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M4 Front Sight Nut


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5.5mm wrench, open ended one. There is no room for a socket.


The Brownells link is a good place to buy the 10 dollar nut. I recommend you buy some other things to make the shipping charge hurt less.

The nut is about 6.60 if you have a LEO account setup. Highly recommended.


I had similar issues with my wrench. I put a drop of blue locktite on it to make up for any lack of torque that I couldn't get on the nut without rounding it.

It along with the nylock washer in it should hold it in place. A 1000+ rounds later, and it hasn't shaken loose yet.

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I think due to the design of the nut, a better quality wrench won't help much. It almost seems as if it has a slight taper on it. I considered grinding the round wrench down at the front so I could pass it below the nut.

I figured I would do that if I couldn't get the nut tight enough with the open ended wrench. Once I began to feel the wrench start to strip past the high spots, I stopped. Bumps, hits and a 1000+ rounds later, it hasn't budged.


If your sight is secure but just chewed up, I'd consider leaving it alone.

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benelli usa just told me on the phone it was a 3mm wrench...

well unless italy has different sizes in mm's than every other country, the person that told you that is WRONG!!!!;) i tried the 5mm= no good and the 6mm= no good. the 5.5mm worked, a little loose which was good as to not scratch the sh-t out of the nut while positioning for the next turn.:) and im pretty sure all mm's are a universal same size.

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Has anyone tried a 7/32" English wrench? It's only about .003" different from 5.5mm and that may make a difference. Different manufacturers may have different tolerances for the opening. I think someone mentioned Snap-On earlier in this thread. I have used their tools to remove nuts, bolts, and screws that were mangled by cheap tools.


Don't be fooled by the Europeans. I was in France and checked out a hardware store. The most common pipe fittings were 12.5mm (we call it 1/2"). There were a lot of bastardized mechanical things that are actually sized by the old English standard but given metric equivalents. I have not checked recently but my Jap cars still had English lug nuts and battery cable bolts.

Edited by OhioM4
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i just wish there was a clear answer to this. i have the new nut tightened on there but its not very tight. i want to tighten it down but 5.5 is going to round it off.


what is the actual size, i wish i had some calipers.

im confused there K, dont you have the same size replacement nut we all have on our M4's?? cause the 5.5mm wrench tightened my nut as tight as i needed it without rounding anything.;) and your new nut should have new nylon and once snug it isnt going anywhere.

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