5/8-24 Threading

I am not a gunsmith so please forgive me some stupid questions. I am looking at maybe making a shim set for timing brakes with .0005 increments between sizes instead of the common .002 increments for current shim options.

When threading the barrel to you thread to class 2A or 3A tolerances? Basically is your max major diameter 0.6238 or 0.6250? If I made my shim set with .6250 ID would I run into issues?

Also why is 5/8-24 UNEF but 5/8-32 UN. I would think that the 32 would be the extra fine.

The UN just tells us that it follows the Unified National standard form and tolerances. UNEF is a standard extra fine. You could call out a 5/8-56 and properly call it a UN, but most drawings would probably call it UNS, for Unified National Special.

I'm no smith either, but I would choose a class 3 fit. The major diameter of a 5/8-24 is 0.6250-0.6185 whereas a 5/8-32 major is 0.6250-0.6190 (both class 3).

I ask about the major not because I want to make the thread but because I want to make a shim (washer) to fit around the thread and was wondering if the washer ID was 0.6250 if I would run into issues with people who thread barrels threading the major to 0.6250. Or if smiths threaded the major to 0.6238 and I would be okay with a 0.6250 ID on the shim.

Maybe I should have made this 2 thread for 2 questions, oh well.

With the UNEF and UN 5/8 threading.
From: http://www.btb.termiumplus.gc.ca/tp...ED NATIONAL EXTRA FINE SCREW THREAD&index=alt
"...The extrafine thread series (UNEF and NEF) is used principally in aeronautical structures where an extremely shallow thread is needed for thin-walled material and where a maximum practicable number of threads is required for a given length."
From: http://www.engineeringessentials.com/ege/fast/fast_page6.htm
"UNEF = Unified National extra fine. Used when length of engagement is limited (Example: Sheet metal)."

What confuses me is that in my Shop Reference for Student and Apprentices book it lists 5/8-24 as UNEF but 5/8-32 as UN. But by the above quotes the finer thread (32) should be UNEF and the coarser thread (24) should be just UN. However they are reversed.

If you make the shims .625" you are covered no matter what the 'smith makes the OD.

Why are the shims needed if you are cutting the threads? Could you just time the brake with the tenon shoulder where it hand snap at TDC and then Rockset the threads?

Bamban said:

Why are the shims needed if you are cutting the threads? Could you just time the brake with the tenon shoulder where it hand snap at TDC and then Rockset the threads?

Click to expand...

I am not making the threads. I am looking at putting together a shim kit for people who already have threads cut. There are currently shim kits available. But they are all .002 or .0022 thickness between each size. I was considering making a kit in which you could assemble a shim set to within .0005 of your desired thickness. I was worried that if I made the ID 0.6250 it would not pass over the threads of some barrels.

The brakes that have to be timed are a pain. Only those that install them know how frustrating it can be. And people who pay for it to be done can't understand why it costs almost as much as a chamber job. A brake that is spiraled or similar where there is no timing, is a snap.

I can appreciate the idea of shims, but a brake needs to be removed and cleaned, and dealing with shims could be troublesome in time. However, in my view, a brake should be installed when the barrel is threaded, when the barrel is dialed in, so the brake hole can be bored in the same alignment. Done this way, there should be no need for shims and the rifle will shoot accurately.

I agree that having a brake timed to the action is the best way. However there are more and more factory threaded barrels, including a new tactical rifle from Ruger. There are also people who want to change brakes. There are brakes that have either a pinch bolt or a nut for timing (I have one of this style). But if the person wants a brake that is the style that needs to be timed then the typical method is to use shims rather than taking a threaded barrel in to a smith to get timed. I just find that the shim kits currently available are lacking in their options for thickness.

If you make your washers to the larger size they will still work well with the smaller...so manufacture to the worst case when it will still cover the other cases.

For the record, peel washers are a pain but work fine. In my case I had a YHM QD brake, so it had to both fit a suppressor and be timed. This isn't realistic for custom fitting or a cone/crush washer so shims or a peel washer are the only real good options.

One of the problems with some shim stock is when the operator puts the brake on and it's just short of TDC, he torques the thing big time and after a while it will deform into the threads, i.e., the shim ID gets smaller and removal of the shim is not an easy task, particularly if it is a thick crush washer. If the ID is made .020 or so, oversize, eventual removal is much simpler.

If the brake turns on a bit past TDC, punch marks, with a sharp punch, near the ID on the shoulder of the brake will allow easy alignment. The marks should be deep enough to move metal upward around the divot and positioned at 90 degree intervals. 45 degree intervals are better. Be sure the marks are inside the area contacted by the barrel shoulder. Done correctly, a brake that is tapered to the barrel diameter will not show any gap, and will time perfectly to 12 o'clock.