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Posted: 1/12/2021 7:22:18 PM EST
Installing a URX 4, can using a large number of shims cause any negative effects? This is the first time I've assembled an upper with a rail that requires them. I think I have used 6 or 7 shims, and it is hand tight at 12 o'clock, so I need at least 1 more shim.
Link Posted: 1/13/2021 8:54:00 AM EST
[Last Edit: 1/13/2021 8:56:09 AM EST by tobycreek]
That’s quite a stack. What thickness are they?  Are they going to create a large gap to the receiver?

I’ve done a few different brands that use shims. At most, I’ve used two. The ALG rails even come with varying thicknesses and you use combos of up to two to get a pre-torque index on their barrel nut wrench.
Link Posted: 1/13/2021 9:30:06 AM EST
Does the barrel nut only have one spot for correct orientation for the gas tube alignment. I’ve never installed a barrel nut like that if so. The most shims I’ve used is two but that is on barrels nuts with multiple provisions for alignment.
Link Posted: 1/13/2021 9:45:51 AM EST
[Last Edit: 1/13/2021 9:47:31 AM EST by IceDiver]
Where does it clock with no shims?

I don't see where the number of shims should have any real impact other than aesthetics.
Link Posted: 1/13/2021 11:29:24 AM EST
If this is limited to an everyday gun I doubt you'll have any issues. But surely there's an alternative if it doesn't sit well with you.
Link Posted: 1/13/2021 11:49:41 AM EST
When the thickness of the shims meets or exceeds the thread pitch, it is too many.
Link Posted: 1/13/2021 12:02:35 PM EST
[Last Edit: 1/13/2021 12:13:41 PM EST by RJeff21]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By harrymank:
Does the barrel nut only have one spot for correct orientation for the gas tube alignment. I’ve never installed a barrel nut like that if so. The most shims I’ve used is two but that is on barrels nuts with multiple provisions for alignment.
View Quote


It’s a URX 4.  There is no barrel nut.  The rail is one piece and threads onto the receiver.

OP, I ended up having to use quite a few of the included shims to get a URX 4 installed on one of my guns.  It still works fine with a heavy ass PVS-30 mounted on the handguard and I’ve never had any POI shift issues with the handguard mounted PEQ.

I’d probably just test it out as-is to see if it causes you any problems.  You could try a different receiver if you have one available.  Or you could use a receiver lapping tool to remove some material from the receiver to see how much you can decrease the amount of needed shims.  I wouldn’t go too crazy with that method, though, as it could cause problems elsewhere.

ETA:  There were a couple companies who offered machining services on URX 4s in the past where they’d remove material from the rear of the rail until it timed without any shims/gap.  Forget who it was, but maybe something to look into as well.
Link Posted: 1/13/2021 12:33:31 PM EST
[Last Edit: 1/13/2021 12:37:32 PM EST by 2apatriot]
Thanks guys. It clocks around 3 or 4 with no shims unfortunately, so it has to be brought back quite a ways. The gap between the rail and the receiver isn't really that big. I've used 3 .007 shims and 2 or 3 .003 shims, I'd have to go take it apart to double check.

@RJeff21 Do you remember what torque spec is for this rail? I bought this used and it didn't come with instructions. Unfortunately the video on KAC's website is FUBAR, and all the links to old threads I can find aren't working for some reason. Best I can find right now is it should be hand tight around 11 and torque to around 60 ft-lbs?



Link Posted: 1/13/2021 2:21:48 PM EST
This is why I only use clamp-on handguards.
Link Posted: 1/13/2021 3:12:25 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By 2apatriot:
Thanks guys. It clocks around 3 or 4 with no shims unfortunately, so it has to be brought back quite a ways. The gap between the rail and the receiver isn't really that big. I've used 3 .007 shims and 2 or 3 .003 shims, I'd have to go take it apart to double check.

@RJeff21 Do you remember what torque spec is for this rail? I bought this used and it didn't come with instructions. Unfortunately the video on KAC's website is FUBAR, and all the links to old threads I can find aren't working for some reason. Best I can find right now is it should be hand tight around 11 and torque to around 60 ft-lbs?

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I believe it’s around 60ft lbs, I can pull the manual later and confirm.
Link Posted: 1/13/2021 3:32:26 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By RJeff21:


I believe it’s around 60ft lbs, I can pull the manual later and confirm.
View Quote


It's got a high torque range (and upper value is even over 100+ if I remember) but 60 sounds right. Basically get to 11 o clock and torque to top dead center and you should be good.
Link Posted: 1/13/2021 3:43:20 PM EST
Here, this should help.

Link Posted: 1/13/2021 5:25:28 PM EST
[Last Edit: 1/13/2021 5:28:50 PM EST by Hydra-shokz]
You can also remove some material off the face of the receiver if you know what you are doing. It has to be done square. It can change the timing of the whole thing which might get you closer with less shims. I have one with 1 shim and zero gap.

Link Posted: 1/14/2021 7:07:17 AM EST
Thanks for the help guys. Looking at the instructions I now can see that a .025 shim is included, which is significantly thicker than any of the ones I have. I bought this rail used and it didn’t have the factory shims. I think I am going to contact Knights to see if I can get a set and get a receiver lapping tool from brownells.
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