

Plus, who reams for npt. Get that tap in there and armstrong it down... broke a 1" npt tap that way, brand new too, sent me half way across the shop. My coworker laughed his ass off. Big ass tapping wrench too. Npt is a whore when it's full depth in material.

Finegrain said:
Errr?

No it's not. NPT is a tapered thread, so even if you could make a "perfect" drill hole, it would only be "perfect" at one depth.

Regards.

Mike
Click to expand...

Eh if it's a little too big...put a few more wraps of Teflon tape :D
 
To answer the question .
NO difference. Even if it a was a very tight tolerance straight thread. No difference.
First this in or just a bit outside the tolerance of making a drill, second they never make on size holes.
This is a crazy worry.
Bob
 
M Code said:
Sir

drill size 0.4375+0.0005= 0.438" instead of 0.4375"

But i think it's not possible since the tolerance in NPT thread very tight, want to make sure
Click to expand...


Not sure what you call tight tolerance, but NPT is gaged as +/- 1 full turn from gage flat ...
IOW, for the 18 pitch thread the pitch dia tolerance is loose enough so much as to have the thread gage depth be varying .110 total!

But to directly answer your question, that .4375 is actually .4396 dia at the L1 gage depth of .2278, so you will be more than fine with .438.
Frankly, I'd probably drill that shit to 11.5mm if it's all the same.
 
I hate pipe threads and in my previous job I worked at eliminating them as they are the #1 source of leaks. Got rid of about 90% and virtually eliminated all leaks. The reason it is so hard to seal them is that the peak of the external thread doesn’t completely seat in the root of the internal thread and viceversa. So you get a spiral leak path that just drips. O-ring, taper to taper, victaulic, Banjo flange, bolted flange, all are much better.
 
garyhlucas said:
I hate pipe threads and in my previous job I worked at eliminating them as they are the #1 source of leaks. Got rid of about 90% and virtually eliminated all leaks. The reason it is so hard to seal them is that the peak of the external thread doesn’t completely seat in the root of the internal thread and viceversa. So you get a spiral leak path that just drips. O-ring, taper to taper, victaulic, Banjo flange, bolted flange, all are much better.
Click to expand...

Well, that's what you get from China anyway.
Guarantee you that an NPT that is made to specs and assembled with the proper sealant ( as it is meant to be assembled ) will not leak.

I know that if I make both female and male pieces, I can make them hand tight only and use Vaseline as a sealing agent, they will seal 120 PSI air.
I have made that test repeatedly!
And you know what? The NPT specs state a very specific flat at the crest of the threads, and they do so for a reason!
 
M Code said:
Hi

Would like to try to drill NPT drill without reamer, so the drill size for 1/4 - 18 is 7/16 = 0.4375 what if i use 0.438 drill so +.0005 .. is there going any problem?

Thnaks...
Click to expand...


None whatsoever. Nobody is ever going to know that you went 1/10th above the ~nominal~ callout.
You could probably go to Ø11.2 or 11.3mm without too much issue.
My $.02

Doug
 
garyhlucas said:
I hate pipe threads and in my previous job I worked at eliminating them as they are the #1 source of leaks. Got rid of about 90% and virtually eliminated all leaks. The reason it is so hard to seal them is that the peak of the external thread doesn’t completely seat in the root of the internal thread and viceversa. So you get a spiral leak path that just drips. O-ring, taper to taper, victaulic, Banjo flange, bolted flange, all are much better.
Click to expand...

I work for a check valve company and one of their basic check valves has NPT thread on both ends (female on the inlet and male on the outlet). These valves are rated 450-6000 CWP.
NPT threads are the better than straight threads for sealing.
 
Used to threadmill NPTs in mold blocks for water and oil lines. No leakage whatsoever is permissible when die casting aluminum. They all worked fine.

Tapped NPTs tend to leak where the flutes stopped.
 
garyhlucas said:
I hate pipe threads and in my previous job I worked at eliminating them as they are the #1 source of leaks. Got rid of about 90% and virtually eliminated all leaks. The reason it is so hard to seal them is that the peak of the external thread doesn’t completely seat in the root of the internal thread and viceversa. So you get a spiral leak path that just drips. O-ring, taper to taper, victaulic, Banjo flange, bolted flange, all are much better.
Click to expand...
I'm curious, was most of the leaks in heat treated blocks?

Sent from my SM-G960U using Tapatalk
 
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