View Full Version : Pipe Tap & Die ????

03-03-2010, 11:58 AM
I'm wondering if anyone knows what the tolerances are on pipe taps and dies. Do they hold close tolerances like screw taps and dies or are they very sloppy like + - .015. I can never find a pipe tap or die that will reproduce the same cut as say a factory thread either on a pipe or preassure gage etc. More on this later...... going for lunch.


03-03-2010, 12:45 PM
Joe...Are you talking about NPS taps or NPT? The NPT fit depends on how deep you run the tap and how tight you screw in the pipe. As to NPS, I don't remember seeing any specs on fits, although there must be some.

03-03-2010, 01:01 PM
I dunno, I've never used a BS tap, only NPT 1/2" or smaller. The only straight threads I've used were for zerks, and those used a standard 1/4 - 28 tap.

I've used/seen in use die heads up to 4", but again, always NPT. Usually Rigid die heads are the defacto standard. Typically nice threads if the inserts are in good shape and nobody cheaps out on the cutting oil or screws up setting the auto retract dies.

Tolerance is a moot point as long as the pitch matches because of the 3/4"/ft taper in the thread form.

03-03-2010, 01:40 PM
Well here is what happens. I have a preassure gage, it has a 1/8" pipe thread on the end that screws into the regulator. Overall length of the thread on the gage is about 3/8" it screws into the hole just about the full 3/8" with minimum effort.
I threadded a piece of pipe with one of my Ridgid pipe dies, after threadding 3/8" of the pipe all I could get to start into the hole was about 1/8" so I figured the I should run the die down the pipe a bit more. so I did, but there was no diff. So I ran the die down furthur untill I had about 1/4" of pipe comming out the back and of the die. That made a little diff. but not much. So my guess is the die is a few thou too big on the overall.
I then ran a small hand die over the thread and it shaved the threads a bit but did not make a huge diff. So I came to the conclusion that the tolerances of these pipe dies are very loose. These arn NPT tapered.


03-04-2010, 12:49 AM
Your regulator and gauge wouldn't be BSP by any chance? 1/8 BSP OD is slightly smaller, and there is 1 TPI difference from the American pipe thread - ask me how I know!


03-04-2010, 12:54 AM
The taps for NPT seem standard........the dies on a Rigid machine like CD is talking about has street and over/under settings so you can pretty much cut the male thread how you see fit.......you can only really do it on pipe or longer fittings though.........close nipples won't work.

03-04-2010, 01:24 AM
See Machinery's Handbook (MHB) page 1860 onward - under "Pipe and Hose Threads" as it deals with US and UK taper pipe threads.

03-04-2010, 08:24 AM
A split die would seem to be the way to go since you could close the die a few thou. and reduce the overall dia. of the taper.
I'll post some pics when I'm done with the project.


03-04-2010, 08:51 AM
Joe, have you tried a 1/8" pipe tap in the regulator? If not then try it and if it won't fit with ease you may have a British pipe thread.

The standard for pipe threads is three turns in by hand for a standard fit. I would bet the tapered pipe taps and dies are ground to some standard.

If your Rigid die is adjustable as some are have you checked it with a 1/8" hydraulic fitting to see if the fitting will go in and stop with some slight penetration out the other side?

I use the hydraulic fittings as a test fitting because they are better quality and more consistent.

03-04-2010, 12:20 PM
It's an american made product, L-Tec flowmeter. I doubt it would be british threads. A regular pipe nipple threads in justa as easily as the fitting I took out. I posted some pics on the welding forum.


03-04-2010, 01:40 PM
JL...Could the gage possibly have a metric pipe thread? A friend used to work for a Swagelok distributor in Jax Fl. and was the traveling rep for the company. He had a situation at a shipyard where a hydraulic steering box required metric pipe fittings. He supplied the fittings and they would not seal properly. He substituted some SAE fittings and they were much tighter and solved the problem.

03-09-2010, 11:48 PM
OK, I finaly repaired the regulator and it works fine. Here is a picture of the two handels I made to assist in unscrewing the regulator cap. As I mentioned in my first post on this subject is the problem I'm having is after threading the end of the two handels 1/8" pipe, all I could get to start in the regulator is about 1 1/2 turns. Not enough ...... I was afraid of tearing out the threads in the regulator. The gage and flowmeter screw in a full three turn before snugging up. My original question was on pipe tap and die tolerances. I tried a couple different dies on the brass handels and found a cheap Tiawan die cut a little smaller dia. on the thread but still not enough to get a full three turns to snugging up. As you can see in the picture I ran the die well past the stopping point before I could get the handel to fit properly but by doing that I just ended up with a straight pipe thread. This is not the first time I have run across this with pipe threads. Factory pipe threads seem to be smaller in overall dia. than the taps and dies. The regulator is clamped in the chuck for holding purposes only, to unscrew the cap. You can see how short the threads are on the nipple, I can't duplicate that fit.



doctor demo
03-10-2010, 12:07 AM
$100 worth of tooling.Time doing the rebuilding $50. Parts $2.Posting pictures on the internet...priceless:D
Of course You coulda picked up a new reg for less:eek:
Job well done Joe!


03-10-2010, 12:12 AM
Compliments on the really nice-looking chuck.

Sorry, I got nothin' for ya on the thread problem. ;)

Except to say I think you have a mismatch on the thread pitch or form.