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Thread: Help needed - sight elevation screw nut

  1. #1
    Boolit Master Norbrat's Avatar
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    Help needed - sight elevation screw nut

    On Monday, 20 shots into a silhouette match, the elevation screw from the rear sight came off and landed on my chest.

    Hmm, thinks I, that could be why I haven't been hitting the targets!

    This is a custom made bolt action Unlimited gun, and while the sight does not have a maker's name on it, I THINK it is a BoMar.

    After removing the sight, I noticed that there is a small 5 sided nut into which the elevation screw fitted. The threads inside this nut are badly worn, not really surprising as this gun has been used for 25 years, and while my loads are relatively mild, the previous owner would have had the 308 stoked up a bit.

    I measure the diameter of the screw to be 0.11", so that should make it a No 4.

    Click image for larger version. 

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    However, the tpi is much, much finer than the expect 48 tpi.

    Click image for larger version. 

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    With the calipers set at 0.1", I count 8 threads, so 80 tpi!

    I don't have a 4 x 80 tpi tap to be able to make a new nut, and I reckon it will be very difficult to find one here in Australia.

    So I am asking for help.

    If there is someone here who has the correct tap, I would really, really, REALLY appreciate it if you could drill and tap a few holes in a small piece of 1/8" flat bar and mail it to me.

    Naturally I am willing to pay, and I will also buy you an adult beverage or three next time you visit us here DownUnder.

    Thanking you all in anticipation.

  2. #2
    Boolit Buddy
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    No worries. I am glad to help out a fellow shooter. I work grave yard shift so I won't be able to get to it until this coming Tuesday or Wednesday ( my days off ). I just need to head out to the hardware store pickup the tap die set (if i dont already have it) and the flat bar. If that works for you send me response via pm. You just pay the shipping what ever that would be.

  3. #3
    Boolit Master Norbrat's Avatar
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    Quote Originally Posted by silverback13 View Post
    No worries. I am glad to help out a fellow shooter. I work grave yard shift so I won't be able to get to it until this coming Tuesday or Wednesday ( my days off ). I just need to head out to the hardware store pickup the tap die set (if i dont already have it) and the flat bar. If that works for you send me response via pm. You just pay the shipping what ever that would be.
    That would be fantastic! But are you sure you could get a 4 x 80 tap from your hardware store?? It would seem to be a highly specialised tool. I can't even find it in the Brownells website.

  4. #4
    Boolit Grand Master leftiye's Avatar
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    Probly Brownell's.
    We need somebody/something to keep the government (cops and bureaucrats too) HONEST (by non government oversight).

    Every "freedom" (latitude) given to government is a loophole in the rule of law. Every loophole in the rule of law is another hole in our freedom. When they even obey the law that is. Too often government seems to feel itself above the law.

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  5. #5
    Boolit Buddy
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    Hmmm haven't thought of that. But I can look

  6. #6
    Boolit Buddy
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    All charts in inches and metric don't show a 4 x 80 ---

    http://www.newmantools.com/tech/pitchconversions.htm

  7. #7
    Boolit Buddy
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    There is 1/4 x 80 nc/nf or 0-80

  8. #8
    Boolit Buddy
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    Got to go to sleep. Will keep checking when I get up

  9. #9
    Boolit Master Norbrat's Avatar
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    It is an odd thread form.

    I can't even find this thread in my Machinery Handbook 25th Edition; listings for No 4 size screws show 36 tpi NS, 40 tpi NC and 48 tpi NF. None are even close to 80 tpi.

    80 tpi is usually a No 0 size screw, which is nominally 0.060" diameter; this screw is definitely 0.110" diameter, so a No 4.

    So this is highly specialised. I doubt you can find a tap just anywhere, even Brownells do not list anything like it.

    I was hoping someone already has such a tap, maybe from a previous project.

    But there might be a few other options. A bloke on eBay in the UK http://www.ebay.com/itm/180568747674 lists a 4 x 80 tap, but he's away on holidays so I can't follow that up just now.

    And we do have a sight maker/supplier here in Australia, so that might be another avenue to try as he may have a suitable spare part.

    BoMar are long out of business, but Kensight/Champion seems to have taken over their designs, so that may be another way, but I've never had a response from them when I've sent a simple enquiry via email.

    Thank you so much for your offer to help, but this is project is more difficult than it looks at first.

  10. #10
    Boolit Grand Master
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    How difficult would it be to go one size larger with a more common thread?
    Don Verna


  11. #11
    Boolit Master Norbrat's Avatar
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    Quote Originally Posted by dverna View Post
    How difficult would it be to go one size larger with a more common thread?
    To change the thread diameter, I would have to make a new screw, and as this one has the detent under the head, that would be quite difficult.

    I think the easiest fix is to make a new nut, but for this requires a No 4 x 80 tap.

    I expect the 80 tpi was chosen to give very fine adjustments.

    This request on this forum was just a forlorn hope that someone may already have the correct tap as it would be simple enough to cut a couple of holes in a piece of flat bar, from which I could file a new nut.

    But it seems this thread type is so obscure that no-one here has the tap needed.

    I plan to leave a message for the eBay seller in the UK. If that is indeed the correct tap, it is inexpensive enough for me to buy it when he gets back from his holidays and make a new nut.

    Thanks again for all your interest.

  12. #12
    Boolit Buddy
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    You may want to check with Novak sights

  13. #13
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    KBC Tools has a #4 x 80 TPI tap listed under their "Special Thread" hand tap section. You can check it out here: http://www.kbctools.ca/

    RRR
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  14. #14
    Boolit Master Norbrat's Avatar
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    Thank you, I appreciate all your help.

    The more options I have to follow up, the better. 😆

  15. #15
    Boolit Grand Master

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    If you change the thread dia or pitch it changes the movement also meaning that the clicks or graduations may no longer be the same minute of angle movement. Another is the larger dia thread may weaken parts it passes thru. The odd threads on sights are common due to this. But the manufacturers have a tool room and cutter grinders that makes just what they need also. We made special taps dies swedges, riveters, chamfering tools and other tooling for the plants needs in the tool room. Special tooling isn't near as big a problem to the manufacturters as it is to the people repairing it is.

  16. #16

  17. #17
    Boolit Buddy
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    Good call red. Still think giving Novak sights a call they may have a sight nut

  18. #18

  19. #19
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    44man's Avatar
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    Indeed a funny size but sight screw movement per click is based on thread pitch and detent distance.
    Things are not as good today with even scopes that do not follow clicks, says 1/4" per click at 100 but you need to spend big bucks for a premium scope. Years ago a Weaver was right on but now stuff is made in China or metric. Common to have double the movement per click. Red dots are horrible.

  20. #20
    Boolit Master fourarmed's Avatar
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    Don't know if this would work, but another possibility would be to wax the screw with Johnson's or some other good paste wax, then form some really hard epoxy around it and make your own nut. Probably wouldn't last for 25 years, but it would be easy to replace.

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