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Thread: Interesting issue with my KAC URX 3.1 build

  1. #21
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    I used a 40 year old American made vice my dad bolted to his hand made solid as hell work bench so the vice isn't the problem either. The one tool i didnt purchase was the KAC vice grip tool because $350 was just a bit much for a rail install tool. I still am leaning toward an issue with either the rail itself or the upper being out of spec. I'll see upon disassembly i guess.
    Last edited by Mrick; 06-13-13 at 19:10.

  2. #22
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by Mrick View Post
    I used a 40 year old American made vice my dad bolted to his hand made solid as hell work bench so the vice isn't the problem either. The one tool i didnt purchase was the KAC vice grip tool because $350 was just a bit much for a rail install tool. I still am leaning toward an issue with either the rail itself or the upper being out of spec. I'll see upon disassembly i guess.
    The problem is that the PRI clamp isn't really what you should be using. That combined with tolerance differences between the threads are probably going to mean you will continue to have an issue. You can try different uppers and dry / test fit everything before applying loctite until you find a good match OR send the upper to Ayan for install. His services are very reasonably priced. I know that you spent good money on the wrench and want to do it yourself, but you can easily recoup the funds in the secondary market.

    IMHO, KAC never should have started selling the wrench. They caved into pressure and started selling it, I understand why. Unfortunately, without a good fixture to secure everything, it's a total crap shoot as to how the rail will align.

  3. #23
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    Frankly the PRI vise fixture is the wrong thing to use so it is difficult to know if it is a rail/upper tolerance stacking issue vs an incorrect installation.

    I think the best DIY tool for URX installation is the YHM Upper Receiver Picatinny Rail Vise Jaws.

    Still, if the face of the barrel extension seems flat and well aligned with respect to the inside of the upper (feed ramps align etc.) then you're probably good to go. It looks good -- I've seen much worse. :^)
    Disclosure: I'm a dealer/owner of an online firearms accessories store.

  4. #24
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    Dang. I just sent off the parts for my upper to be assembled by rainier arms with a 3.1 rail, including a krieger SS from compass lake and and a colt upper from SAW

    At least they have a good rep, and claimed to have all the parts to properly assemble it. I'll see for sure when it gets back.

  5. #25
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by eesmith4 View Post
    Dang. I just sent off the parts for my upper to be assembled by rainier arms with a 3.1 rail, including a krieger SS from compass lake and and a colt upper from SAW

    At least they have a good rep, and claimed to have all the parts to properly assemble it. I'll see for sure when it gets back.
    I didn't know that Rainier did URX work. Cool.

  6. #26
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    Ayan's type of vice block works best.

    The URX can "kick off" due to several reasons - play between the threads of the URX and the Upper, or being pushed by the URX rubbing on the Upper.

    The other issue that can cause this is that the upper it self may not be true where the barrel extension mates with it - and the barrel is kicking over.
    Kevin S. Boland
    Manager, Federal Sales
    FN America, LLC
    Office: 703.288.3500 x181 | Mobile: 407-451-4544 | Fax: 703.288.4505
    www.fnhusa.com

  7. #27
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    Quote Originally Posted by scottryan View Post
    You need real old school american made vice for this with atleast a 6" jaw. An old school one where the jaws are straight and no play, using the KAC fixture is good.

    I use an old Columbian vice that belonged to my great grandfather that never really got used for anything and it is still like new and straight.

    A mill vice with custom jaws like what Ayan uses is also good.

    Harbor Freight / Lowes / Home Depot china shit isn't going to cut it for this.

    For a URX install, everything needs to be perfect. You need a high end barrel, a name brand flattop receiver, a real vice, and the KAC tools.
    +1... No way in hell the PRI vice block combo thing can hold the URX & upper receiver in alignment. you may need several upper receivers to test fit, and then you still may have to machine the end of the URX to get it timed flush. Then Cerakote the machined area... I think the finished product is worth it though.

  8. #28
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    Just FYI, I took the upper apart and reassembled it and I can reproduce the problem. I think the OP did a fine job installing the rail -- maybe slightly excessive loctite :^) -- but I don't think the issue was his fault.

    The rail is perfectly aligned while it is in my vise but the moment I take it out, it skews about 6/100" measured near the end of the rail.

    I think I have a fix for it. We'll see in the morning.

    -ayan
    Disclosure: I'm a dealer/owner of an online firearms accessories store.

  9. #29
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by CharlieMike View Post
    Just FYI, I took the upper apart and reassembled it and I can reproduce the problem. I think the OP did a fine job installing the rail -- maybe slightly excessive loctite :^) -- but I don't think the issue was his fault.

    The rail is perfectly aligned while it is in my vise but the moment I take it out, it skews about 6/100" measured near the end of the rail.

    I think I have a fix for it. We'll see in the morning.

    -ayan
    Ayan, please update us on your fix. Very interested in seeing what you find out.

  10. #30
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    Quote Originally Posted by CharlieMike View Post
    Just FYI, I took the upper apart and reassembled it and I can reproduce the problem. I think the OP did a fine job installing the rail -- maybe slightly excessive loctite :^) -- but I don't think the issue was his fault.

    The rail is perfectly aligned while it is in my vise but the moment I take it out, it skews about 6/100" measured near the end of the rail.

    I think I have a fix for it. We'll see in the morning.

    -ayan
    I'd just like to take a second to thank Ayan for contacting me and offering to help out with the upper. He is truly a class act and I'll surely be doing business with him going forward.

    Rob

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