All:

I'm trying to shave some weight from my AR10B A4 Rifle. The rig has been wearing an Armalite FF tube for a number of years now and I am looking to swap it out for an MWI 12" Keymod tube. So a while back the retaining nut loosened on me (this is the nut that holds the FF tube on NOT THE BARREL NUT). And I treated it with a little blue Loctite. It won't budge now. I'm using Armalite's upper receiver vice block and have tried a strap wrench (the kind you wind up and use a ratchet or open end to turn) and WD40. I put a little heat on it also. Can't get it to budge. So I see my options as:

1) Try a proper strap wrench like this: Rigid strap wrench

2) Sacrifice the nut by using a cutting disk to split it.

3) Get serious about applying heat (Not sure if Blue Loctite fails with heat anyway and significant heat will melt my vice blocks)

Advice?

WD40 won't do anything to LocTite.  Heat is your friend.

You don't need the upper in a vice block for this, just have it set up where you can apply plenty of heat to the nut - as precisely as possible.  LocTite melts around 450ºF, so you can get away with using a heat gun for this.  Just take your time and keep the heat as even as possible on the nut.  I can't remember if you'll actually see the threadlocker drip out, but with the kind of mass of metal you're working with, it shouldn't take too very much time for you to be able to break that nut loose.  Once LocTite has been heated, it is done - it won't re-set and lock the nut again.  So give it plenty of heat, rig the upper in something to hold it, and try that "proper" strap wrench on the nut.  If it doesn't budge yet, heat it up again, for a bit longer this time, and try again.

For what it's worth, Blue LocTite is serious stuff.  It's tenacious, and holds tight.  You may need to heat that nut several times before you get it to let go, but you'll be fine with just heating it for a bit longer with each try.

I'm looking forward to hearing how you do.

You might be confused with red loctite. Blue loctite is meant to be able to be removed with handtools only; no heat is needed. I'm not sure how much of this stuff you used but I have used a ton before and it has never been that hard to remove, even on really small bolts where a lot of torque can't really be applied. Since you are dealing with a barrel nut, crank on that thing. I don't see why you can't pop it loose....

Blue is a "medium" strength thread locking product, and it's intended (according to LocTite) for 1/4" to <1" fasteners.  With enough applied to a large-ish handguard retaining nut, there's plenty of grip to work against.  Heat will help - and the OP obviously had already tried to strong-arm it off.  The potential need to heat more than once is based on the mass of metal that will need to heat up to break down the LocTite.

I was in the EXACT same position as you are.  Here is the problem, when you heat this with a torch both the aluminum parts heat up and expand at the same rate so you don't get anything out of it.  Yes, I have the Armalite aluminum FF tube as well.

Eventually enough heat will break down the loc-tite.  What I finally had to do was bite the bullet somewhat.  I ended up using large channel lock pliers on the nut because the strap wrench wasn't getting it.  I have two different sized strap wrenches you're showing in your link.

The only way I even got it to the point where it would turn was by cycling the entire upper into a freezer (cycling means 3 different attempts) and then heating just the nut toward the muzzle side.  The receiver will heat up as well but will be much cooler so you don't have that same rate of expansion.  I was using a propane torch on my multiple attempts.

I have used Channel-loc with an old leather belt before.

A plan is forming...

1) Use C clamp to position upper on my bench.
2) Mapp gas torch to heat the nut to 488*F using Markal heat stick to measure temperature.
3) Allow to cool
4) Use proper strap wrench.

Nut was only hand tight when Loctite was applied so, as long as it doesn't re-set when it cools this sounds like a good plan fwd.

Can anyone tell me what temperature I would NOT want to exceed?

Thanks for all the help so far. Good ideas all.

Good plan.

Mapp can get hot fast so either be real careful or use a colder fuel if you have it available. Before you put your flame to valuable parts, practice on something else of similar proportions that isn't important if it gets overheated.

Blue Locktite will not be very difficult on the scale of being compared to other types, so just be patient and you will be fine.

The manufacturer of LocTite says to heat to 482ºF and disassemble while hot.  Read it here at the bottom of the page.  Glad I checked.

I agree that a MAPP torch will get stuff hot really fast, but my heat gun gets to 750º on its low setting, and it won't set fire to anything....  Remember that you're not trying to weld anything, just heat up the whole assembly.  Yes, all the parts will expand while they're hot, but they should do so evenly enough that it won't be a factor.  Using the 488º indicator will work fine.  In this case, I'd use big Channellock pliers with leather or something like that attached to the jaws instead of a strap wrench to unscrew the nut.

This is the method I used. It was successful:

1) Heat nut using Mapp gas torch and 488* heat stick.
2) Once hot enough, use belt and 12" channel locks on nut and strap wrench on handguard to hold back up.
3) Swear
4) Establish movement of nut
5) Once cool enough to mount in receiver blocks, lock in vise and finish removal.

Took me two tries. Thanks for all your help.