Question on reading thread tap info

[Chill Wills]

Over time I have accumulated taps both large and small. Handy!

For years I have had a 7/8-14 tap. Looks new. I went to use it making a handloading adapter for a friends powder measure. All good so far. After drilling the 13/16ths tap hole the tap won't start. SPEED BUMP In no time I see it is a LEFT HAND tap! Duh. Where did this tap come from ??? No mater. I set the job aside.

I got a three tap set, NOS $20 on ebay. They arrive. I tap the hole. Works like a charm - New Tap and all. Super! ....Tested it on the 7/8-14 part. Won't go. I clean the threads and run the tap in again. No Go.

Testing other 7/8-14 sizing dies in the new part, a few sizing dies screw in and most don't. Applying a wrench, I learn all go together as an interference fit.

Okay. You read this far :popcorn: Here is a picture of the markings on the tap. Does the tap info I got call out a small tight fit? Should I have know from the info that this tap is non-standard?

[CastingFool]

According to my Cleveland tap drill sizes chart, you used the correct drill bit, for 67% threads. Is your drill bit cutting to the correct diameter?

[Chill Wills]

According to my Cleveland tap drill sizes chart, you used the correct drill bit, for 67% threads. Is your drill bit cutting to the correct diameter?

Good question.
Yes it does. And even a little larger.

[CastingFool]

Any way you can check the pitch diameter of the threaded part? When I worked in a machine shop, we made a lot of threaded studs, with one end cut for an interference fit, while the other end was threaded with a class 2 fit, meaning std fit for ease of assembly/disassembly. The d
Interference fit had a larger pitch diameter.

[CastingFool]

I did a search for the meaning of the markings on taps. Found an explanation in the practical machinist site. If I understand what I read correctly, your taps taps should cut a normal thread, not undersized. However, I can't quite figure out what the - 012 means

[Winger Ed.]

On a few occasions, with the same bit and the same tap, I've had a interference fit where a hole right beside it was fine.

I've just run the tap in & out a few times- cleaning/blowing out chips it in between, using plenty of oil, and that fixed it.

[Chill Wills]

On a few occasions, with the same bit and the same tap, I've had a interference fit where a hole right beside it was fine.

I've just run the tap in & out a few times- cleaning/blowing out chips it in between, using plenty of oil, and that fixed it.

Yes. That was my first response to the newly cut thread giving me trouble.


I dual posted this to a gunsmith section on another forum. It was suggested I measure across the full threads on the tap to see if they were really 0.875" I should have thought to do that right away!
Anyway... 0.866" across the full sized threads on the Tap in question. For comparison I also measured the 7/8-14 Left Hand tap, = 0.876"

The __012 marked on the lowest line does not = the 0.009" smaller I measured across the tap, but there are a lot of ways that can be accounted for. ...but it is close.

[CastingFool]

Now we all know the answer. Good job!

[country gent]

Threads are measured on the Vee at a certain dia ( pitch dia) not the od or id. usually special mics with vee anvils to match a range of pitches. Or a set of 3 wires. The -.012 is more than likely a .012 undersized tap. I have seen taps both + and - in .005, .010, and .015. + taps are usually used when the part is to be hardened so if thread warps or shrinks it still fits, Minus are for threads to be ground or lapped in.

Another thing is some cutting fluids affect how a tap cuts to size. The original tap magic was known to make taps cut to size or a couple thou under in certain materials. A thin wall may also stretch and come back resulting in a tight thread.

Im betting these taps were a production tap used as roughers to pre tap then finish with a full sized for optimum finish.

[Chill Wills]

Threads are measured on the Vee at a certain dia ( pitch dia) not the od or id. usually special mics with vee anvils to match a range of pitches. Or a set of 3 wires. The -.012 is more than likely a .012 undersized tap.

That's right. Knowing I have limited resources, I don't think the man suggesting I measure threads with a micrometer was thinking I was getting anything more than a good idea of what I had. In this case the choice was a standard tap OR, some other grind consistent with the __012 hand scribbled on the lowest line. And that was good enough to confirm I made a mistake in thinking I was buying a standard 7/8-14 RH tap. Who da thunk. Ebay can be risky.

I agree, the tap was purpose ground under size for some job, we can only guess what.

It is interesting to see the thread fit this tap made when its threads are tried on a bunch of reloading dies, RCBS, Lyman, Redding, Bonanza and Lee for the most part. Almost all Lee dies fit this undersized thread. Some RCBS dies will, half and half maybe. The majority of newer Lyman dies mate up, older ones do not. None of the Redding dies will even start.

Seeing the poor or loose fitting threads on some loading dies helps make a case for loading with the dies floating.

I'm likely going to have to track down another 7/8-14 die and chase out the threads.

[Cap'n Morgan]

Perhaps the taps were made to be chased with a full size tap? This will produce a more smooth and accurate thread.

[CastingFool]

Doesn't eBay have a buyer protection plan? Did the listing mention anything about the taps being undersize? If not, you may be able to get your money back, cuz the items you received were not as advertised. You would have to word your request carefully. Just a thought.

[Chill Wills]

Doesn't eBay have a buyer protection plan? Did the listing mention anything about the taps being undersize? If not, you may be able to get your money back, cuz the items you received were not as advertised. You would have to word your request carefully. Just a thought.

That's right. Yesterday afternoon I contacted the seller and described the problem in detail. I did not get any kind of reply but rather, right away received a refund. As soon as I have a correct tap I can complete the part.

[alamogunr]

A few years ago member here offered 7/8-14 taps for either free or shipping only. I don't really remember. I do remember thanking him but should have gone further and named him as a "Straight Shooter +. I've used it to follow threads on new presses etc.

I bought a Redding T7 awhile back that the turret holes seemed kind of rough. The tap didn't do anything to help. I had a couple of 7/8-14 bolts on hand, so I used some valve grinding compound on them to try to smooth out the threads in the turrets. I'm not running Redding down because I know from past experience that cast iron is difficult to get good finish unless you take extra care.

It took awhile to get a finish on the 7 holes of each turret. Still not perfect but at least the dies don't grate on the threads.

Reading this thread, I'm not sure that I would have known what to do if my tap was undersized. Good luck to Chill Wills.

[akajun]

Most cheap budget taps are not made to tolerance and will cut oversize or undersize threads, it’s a crap shoot. Combine that with most dies and presses are made with tight tolerances probably 2a maybe even 3a and you’ll understand why the taps your using don’t work.
You need to figure out the h limits you need on the tap and buy a quality tap.
Or single point in a lathe till you get the fit u want

[rockrat]

Do you live on the Front range or Western slope?

[rbuck351]

Those are clearly marked 012 undersize.
For chasing threads that are a bit buggered (not .012 under) you can use a bolt of proper size, screw it in a couple of turns then tap on the top of the head with a small hammer. Turn it in another turn or two and tap some more. Continue on.