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Posted: 3/29/2025 8:37:20 AM EDT
[Last Edit: wormraper][Edited]
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Now I have a question for my own build instead of my buddys. this is one of those "hmmm, should I make a big deal out of this and ask for it to be "fixed" or just accept it as tolerance stacking long story short, I picked up a BCM socom upper a couple months back and stuck it on a spare PSA lower I had lying around as a place holder. Anyways, I finally got around to digging out one of my nicer built Sionics lowers and realize that that it wouldn't work on that lower. The rear takedown pin wouldn't go all the way through the lug on the upper. it will go 3/4 of the way through, then just stick hard as a rock. Trying to figure out whether was the lower or the upper that was in question, i swapped uppers and lowers with the rest of my collection until i relealized that it was an issue between ANY sionics lower and this upper. The upper mates firmly with every other brand of lower (Aero, PSA, bcm, SOLGW) and the sionics lowers all mate up with all my other uppers, meaning it's some sort of tolerance stackign with that rear lug on the upper and however the Sionics lower takedown/pivot pins are made that is the issue. out of 5 Sionics lowers they all exhibit that same "takedown pin won't go all the way through" with that ONE upper only. I've narrowed down the exact issue to the left side pin and hole don't align perfectly, so that when the pin is 80% of the way through it hits the edge of the receiver pin hole and won't go through anyways, now that i've narrowed it down, my question is this. Should i make an issue of this with bcm and see if they'll take it back and make sure the rear takedown lug on the upper is within specs, or is it one of those things that would be left to "whatever dude, it just doesn't like this one brand of lower, tolerance stacking happens", especially since it does work with bcm (and every other brand i had in my collection other than my sionics ones) lowers no problem (and i actually have a couple bcm lowers on the way that weren't affected by the price hikes yet). My concern was sending it in to bcm and them saying "well it works with our lowers, not our problem" since it only happens with one brands lowers...but on the other hand I didn't want to ignore something if it was something that SHOULD be addressed as it is the ONLY upper out of my 5 BCM uppers that do that |
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How often do you play musical chairs with upper and lowers? Is fitting on all lowers required, or just OCD? I would start with some different take down pins. The offending pin hole could be honed slightly to fit. I would just run the upper on whatever it fits onto. |
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Originally Posted By M4BlackRifle: How often do you play musical chairs with upper and lowers? Is fitting on all lowers required, or just OCD? I would start with some different take down pins. The offending pin hole could be honed slightly to fit. I would just run the upper on whatever it fits onto. The fitting on all lowers thing was more of a "if it's in spec, it fits, if it's not, it's not in spec" type thinking. And yes, a little OCD lol. Since it works with BCM lowers (and all my others outside of my sionics) like butter I'll just slap one of the new ones in the mail onto it as soon as it comes in and call it a day |
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Tolerance stacking is a thing, but I think you already guessed correctly that BCM will find it is within specification. I know all about the OCD thing. If you only had one rifle, that issue would need to be solved, but I wouldn't spend the time or effort to send it back. |
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Originally Posted By M4BlackRifle: Tolerance stacking is a thing, but I think you already guessed correctly that BCM will find it is within specification. I know all about the OCD thing. If you only had one rifle, that issue would need to be solved, but I wouldn't spend the time or effort to send it back. yeah, that's kind of my thought process. I actaully AM clinically OCD (not "oh hehe, I like things perfect, I'm OCD", but actually OCD and need to go to a therapist once a week to manage it type OCD) which is why I posted the question. On one hand I can over analyze things to death and make mountains out of mole hills, but on the other hand when I try and manage my triggers I can go the opposite direction and say "this is fine, I'm just being OCD" when it's important. So I posted it here for your guys opinions to see whether I was making a mountain out of said mole hill, or it was an actual issue. |
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If it absolutely needs to fixed so you can go to sleep at night, get a different upper that fits and swap everything over. I have some BCM blem uppers with no visible or dimensional defects, and some Dogbone uppers made by Demco that are said to be mil-spec GI contract overruns. https://www.ar15.com/forums/ar-15/What-is-a-Dog-Bone-M4-upper-receiver--/118-767136/ |
