View Full Version : UNS threads

05-05-2004, 01:20 AM
Hey guys! So tell me...what is the UN series thread all about? What makes it different from other threads? Why can't I find taps in my KBC catalog? Seems the auto industry uses this thread for tie rod ends. Any idea why? I did a google search for this and found a few tidbits but nothing to explain the ins and outs. Any info on internal 7/8 X18 UNS thread cutting on a lathe? Thanks in advance!

05-05-2004, 02:49 AM
UNS stands for UNified Special. Here is a table of sizes.


They are basically bastard sizes.

Here is another table that lists 7/8 X 18


and another one. They have the same thread form as UNC and UNF.


Use your regular thread cutting tools and cut at 18 tpi.

As to the "why" part I would assume it would give fine adjustment capability by using 18 tpi on a 7/8 shaft.

[This message has been edited by Evan (edited 05-05-2004).]

Forrest Addy
05-05-2004, 07:03 AM
There's good reason for UNS threads.

One is that some applications force use of a special thread because of space - bearing nuts, threads used in optical apparatus, and measuring threads are but a few examples.

Another is to force purchasers to use the manufacturer's replacement parts - a vile practice widely used in valves and switches.

There's conservation of materials and weight savings.

Note that the manufacturing data for UNS threads are claculated from the same formula as UNC and UNF threads. It's a consistant system that allows the design of screw threads over a wide variety of applications and sizes. Ingenious really, considering it was developed by a committee.

Paul Alciatore
05-05-2004, 09:08 AM
<font face="Verdana, Arial" size="2">Originally posted by Forrest Addy:
There's good reason for UNS threads.

One is .......

Another is to force purchasers to use the manufacturer's replacement parts - a vile practice widely used in valves and switches.


And by manufacturers of measurement instruments and gauges like Starrett! And not just replacement parts but accessories also.


Paul A.

05-05-2004, 02:05 PM
Thanks for the info and links guys! Any idea where to get taps for this thread? I have a couple of pieces to do that are way too long for my lathe and won't fit through the headstock....Thanks.

05-05-2004, 02:12 PM
KBC Tools, page 191, Part number 1-372-324, $37.12 Cdn each.



05-05-2004, 02:16 PM

7/8 X 18 tpi KBC part number is 1-372-324

Price is $37.12 cdn on page 191.

05-05-2004, 06:52 PM
Mike and Evan...Thanks! I actually did see that in there but wasn't sure if it was the right one. Sorta thought there might have to be a UNS designation there somewhere.

05-05-2004, 08:55 PM
Firearms makers have long been facinated by special therads. Scope mounting holes are either 8x40 or the more common 6x48 thds. The receiver ring is thin due to the barrel threads and hence the tighter thd.for the scope mounts. Instrument and tool makers follow the same theory. It just gives a little more bearing surface. They are however the pits to deal with if over tightened and the heads stripped off as some are wont to do!