View Full Version : Upper and lower mating tolerances
WaterChunker
07-30-14, 22:14
Ok, so I just finished my first build with a stripped spikes upper and lower. This entire gun was pieced together. I am having a serious issue with the tolerances being very tight. I mean in regards to the last 1/16 of space between the upper and lower. It's so tight it literally snaps into place. It almost seems like there is maybe an issue with the back of the bolt carrier and the buffer? The take down pins are also ridiculously tight, but I have read this is not uncommon(its luck of the draw). However I have never handled an AR with the upper and lower so tight it literally snaps together. Any ideas as to what this could be or if their is a fix?
Lucky. Always seems like they are too loose for me.
wildcard600
07-30-14, 22:39
probable tolerance stacking making things tight. had a similar issue with my first build of a BCM upper mated to a DTI lower. i had to use a delrin punch to knock the pins out for dissasembly. after shooting for a bit it loosened up and now is fine. it also used to "snap" into place when seating the upper.
Eurodriver
07-31-14, 06:29
Lucky. Always seems like they are too loose for me.
No such thing as too loose. That's preferable to too tight every day.
Lucky. Always seems like they are too loose for me.
I'm trying to decipher if that's a personal issue or if the lowers that you've been trying to "mate" with have all been used & abused. Either way, that seems like TMI.
I do agree with Euro that the loose ones are definitely easier to mate with.
;)
Eurodriver
07-31-14, 08:15
Well, if that's not "Dicking the dog", I don't know what is. :cool:
I'm trying to decipher if that's a personal issue or if the lowers that you've been trying to "mate" with have all been used & abused. Either way, that seems like TMI.
I do agree with Euro that the loose ones are definitely easier to mate with.
;)
Ya got me.....you caught the tater...
That's what she said right? Guess I kinda walked face first into that one.
Seems like I have to put wedges in mine always. Even other handling people's rifles or handling them at the lgs. I never never handled a tight one without play.
Berserkr556
07-31-14, 09:26
No such thing as too loose.
If my memory serves me well there is such a thing as too loose. If you can slip a 20 thousandth feeler gauge between the upper and lower one or both are out of spec.
WaterChunker
07-31-14, 09:26
Well it definitely sucks having to eat a can of spinach every time i want to push out my take down pins or need to mate my upper and lower.
Eurodriver
07-31-14, 09:44
If my memory serves me well there is such a thing as too loose. If you can slip a 20 thousandth feeler gauge between the upper and lower one or both are out of spec.
Can't tell you what the armorer standard is for replacement, but I can tell you that I've seen a Marine use a rifle which could fit a credit card in the space between the two halves still earn an expert award on the KD course of fire. To me, its a non issue as 999 out of 1000 times the people who worry about these things worry even though it is in spec.
Any ideas as to what this could be or if their is a fix?
I've had some builds that were tighter than others. Shooting & disassembly of the two halves will naturally loosen things up over time. I wouldn't worry about it.
I have also read here somewhere that there is an actual spec for what is "too loose" however, it escapes me right now.
Army TM9-1005-319-23&P
Air Force TO 11W3-5-5-42
Page 3-68
d. Test
1. With the upper receiver attached to the lower receiver, and the pivot pin and takedown pins in place, perform the following test:
(a) Apply hand pressure to push the upper receiver as far to one side as possible.
(b) Attempt to insert a 0.020" inch thickness gauge between the pivot pin lugs of the upper and lower receivers.
(c) If the thickness gauge penetrates to the pivot pin at all accessible locations, repair by replacement of the upper receiver (see (b) below) or replacement of rifle is required.
2. If the rifle fails the above test, remove the upper receiver and install a "NEW" upper receiver and perform the test again.
3. If the rifle now passes the above test, it shall be considered serviceable and continue in use.
4. If the rifle fails the test with a new upper receiver, this failure shall be considered a shortcoming. This shortcoming requires action to obtain a replacement rifle. Once a replacement has been received, evacuate the original rifle to depot for overhaul.
This leaves a lot of room for loose upper to lower receiver fit on a functional rifle or carbine.
justin_247
07-31-14, 10:33
Ok, so I just finished my first build with a stripped spikes upper and lower. This entire gun was pieced together. I am having a serious issue with the tolerances being very tight. I mean in regards to the last 1/16 of space between the upper and lower. It's so tight it literally snaps into place. It almost seems like there is maybe an issue with the back of the bolt carrier and the buffer? The take down pins are also ridiculously tight, but I have read this is not uncommon(its luck of the draw). However I have never handled an AR with the upper and lower so tight it literally snaps together. Any ideas as to what this could be or if their is a fix?
If it doesn't loosen up enough to pop out the takedown pins by hand after a few dozen times, you can take a file and *gently* sand a slight amount on the bottom part of the hole in the takedown lugs. Be very, very gentle and careful. But, whatever you do...
DO NOT PERFORM ANY SANDING ON THE LOWER RECEIVER.
Honestly, it'll probably loosen up with time. But, in general, I prefer to be able to pop the takedown pins by hand.
