BCM thermo fit upper fun

[mv1arms]

Installing the thermo fit blem upper from BCM was quite a challenge. Took about 10 iterations of cooling the barrel extension under the cold water faucet, then quickly heating the upper receiver, and pounding on it with a mallet to get the barrel in. I also used some green loctite 620.

The barrel is proof research stainless and the extension measured at 0.9995. The inside of the receiver was 0.9990 before ceracote.

Looking forward to some accuracy testing with it because that thing isn't getting pulled any time soon. It wasn't very fun to install!

[BlogSarge]

Yikes, that’s tight.

Did they ceracote inside the barrel extension hole on the receiver? I’d have thought they would have masked it off if it was being coated before assembly.

[mv1arms]

I think the inside was ceracoted, ya. I probably should have requested that be left alone.

[mv1arms]

Oh I should also mention that my factory criterion barrel was a very sloppy fit inside the BCM upper and slid right in. I couldn't shoot anything under 1 inch with that barrel.

[BlogSarge]

I think the inside was ceracoted, ya. I probably should have requested that be left alone.

Well, at least you have a tight, solid fit!

The proof will be in the precision. Post up to let us know how it does.

[rabiddawg]

I’m no expert but running cold water over metal is a waste of time.

Put barrel in freezer over night. If it doesn’t slide in put the upper in the oven at 180 degrees.

[A5BLASTER]

I’m no expert but running cold water over metal is a waste of time.

Put barrel in freezer over night. If it doesn’t slide in put the upper in the oven at 180 degrees.

It's like someone should have told him that you have to freeze the barrel. And to lose the stupid glue.

[mv1arms]

Three cold water actually helped because after a while the barrel extension would heat up and the thing would no longer budge. Definitely doing the freezer next time.

[Old Bob]

Three cold water actually helped because after a while the barrel extension would heat up and the thing would no longer budge. Definitely doing the freezer next time.

I recently installed a loose fitting barrel using the thermal method & .002" stainless steel shim stock. The cold barrel went in only so far into the heated upper before I could no longer push them together by hand. It was about a quarter inch shy of being fully inserted so I threaded on a barrel nut & used that nut to pull the barrel the rest of the way into the upper. Torqued to 45ftlbs it's a good, tight, solid connection. Here's hoping it will shoot gnat's eye groups.

[StoneHendge]

Luckily you have a Proof barrel and you won't have to pull it because chances are you won't get it out. When the barrels shot out, it will make a unique tomato stake with the receiver on top.

[mv1arms]

unique tomato stake with the receiver on top.

No doubt

[BlogSarge]

The freezer for the barrel and a heat gun for the receiver are the way to go. Or just leave it outside like I did last winter. It got down to about 10 F. I used a paint stripping gun to heat the receiver til it was good and hot. The barrel slid right in kind of snug. After a few seconds the barrel was in for good.

If it ever needs to come out, a suitably sized dowel in from the rear of the receiver and a small sledge should do it.

[PVBoom]

I think the inside was ceracoted, ya. I probably should have requested that be left alone.

Um, irrelevant.
BCM uppers (and Noveske, VLTOR, JPE) are machined to require thermal fitting.

If you are getting one, you will be thermal fitting, and using any simple liquid to chill the barrel is a waste of time.

[mv1arms]

Um, irrelevant.
BCM uppers (and Noveske, VLTOR, JPE) are machined to require thermal fitting.

If you are getting one, you will be thermal fitting, and using any simple liquid to chill the barrel is a waste of time.

Ceracote adds 0.001 thou so not sure how that's irrelevant. Also thanks for explaining that the method that worked for me was a waste of time, very helpful.

[tdbru]

if it's available, LN2 works pretty well at chilling things down. otherwise a deep freeze makes the barrel cold enough it hurts to hang on to it with bare hands.
-tdbru

[Mike98632]

if it's available, LN2 works pretty well at chilling things down. otherwise a deep freeze makes the barrel cold enough it hurts to hang on to it with bare hands.
-tdbru

I've heard dry ice -109 F. Don't handle it or the barrel bare handed. I would also consult with someone who knows what it might do to the barrel. I don't think anything but I'm no expert

[teppou]

Oh I should also mention that my factory criterion barrel was a very sloppy fit inside the BCM upper and slid right in. I couldn't shoot anything under 1 inch with that barrel.

Hmm, I had the same sloppy fit on a Criterion and BCM, with similar results... Makes me wonder if I should pull it and start over.

[Laman]

My Monster 20" 6 ARC barrel was a tight fit for the BCM upper. A propane torch from a distance and a couple of taps with a wooden mallet and it was in. If there is any wiggle I don't have any instrument that would measure it. I hope to be able to try it out next week.

[drewthebrave]

I've used a handful of BCM uppers on builds, and it seems like every fitting is a little different. My LaRue PredatAR, Bergara SPR, and Alexander Arms barrels took a heat gun and a few light taps of a mallet to get set. A Wilson Combat SR barrel was juuust able to slide in without any thermal fitting needed at all, so I bedded with some Loc-tite to take up the very minimal slack. FWIW, that same WC barrel wiggled like crazy in a standard upper receiver.

The toughest assembly was an Anderson Mfg Grendel barrel that I couldn't get past 1/8" into the receiver with heat alone. So I put the barrel in the freezer overnight. I heated the receiver over my stove (right next to the fridge) to get everything together while the temp differential was most extreme. It got the barrel about 75% of the way in to the receiver before it got stuck. Each part was literally just seconds away from their heat/cold source when it stopped moving. I hammered it the rest of the way with a mallet before the fit had time to get much tighter. Despite the tight fit, that particular barrel wasn't a great shooter, whereas all the others were solid performers.

[Old Bob]

I've used a handful of BCM uppers on builds, and it seems like every fitting is a little different. My LaRue PredatAR, Bergara SPR, and Alexander Arms barrels took a heat gun and a few light taps of a mallet to get set. A Wilson Combat SR barrel was juuust able to slide in without any thermal fitting needed at all, so I bedded with some Loc-tite to take up the very minimal slack. FWIW, that same WC barrel wiggled like crazy in a standard upper receiver.

The toughest assembly was an Anderson Mfg Grendel barrel that I couldn't get past 1/8" into the receiver with heat alone. So I put the barrel in the freezer overnight. I heated the receiver over my stove (right next to the fridge) to get everything together while the temp differential was most extreme. It got the barrel about 75% of the way in to the receiver before it got stuck. Each part was literally just seconds away from their heat/cold source when it stopped moving. I hammered it the rest of the way with a mallet before the fit had time to get much tighter. Despite the tight fit, that particular barrel wasn't a great shooter, whereas all the others were solid performers.

When it gets this close (75%) use a barrel nut & barrel nut wrench to pull the barrel the rest of the way into to upper. No need to smack it with a mallet.