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British Imperial threads?

Discussion in 'Gunsmithing and Repairs' started by Kerf, Aug 14, 2013.

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  1. Kerf

    Kerf Member

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    I’m making a new stock for a Vickers Armstrong martini type barreled action and the action screws are giving me a fit. The action screws seem to be threaded ¼”- 26, a non-standard screw pitch. I say, “seems to be” because the screw goes into a thread checker of a ¼”-28 loosely and at ¼”-32 it’s very tight. The major diameter on the threads is .234, while the minor diameter between the threads is .201. Not exactly what you would call a quarter inch bolt. Normally, a quarter inch bolt would measure .245/.222 m/l.

    These “action screws” go through a sleeve, not unlike a pillar-bedding sleeve, which is threaded internally and externally. The external threads of the sleeve screw into the wood stock, (like a threaded insert used in knock down furniture), and the internal threads fit the screw itself for a few turns, then the screw slips through and goes on to thread into the barrel action itself. Basically, the internal threads in the sleeve act as a “keeper” so you don’t lose the screws when you loosen them to take the stock off the barrel.

    So, my question is: can I convert this over to a ¼”- 28 standard size by simply running that size tap, (as one consort suggested,) or will that just bugger up the situation? I would also have to rethread the sleeve internally and the screw itself. If that isn”t going to work perfectly, I’d rather not muck up the situation, just for the sake of expediency. I’d have to find another more suitable solution, if that’s the case.

    One solution I considered was, basically, discarding the British screw and sleeve, drilling new holes in the barrel and action next to the old ones, and making new sleeves. That way, I can drill a hole in the butt of the rifle, and insert the old hardware (screw & sleeve) for the next owner to make their own decision how they choose.

    I also considered doing the next owner a favor by using J B Weld to fill the threads on all the parts and re-cutting new threads, thereby using the old hardware with new threads.

    Just not sure which is the best solution from a mechanical point of view. And, I’m not certain any of the above solutions would work at all. Any opines you might have would be greatly appreciated.

    Thanks, kerf.
     

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  2. Jim K

    Jim K Member

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    With the understanding that I don't have the parts and the rifle in hand, my first thought is what is the problem? Are the current threads in bad shape? The threads that go into wood shouldn't be a problem (I assume the sleeve is slotted for a screw driver) and the screw threads look OK. Unless the receiver threads are bad, I don't see a problem. Just replace the stuff in the new stock. I wouldn't change anything just to use a different threaded screw if the old one works OK.

    One thing not to do is to use JB Weld where there is any stress; the stuff is OK for what it was made for, but it has a reputation for being something like magic. One gentleman even suggested using it to patch a split shotgun barrel!!!

    Jim
     
  3. Kerf

    Kerf Member

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    I guess I should have mentioned that I'd like to use some inletting screws for fitting the barreled action to the stock. I'm working with a stock blank which is way over sized and would need to use some 2 1/2" headless screws while fitting. You end up taking the barreled action in and out a million times while fitting and the inletting screws are there to facilitate that.

    I don't like the idea of altering threads on the gun, I think they should remain unaltered.

    I think I'll try turning down a 1/4-28 bolt and see if I can get that to work. Or, I'm considering using hot melt glue to temporarily hold a bolt in place; that might work. I'll figure some thing out.

    kerf
     
  4. NCsmitty

    NCsmitty Member

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    The thread gauge pic looks like the screw is closer to 24tpi to me. The thread of the bolt is running shorter than the 26tpi gauge. Do you have a #12-24 screw to try?


    NCsmitty
     
  5. BBBBill

    BBBBill Member

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    That would make it closer to 28 TPI, not 24 TPI.
     
  6. Jim K

    Jim K Member

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    I don't know how much it would be worth to you, but there are makers of odd-size taps and dies. One such is

    http://catalog.tapcotaps.com/

    Google "taps and dies odd size" and you will come up with others.

    Jim
     
  7. Kerf

    Kerf Member

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    Thanks for all the replies...

    I hesitated to post a photo of the threads with the gauge on it knowing in advance that it would get misinterpreted due to the various angles of the photos. I'm not a photographer. But, I did photograph the 24 and 28 tpi, and they are obviously foo fine and too coarse. Definitely it's 26 tpi.

    And, I somewhat managed to solve the problem by turning down a blank bolt slightly and then threading to 28tpi. Not perfect, but enough for a few threads to grab and with some Locktite, I think it will hold together for my purpose.

    Later today I intend to contact some of the custom die makers, as suggested (thanks), to see what's available. I know there's the BSC (British Standard Cycle) which is 26tpi. Never hurts to have something odd ball on hand for the next one.

    Thanks, everyone,

    kerf
     
  8. brickeyee

    brickeyee Member

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    Buy some steel or even brass rod and order the correct die to thread it.

    Just make some short piece up.

    If you just want headless screws for inletting, even something close and only finger tight might work.
     
  9. LemmyCaution

    LemmyCaution Member

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    Try an M6x1.0 metric machine screw. I'll bet you'll find it works perfectly.

    1.0 thread pitch is ~26tpi

    The major diameter on M6 is 0.2351

    The minor diameter on M6 is 0.2281
     
    Last edited: Aug 15, 2013
  10. NCsmitty

    NCsmitty Member

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    Thanks for the correction, BBBBill, that was a piss poor assessment on my part. Sometimes I should keep my fingers off the keyboard until my brain's in motion.


    NCsmitty
     
  11. Kerf

    Kerf Member

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    LemmyCaution wins the box of cigars...

    down comes the duck, etc.... Thanks a million, free beer next time we meet.

    Looking at your spec's that were so close to what I needed, I decided to purchase some M6X1.0 x 70 bolts to try out and bingo. Just like you said, they went right in, fit perfectly; problem solved thanks to you.

    Let me say in my own defense, the first thing I tried was metric. Why my metric bolt wouldn't fit the metric checker doohickey at the hardware store, I dunno. I'm just extremely relieved and thankful for that help provided. Big load off my mind.

    kerf
     
  12. LemmyCaution

    LemmyCaution Member

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    Glad I could help you avoid mucking around with custom dies.

    Best regards,
    LC
     
  13. brickeyee

    brickeyee Member

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  14. BBBBill

    BBBBill Member

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    No worries, mate! I'm subject to errors myself. Team effort!
     
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