First time drill and tap, not sure

I'm doing some extra mods while I have my truck apart, and one of them requires drilling and tapping for a 3/8 npt fitting. I am using a piece of scrap metal for practice as this is the first time using it. I followed the chart in the tap and die set, drill a 9/16 hole, and use the corresponding tap. However, when I go to screw in the 3/8 NPT fitting it started getting tight at half a turn. I ran the tap 2/3 of the way in so I know it's deep enough. When I backed the fitting out it looks like the tapped hole is flattening out the threads on the fitting.



Is this right or am I missing something?

run the tap further in would be my suggestion.

Yup. Npt is tapered so you may have just not ran it deep enough.

Ummm, ok, I just went to run the tap deeper and realized I used a 5/8-18 tap. Now, I grabbed the 9/16-18 tap and it will go right through the hole, not catching any of the threads I cut, but the 3/8 npt fitting is still grabbing the hole threads. I am thoroughly cornfused???? Why would the 3/8npt fitting be grabbing the threads cut by a 5/8 tap?

9/16 tap and fitting

5/8 tap and fitting


It looks like the 9/16 tap is too small for the fitting. And I ran the 5/8 tap all the way, no change.

r u sure the fitting is 3/8 NPT ?

You need to use a 3/8 pipe thread tap. Tap drill for it is 37/64 but 9/16 would work. Your taping with a 5/8 18 straight tap that will never work. Pipe threads are tapered. Get the correct tap.

More info .... a 3/8 npt has a nothing to do woth its od. rough outside diameter is .675

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Zmann said:

r u sure the fitting is 3/8 NPT ?

Click to expand...

Yes, unless Summit sent the wrong part in the correct packaging.

earlybroncoboy said:

You need to use a 3/8 pipe thread tap. Tap drill for it is 37/64 but 9/16 would work. Your taping with a 5/8 18 straight tap that will never work. Pipe threads are tapered. Get the correct tap.

More info .... a 3/8 npt has a nothing to do woth its od. rough outside diameter is .675

Click to expand...

OK, after reading your post and a bit more surfing I got it. My taps are all straight.

I tried to find a 37/64 in town, but nobody stocks them so I used the 9/16 I have.

I get that the 3/8 npt has nothing to do with the od, but just looking at the tap I have and the od of the fitting led me to ask. Anyway...

Since nobody has the right tap in town, I ordered it online, and a 37/64 bit just to have it. Hey, at least I tried it on scrap first. Thanks for the help guys.
:toast:

Glad to help sorry if I came of harsh lol

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earlybroncoboy said:

Glad to help sorry if I came of harsh lol

Click to expand...

cry, cry, tears falling..... :grouphug:

Don't sweat it, no offense taken. If a guys skin is that thin, turn in your man card.

When I don't understand something, like this, I compare stuff... the fitting to the tap to the bit and it just didn't look like it all would match up. When I get the right tap tomorrow I'll do the same, and then drill and tap the scrap I have and test the fitting.

Good recourse here for basic information on the different types of taps, if you're so inclined.
http://www.threadtools.com/Files/Taps technical info.pdf