Word of Caution: Broke My Bbl Bolt This Evening.

[Sean K.]

Word of caution:

I broke off the head of my bbl bolt on the 10.5" today upon reassembly. I've taken the bbl off probably a dozen times now. I'd highly recommend anyone taking their bbl off that frequently to go ahead and replace the socket head allen screw when you get up around 1/2 dozen times or so.....they are high quality YFS fasteners (typically referred to as "grade 9"). My newer PDW upper has a H-K USA labeled 5/16-18 socket head screw. YFS is typically high quality....not sure why there was a change.

I always just use a standard allen L shaped 1/4" wrench (5.625" long, so over leverage wasn't an issue) and it snapped the head off. The bolt was not defective. My gut feeling in working with a lot of fasteners in the aerospace industry is that over time, the bolt continues to stretch in the threaded portion and eventually, it just snaps. IMO, this was just run of the mill metal fatigue though I admittedly gorilla fist fasteners.

I'm considering going to a set screw instead of the socket head screw next time around (after I drill and easy out or tap the current clusterfuck) as this problem cannot occur with the set screw....though I could conceivably strip the head....but allen head set screws would also act like a self centering pilot hole if that happened.

Anyone know off hand what the correct length is for the set screw? Anyone know what size allen bit fits this set screw? I'm concerned that you may overtorque the allen wrench itself if it is significantly smaller than the 1/4" wrench used on the socket head screw if I were to tighten to 200-250 in-lbs. Finally, anyone know if the set screw (or the socket head allen screw for that matter) has a special degree chamfer on the end that mates to the bbl or if it's just a standard taper? I'd like to just pick this up at the local hardware store tomorrow as opposed to having to order from Kermit.

Thanks,
Sean

[aziator]

Sean, PM Bravo. He ended up purchasing 100 set screws at one point, I believe they already have the correct chamfer on them. I think it is a 5/23" allen wrench, same as the gas block. If you get one from the hardware store you will just have to grind down the point to get the slightly beveled chamfer you are looking for. I don't know length off the top of my head but I will search.

[Sean K.]

Sean, PM Bravo. He ended up purchasing 100 set screws at one point, I believe they already have the correct chamfer on them. I think it is a 5/23" allen wrench, same as the gas block. If you get one from the hardware store you will just have to grind down the point to get the slightly beveled chamfer you are looking for. I don't know length off the top of my head but I will search.

Shoot....so it's not a standard chamfer? That sucks. Oh well....such is life I suppose.

The gas block set screw bit is pretty darn small. I'd worry about twisting the allen bit off if torquing it to 250 in-lbs. Maybe I should just stick with the allen head and replace it every few times I take it off.

Sean

[aziator]

Do you have a Fastenall near you??

https://xcrforum.com/index.php/topic,...0.html#msg4910

Mine has been fine, haven't stripped it yet. It is a 5/8" bolt.

[Sean K.]

Do you have a Fastenall near you??

https://xcrforum.com/index.php/topic,...0.html#msg4910

Mine has been fine, haven't stripped it yet. It is a 5/8" bolt.

Not sure on Fastenal....I'll check. I'm pretty sure there's one by my old work and I have to go by Grainger tomorrow for a file set anyway.

Thanks for digging that up....I'll have to look into it more tomorrow after I get the broken piece out. Not sure which way I'll go at this point. I'm thinking the larger allen head bit may be easier to come up with if the SHTF verses having the hardened short bits on 3/8 chrome plated sockets (the ones that you can put on a 3/8" head ratchet).

Sean

[Sean K.]

Hey Chris,
After reading Bravo's post.....does the "cone nose" indicate that the chamfer is correct for mating to the dimple in the XCR bbl? I'm assuming he didn't have to grind the taper into the leading edge of the bolts, right?

Sean

[DSM]

Sean, I got a couple set screws from Bravo awhile back. They are pointed so you'd need to grind down the tips to the right length. The chamfer angle was fine. As far as torque limits I'd have to dig up the charts but I think the set screw would max out at 200 in/lb, IIRC.

[Sean K.]

Sean, I got a couple set screws from Bravo awhile back. They are pointed so you'd need to grind down the tips to the right length. The chamfer angle was fine. As far as torque limits I'd have to dig up the charts but I think the set screw would max out at 200 in/lb, IIRC.

If you don't mind looking that up, I'd appreciate it. The set screw seems a good way to avoid the issue I've experienced, but only if you can torque it enough to actually retain the bbl.

Thanks in advance.

Sean

PS. thanks for the info on the pointed tips....That's no big deal. I can grind that down enough to make it work. Putting a chamfer on one by hand is a little more tricky; doable but tricky.

[aziator]

I torque the piss out of mine, never had the bolt or barrel come loose on me.

[LoadedDrum]

I have uninstalled and reinstalled my barrell many times without problems.

Are you completely removing the bolt from the upper or just backing it out enough to remove the barrel?

I do the latter and never the former.