slightly different gap in upper and lower receiver question.

[msedward43]

Ok, I have a question. I have 2 spikes lowers that fit like normal with every upper I have, except there is a slight gap between the back of the upper and the part of the lower that curves upwards and becomes the buffer tube support area. ESTIMATED (my calipers aren't the greatest) 0.013-0.015 in gap. I noticed it only because when shotgunning the lower and upper together like normal I noticed it didn't seen to have as much friction between the bolt edge scraping along the buffer as it pushes it back. That's when I checked the gap and noticed I could see light between it and fit 3 post it notes stacked together between the gap (add a 4th and it becomes impossible). Now I'm not too caring about horizontal gaps, but that vertical gap between the bolt and the buffer area just makes me wonder if with that gap the bolt isn't pushing the buffer back far enough and could possibly beat the retaining pin up, but I wasn't sure if that gap was worth me bitching about to Spikes (especially since both lowers are fully built up with SOLGW and BCM parts kits with LaRue triggers) or if that amount of gap was just a "who cares" sort of thing. Like I said, horizontal gap I wouldn't worry about, but that vertical gap just makes me worry about the buffer not being pushed back far enough (I can see it being pushed back a hair from the retaining pin as the upper closes if you watch carefully)

https://i.imgur.com/C713U4J.jpg

https://i.imgur.com/zIUMNOc.jpg

[26 Inf]

(I can see it being pushed back a hair from the retaining pin as the upper closes if you watch carefully)

If the end of the BC is pushing the buffer back off the buffer retaining pin you should be good to go.

Other than it being a PITA to manage the buffer as you open and close the upper, you don't really need the buffer retaining pin.

[msedward43]

If the end of the BC is pushing the buffer back off the buffer retaining pin you should be good to go.

Other than it being a PITA to manage the buffer as you open and close the upper, you don't really need the buffer retaining pin.

got it, thanks. I believe it's lifting it off the pin just slightly guaging by inspecting it clamshelled closed with the charging handle off and out of the way, but wasn't sure by how much... I just wasn't sure if that much (or little) gap was worth stripping the lowers down and sending them back to Spikes or whether it was just my OCD working.

[Stickman]

I would first have concern over what parts were indexed during the machining operations. If the end is off, the other dimensions are most likely off as well. More than simple a matter of tolerance stacking, this may be an issue of dominoes, where you don't know what is wearing or causing hidden unknown games inside. While it obviously isn't going to have affect on headspace, it clearly isn't correct.

The above being said, I agree with 26 for at least the immediacy of the weapon usage.

[Stickman]

If the end of the BC is pushing the buffer back off the buffer retaining pin you should be good to go.

Other than it being a PITA to manage the buffer as you open and close the upper, you don't really need the buffer retaining pin.

That was my first thought, but the more I kicked the idea around, the more the overall issue bugged me. Neither of us would find it acceptable if that were the 3rd hole on a lower we just forked good money out for.. .. My concern is that there are hidden variables, and no matter how you slice it, none of us are going to say "that is grade A ok".

ETA- The more I look at that pic, the more I wonder what area is actually out of tolerance, the upper, the lower.... both? I still don't see it as creating any short term problems though.

[msedward43]

That was my first thought, but the more I kicked the idea around, the more the overall issue bugged me. Neither of us would find it acceptable if that were the 3rd hole on a lower we just forked good money out for.. .. My concern is that there are hidden variables, and no matter how you slice it, none of us are going to say "that is grade A ok".

lol, yeah that's been my concern too. hidden variables. Ironically I have TWO Spikes lowers with the forged trigger guard and they BOTH do this identically (bought a year apart). I guess the OCD in me is going to have to swallow the loss and return the lowers to spikes (and bang out or lose out on all the internal guts of the lower that can't be taken off easily)

upon inspecting them closely it SEEMS that the pin holes for the takedown and pivot pins were drilled a bit off, as the lower will lock back into position with no gap if the pins are pushed out, but then they pins holes from the lower to the upper are misaligned (it seems they were drilled 0.015-0.018 in too far forward)

oh, and this gap happens with every upper I have (bcm, Sionics, SOLGW, even cheapo PSA uppers or Aero uppers)