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Originally Posted By M4BlackRifle: If it absolutely needs to fixed so you can go to sleep at night, get a different upper that fits and swap everything over. I have some BCM blem uppers with no visible or dimensional defects, and some Dogbone uppers made by Demco that are said to be mil-spec GI contract overruns. https://www.ar15.com/forums/ar-15/What-is-a-Dog-Bone-M4-upper-receiver--/118-767136/ haha, nah I'm good man. For me the fear of "the unknown" is my biggest trigger. if I KNOW something's OK, I can live with imperfections. Especially since this is a 14.5 inch Socom BCM upper, which almost never comes back in stock and I'd be without it for ages. if all that's the problem is that I can't fit one brand's lowers to it, my fear is assauged. Now I just have to wait for those 4 BCM lowers I got from Rooftop Defense to arrive on Wednesday and I can complete this sucker. |
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Knowing that BCM parts tend to manufacture toward "tighter" fits and reduced play, I'm not surprised to read this. They undersize their receivers for barrels. Their pistol grips are so tight on some lowers they require a rubber mallet as a standard installation procedure. I have a couple of their blem uppers, but haven't encountered the issue you're having. I've noticed they tend to have fairly low play with lowers compared to many other receivers. If you are a one upper to one lower guy, build it on a lower it works with. I'd just swap around lowers to get a pairing you like where everything works. If you use multiple uppers with a given lower, swap around lowers such that any lower you want to pair a BCM upper with is not one with tolerance issues. Also, trying a different takedown pin doesn't seem like a bad idea. Optionally, you could modify parts if you really want that one upper with an uncooperative lower. If modifying parts, I'd alter the hole for the takedown pin in the rear lug on the upper by grinding the bottom of the hole slightly lower. Those lugs are fairly thick, and so removing a few thousandths shouldn't weaken it much, but will provide enough clearance for the takedown pin. It will, however, make this upper looser on other lowers. Best solution, in my book: shuffle your lowers such that whatever uppers you pair with whichever lowers won't include matching these two parts. Part of the reason I'd go this route is I like the tight fit that BCM provides. |
| I’ve had that happen on one build once, don’t recall who made the upper, but I lightly reamed the rear takedown pin on the upper until it fit, it’s the tightest upper and lower fitment I’ve ever had. It functions great but i keep it as a matched set now, it’s a little loose if I swap it on others. |
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Originally Posted By PreacherOfGadget: Knowing that BCM parts tend to manufacture toward "tighter" fits and reduced play, I'm not surprised to read this. They undersize their receivers for barrels. Their pistol grips are so tight on some lowers they require a rubber mallet as a standard installation procedure. I have a couple of their blem uppers, but haven't encountered the issue you're having. I've noticed they tend to have fairly low play with lowers compared to many other receivers. If you are a one upper to one lower guy, build it on a lower it works with. I'd just swap around lowers to get a pairing you like where everything works. If you use multiple uppers with a given lower, swap around lowers such that any lower you want to pair a BCM upper with is not one with tolerance issues. Also, trying a different takedown pin doesn't seem like a bad idea. Optionally, you could modify parts if you really want that one upper with an uncooperative lower. If modifying parts, I'd alter the hole for the takedown pin in the rear lug on the upper by grinding the bottom of the hole slightly lower. Those lugs are fairly thick, and so removing a few thousandths shouldn't weaken it much, but will provide enough clearance for the takedown pin. It will, however, make this upper looser on other lowers. Best solution, in my book: shuffle your lowers such that whatever uppers you pair with whichever lowers won't include matching these two parts. Part of the reason I'd go this route is I like the tight fit that BCM provides. yup, I got in the 4 BCM lowers I ordered from rooftop defense last night.. they all fit it nicely so I just tossed one of those on it and called it a day. |
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Originally Posted By M4BlackRifle: Tolerance stacking is a thing, but I think you already guessed correctly that BCM will find it is within specification. I know all about the OCD thing. If you only had one rifle, that issue would need to be solved, but I wouldn't spend the time or effort to send it back. Tolerance stacking, in this sense, is NOT a thing . . . . With properly designed and dimensioned design, parts at the maximum material condition (MMC, ie, biggest) will properly mate with parts at the least material condition (LMC, smallest). And the M16 is properly dimensioned. In 1967, the Army and Colt did a detailed tolerance evaluation of the then current drawing and found over two hundred potential interference points, that is places where MMC and LMC parts would not fit together. They then when through and fixed all the tolerances. A certain percentage of all contract weapons are subjected to interchangeability evaluation, where ten weapons are disassembled to the smallest parts, the parts all mixed up, and then reassembled, they have to fit back together without any problems and function properly I you have two new parts that do not fit together, one of them is not made to the proper dimensions. There are over 400 gages and fixtures for gauging all mating parts, I doubt even BCM has all of them, so |
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