WaterChunker
07-31-14, 10:46
Well I did some playing around and what's going on is the back of the bcg is pushing against the buffer. So the last initial squeeze i am making that I have described as "snapping" into place is the back of the bcg pressing against the buffer.
justin_247
07-31-14, 10:49
Well I did some playing around and what's going on is the back of the bcg is pushing against the buffer. So the last initial squeeze i am making that I have described as "snapping" into place is the back of the bcg pressing against the buffer.
That's perfectly normal.
WaterChunker
07-31-14, 10:56
Yea, I have just never had a gun this tight before I guess, so it took me by surprise. Now I did forget to mention that I cerakoted it,..but I highly doubt it would drive the tolerance off this much. All assembly was a little tighter then normal but nothing drastic. First time I have dealt with this lol. I'll find out how she shoots next weekend.
The back of the BCG should glide the buffer back with NO snapping action AT ALL. Your RE and/or Buffer detent hole may be out of spec... The RE may also be threaded in a full rotation TOO MUCH because of an out of spec buffer detent hole.
This "some are tight... no big deal" crap is too quick of an assumption with all the garbage being rushed to market lately.
The wear spec of .020 seems like alot. I guess I just need to stop thinking about it.
The play always seems more noticeable to me when using a vertical grip.
WaterChunker
07-31-14, 12:05
The back of the BCG should glide the buffer back with NO snapping action AT ALL. Your RE and/or Buffer detent hole may be out of spec... The RE may also be threaded in a full rotation TOO MUCH because of an out of spec buffer detent hole.
This "some are tight... no big deal" crap is too quick of an assumption with all the garbage being rushed to market lately.
I hope this is not the case as its a registered lower, i have also notice if i squeeze and upper and lower together i can push out the take down pins by hand without the need for tools. It most definitely is not "gliding" on across the buffer.
Can you get a pic of the buffer retaining area from above and to the side of the lower with the upper removed... or at least with the receivers opened up???
(the back of the bolt carrier should be flush with the back of the upper receiver. The Buffer face should stick forward a little. Then the back of the Carrier (when closing the receivers) should push the buffer back to that it's not touching the buffer retainer when the actions are closed.
If the back of the carrier is hitting the TOP of the buffer at all when you close the receivers, there are problems. It should only contact the face/front of the buffer... nowhere else.
WaterChunker
07-31-14, 12:50
Can you get a pic of the buffer retaining area from above and to the side of the lower with the upper removed... or at least with the receivers opened up???
(the back of the bolt carrier should be flush with the back of the upper receiver. The Buffer face should stick forward a little. Then the back of the Carrier (when closing the receivers) should push the buffer back to that it's not touching the buffer retainer when the actions are closed.
If the back of the carrier is hitting the TOP of the buffer at all when you close the receivers, there are problems. It should only contact the face/front of the buffer... nowhere else.
Yea I will post some up in a sec
WaterChunker
07-31-14, 13:12
http://i732.photobucket.com/albums/ww325/grinchx04/sbr_zpsb0a849a7.jpg (http://s732.photobucket.com/user/grinchx04/media/sbr_zpsb0a849a7.jpg.html)
http://i732.photobucket.com/albums/ww325/grinchx04/sbr2_zps72181146.jpg (http://s732.photobucket.com/user/grinchx04/media/sbr2_zps72181146.jpg.html)
http://i732.photobucket.com/albums/ww325/grinchx04/sbr3_zps22f5e6bc.jpg (http://s732.photobucket.com/user/grinchx04/media/sbr3_zps22f5e6bc.jpg.html)
http://i732.photobucket.com/albums/ww325/grinchx04/sbr4_zps698574ad.jpg (http://s732.photobucket.com/user/grinchx04/media/sbr4_zps698574ad.jpg.html)
http://i732.photobucket.com/albums/ww325/grinchx04/sbr5_zps72f26e21.jpg (http://s732.photobucket.com/user/grinchx04/media/sbr5_zps72f26e21.jpg.html)
http://i732.photobucket.com/albums/ww325/grinchx04/sbr6_zpsa599e973.jpg (http://s732.photobucket.com/user/grinchx04/media/sbr6_zpsa599e973.jpg.html)
this is as far as it goes without having to use a a hulk grip to seat them together at which point they "snap" together.
http://i732.photobucket.com/albums/ww325/grinchx04/sbr7_zps9cfe95f7.jpg (http://s732.photobucket.com/user/grinchx04/media/sbr7_zps9cfe95f7.jpg.html)
Those are filtered at my work... maybe some of the other guys can offer an opinion. I'll look at them from home in a few hours.
Looks like a non issue to me but I prefer mine to have a snug fit.
Are you having issues when shooting?
WaterChunker
07-31-14, 14:05
Looks like a non issue to me but I prefer mine to have a snug fit.
Are you having issues when shooting?
