Is it time to try some barrel nut shims?

[Circle_10]

Tried assembling an upper today but just couldn't get the barrel nut to align properly to install the gas tube no matter what. Everything is greased up with Aeroshell, I tightened the nut to 30lbs three times and backed off three times before trying for final installation. The nut aligns ok at finger tight, but as soon as I torque it to 30lbs the notch has moved to far. Trying to get the next one to line up was also unsuccessful, I torqued the nut incrementally from about 32lbs right up to 80lbs, stopping every 5 to 10 lbs or so, checking at each interval for gas tube clearance and just couldn't get that next notch to move far enough along to line up either.
I spent about two hours backing the nut off, re-torquing, backing off again, re-re-torquing etc etc.. every 5-10lbs and still no go.
I have a couple other barrel nuts but the barrel in question has a fixed​ FSB pinned in place so unless I remove the FSB I guess I'm SOL as far as just switching nuts goes.

Are barrel nut shims my only real option at this point?

[georgeib]

Just muscle up and take it to the next notch. It will be fine.

[christopher.dow]

I must disagree with georgeib, as my understanding of the torque spec is 30-60 ft-lbs. I've solved this problem two ways: shims (both my Aero handguards needed the ones provided) and lapping.

The first time I lapped, I used wet/dry sandpaper as per recommendation from this forum. It worked fine. Put the paper on a flat surface and do figure eights for a while, then try again to get it to line up within the torque specs. That takes a while. I've since bought a lapping tool from Brownell's, and that is much faster. I run it until the front face is all exposed aluminum, then use the same procedure: rinse, wash, repeat until it lines up properly with the specified torque.

The Rifles with the Aero shims are among the most accurate I have (although neither has an Aero barrel). Using either approach, the rear irons zero very close to the middle. So, I've gotten a good mid-windage zero and proper alignment within torque spec either way.

I have a slight preference for lapping because it just feels more like the right way to do it, but I have not generated data to support that feeling.


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[Circle_10]

I must disagree with georgeib, as my understanding of the torque spec is 30-60 ft-lbs. I've solved this problem two ways: shims (both my Aero handguards needed the ones provided) and lapping.

The spec is 30-80 ft-lbs, and I went right up to 80 and still no joy so I'm not going to try and force it beyond that. I guess I'll order some shims and the stuff to do some lapping and hopefully one of the two works. I'll try the shims first and see what happens.

[556BlackRifle]

Is it a proprietary barrel nut (if so what brand) or standard mil-spec style? Have you tried wiping most of the aeroshell off the threads to see if that helps? Are you using a good torque wrench or a harbor freight knockoff?

I had one like that a while back with a mil-spec barrel nut. Grabbed another nut and it worked fine.

If the nut is proprietary to a specific handguard, and cleaning the excess grease off doesn't help, shims are probably your only option.

[Circle_10]

It's just a standard barrel nut. It's for a retro build. I can try cleaning the grease off but I was under the impression everything should be generously greased during installation.
Torque wrench is a Tekton brand.

I have other nuts I could try but don't really want to mess around removing and reattaching the pinned FSB in order to swap out the nuts. I'm still new to assembling uppers so I'm leery about dicking around with the front sight.

[christopher.dow]

It's just a standard barrel nut.
8<
don't really want to mess around removing and reattaching the pinned FSB in order to swap out the nuts.

A standard nut should time at proper torque without either lapping or shims (I think that's why the spec has 30 ft-lbs of range). It's likely the receiver is a little out of spec. 556BlackRifle's suggestion is definitely worth a try.


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[556BlackRifle]

It's just a standard barrel nut. It's for a retro build. I can try cleaning the grease off but I was under the impression everything should be generously greased during installation.
Torque wrench is a Tekton brand.

I have other nuts I could try but don't really want to mess around removing and reattaching the pinned FSB in order to swap out the nuts. I'm still new to assembling uppers so I'm leery about dicking around with the front sight.

I'm not saying remove it. Wipe it off. There will be enough residual grease left behind to do the job and if it changes things enough, you may find a spot where it aligns going over 30 ft lbs. What brand torque wrench? It may be okay or the values may be jacked. If it's a chicom knockoff, you might want to have it checked before relying on it.

[Circle_10]

This is the wrench I'm using.
https://www.amazon.com/gp/aw/d/B00C5...40_FMwebp_QL65

I ordered some barrel shims as well but I'll clean off some of the grease and take another crack at the upper tonight maybe. If it still doesn't work than I'll wait for the shims, if it does than I guess I'll have some extra shims kicking around if I ever need them.

[MistWolf]

Sometimes an upper won't clock correctly within the torque spec. I'm currently working with an upper that won't clock right, even after trying three different barrel nuts and two different barrels