[26 Inf]

lol, yeah that's been my concern too. hidden variables. Ironically I have TWO Spikes lowers with the forged trigger guard and they BOTH do this identically (bought a year apart). I guess the OCD in me is going to have to swallow the loss and return the lowers to spikes (and bang out or lose out on all the internal guts of the lower that can't be taken off easily)

upon inspecting them closely it SEEMS that the pin holes for the takedown and pivot pins were drilled a bit off, as the lower will lock back into position with no gap if the pins are pushed out, but then they pins holes from the lower to the upper are misaligned (it seems they were drilled 0.015-0.018 in too far forward)

oh, and this gap happens with every upper I have (bcm, Sionics, SOLGW, even cheapo PSA uppers or Aero uppers)

Stickman makes good points. From what you describe, you've narrowed it to the lowers and as Stickman noted, hard telling what issues might crop up with tolerances.

It sure sounds as if you've had the first one long enough to shoot it some. Any problems with function?

P.S. Now you've made me paranoid - it will take me most of the afternoon to look at my rifles and play swap the uppers.

[msedward43]

Stickman makes good points. From what you describe, you've narrowed it to the lowers and as Stickman noted, hard telling what issues might crop up with tolerances.

It sure sounds as if you've had the first one long enough to shoot it some. Any problems with function?

P.S. Now you've made me paranoid - it will take me most of the afternoon to look at my rifles and play swap the uppers.

the first one has been shot a bit (I didn't get to shoot as much as I wanted this year since I had to buy a new car, and that sucked fun money away at a rapid pace) and FUNCTION seems fine... it's just showing wear on the buffer pin and buffer a bit (only 200 rounds)... the second one was just mated to the an upper and that's when I noticed it was even WORSE, having no friction at all on the buffer and the bolt carrier when I clam shelled them shut. Then I went OCD and checked EVERY upper and lower combination on ever other lower (I have 18 lowers and 16 uppers atm)... NONE of them but these two spikes lowers show any issues like this and as Stickman said. it's not SUPPOSED to have that big of a gap and I'm nervous about what else is under the surface...

I talked to John at Spikes and he offered to exchange them with new lowers and test fit uppers on them to make sure this doesn't happen a second time.

[lysander]

Ok, I have a question. I have 2 spikes lowers that fit like normal with every upper I have, except there is a slight gap between the back of the upper and the part of the lower that curves upwards and becomes the buffer tube support area. ESTIMATED (my calipers aren't the greatest) 0.013-0.015 in gap. I noticed it only because when shotgunning the lower and upper together like normal I noticed it didn't seen to have as much friction between the bolt edge scraping along the buffer as it pushes it back. That's when I checked the gap and noticed I could see light between it and fit 3 post it notes stacked together between the gap (add a 4th and it becomes impossible). Now I'm not too caring about horizontal gaps, but that vertical gap between the bolt and the buffer area just makes me wonder if with that gap the bolt isn't pushing the buffer back far enough and could possibly beat the retaining pin up, but I wasn't sure if that gap was worth me bitching about to Spikes (especially since both lowers are fully built up with SOLGW and BCM parts kits with LaRue triggers) or if that amount of gap was just a "who cares" sort of thing. Like I said, horizontal gap I wouldn't worry about, but that vertical gap just makes me worry about the buffer not being pushed back far enough (I can see it being pushed back a hair from the retaining pin as the upper closes if you watch carefully)

https://i.imgur.com/C713U4J.jpg

https://i.imgur.com/zIUMNOc.jpg

The tolerances allow for a gap as small as 0.001" and as large as 0.013".

I wouldn't get excited about an extra 0.002" . . . .

[msedward43]

update: Spikes took them back and swapped them out for two more lowers. 1 fits a bit tighter, and the other distinctly tighter when clamshelling them shut. One is still a bit wider of a gap than my other lowers, but seems to push the buffer back well enough. According to the rep they hand fitted several uppers onto multiple lowers before they chose the tightest fitting lowers. I guess Spikes lowers are a little looser than the Aero and Sionics and Mega lowers I'm used to.

to break it down

1. The lower that was showing signs of this (the oldest) and wasn't AS bad is about the same with the new lower, maybe a little better. But then again it was locking onto the buffer pretty decently to begin with (and I noticed fits tighter with different uppers to, so there's some variances there and I can pair it with a tight fitting upper to make up for some of it)

2. the one that I had the most worries about and showed the pictures of that had almost NO friction between the back of the bolt and buffer is actually now the best. It's the tightest fit of the two and is distinctly better.

not super tight fitment back there, but seems reasonable now compared to how they were