Function test shows it to be good. I won't get a chance to fire it until next Saturday. (and yes before someone says it, rear sight is on backwards, i just threw it on and locked it with my finger to keep it together till i get ahold of my appropriate size flathead)
Get out and shoot it and it all functions well enjoy your new build. All looks fine from me from the pics. Also don't sweat the snug fit since its not like you'd need to do a speedy break down to change a trigger pack in the middle of a firefight. I have one rifle that has a very snug fit and I simply push in on the pin enough to expose enough of the other side to lift with the rim from a cartridge. Easy peasy.
No. Tight pins are one thing. If the receivers WON'T shut smoothly, there's something wrong. If the RE isn't in the right spot or the Buffer detent isn't positioned properly, there can be damage to the components of the gun.
It could also be the upper receiver's rear lug that is too wide for the inside of the lower. I'll look at the pics, and then take a pic of the buffer engagement when I get home in a while.
Another thought on this....
When you close your receivers. Are you leaving the hammer cocked? Depending on a TYPE 1 or TYPE 2 hammer... If you leave the hammer down, the firing pin can float back and hit the decocked hammer and cause hang up.
You need to either close the action on a cocked hammer, or point the muzzle down so the firing pin floats forward.
Dumb question.... Have you tried closing it without the BCG, buffer and of course spring out of the gun??
Seems a little tight on the buffer. Just going by the pics it's a lower issue and not an upper. You got some excellent advise above. Use it and even if its registered, call spikes. See if they can fix it with the same serial number. If not, well you will just have to wait again on the damn ATF. These things can be a pain but its what we have.
ETA: looking at pics, it doesn't look that bad. Then again I'm not pulling the pins. Your call.
WaterChunker
07-31-14, 15:31
Dumb question.... Have you tried closing it without the BCG, buffer and of course spring out of the gun??
Yes, when I take out the bcg its closes without any problems. It's just the last 1/16 that seems to get tight,..once mated everything is functioning properly.
WaterChunker
07-31-14, 15:52
Seems a little tight on the buffer. Just going by the pics it's a lower issue and not an upper. You got some excellent advise above. Use it and even if its registered, call spikes. See if they can fix it with the same serial number. If not, well you will just have to wait again on the damn ATF. These things can be a pain but its what we have.
ETA: looking at pics, it doesn't look that bad. Then again I'm not pulling the pins. Your call.
As far as squeezing the pins, I have it down now. I just squeeze the upper and lower together, push the take down pin out and while still squeezing the upper and lower together i pull the take down pin out on the hubbed side.
Eurodriver
07-31-14, 16:54
This "some are tight... no big deal" crap is too quick of an assumption with all the garbage being rushed to market lately.
Just to clarify, if you were speaking to me, I was talking about looseness only.
this is as far as it goes without having to use a a hulk grip to seat them together at which point they "snap" together.
Ok... I see the pics now. That is odd. You're beyond all the buffer/RE type issues so that's good. Now if the receivers close fine with the BCG out, I'm thinking the hammer is hitting something. Have you tried closing them with the BCG in and the hammer COCKED???
WaterChunker
07-31-14, 17:14
Yea, same issue cocked and un-cocked.
WaterChunker
07-31-14, 17:31
Ok, with the bolt carrier out and the buffer in it mates no problem. With the bolt carrier in and the buffer out its hard to close and "snaps" together with applied force.
Try putting a thin film of grease on the face of the buffer and see if that helps.
WaterChunker
07-31-14, 18:15
Ok I put in a cheap semi auto bcg and it mated with zero issues. I put my bcm fa bcg back in it and back to the mating issue. I'm over here at my fathers house, I took the BCM BCG out of his rifle and put it into mine and it mated with zero issues. I took my BCM BCG and put it into his rifles and it closed with zero issues. Very weird.
Yes, when I take out the bcg its closes without any problems. It's just the last 1/16 that seems to get tight,..once mated everything is functioning properly.
Edit: saw your recent posts.
Do you have a friend who has another FA BCG you can try on yours?
So the only issue is with your BCM BCG? Try swapping bolts only. I'm wondering if something is going on with the bolt lugs.
WaterChunker
08-01-14, 11:08
i may have to try this actually.
The standard per the TM is a .020 feeler gage inserted to the front and rear lugs at multiple points. We rarely checked this and if it failed you tried a new upper.
Can't tell you what the armorer standard is for replacement, but I can tell you that I've seen a Marine use a rifle which could fit a credit card in the space between the two halves still earn an expert award on the KD course of fire. To me, its a non issue as 999 out of 1000 times the people who worry about these things worry even though it is in spec.
Check to see if the carrier is contacting the buffer retainer pin as your closing the two halves
Check to see if the gas tube is hanging up the gas key, preventing the BCG from easily closing completely
I saw some of this recently and we determined that the upper was out of spec.
Sorta off topic, but found a way to tighten upper and lower. Already use accuwedge, and still get some slop, but after starring at the upper fora few minutes a light bulb went off I'm my head. I put a thin o ring on the front take down pin lug. Nice and solid now.
I'm sure it's been done before, but just wanted to add to the loose upper lower discussion
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