My Aero M4E1 upper doesn't like my Larue Tactical billet lower....

[pag23]

So bought a complete Aero upper with the Atlas MLok... M4E1 and had a local LGS assemble a stripped billet Larue lower...

Now these parts aren't going together at all in regards of a build, I was just trying out the lower build on the upper.. I am awaiting a Larue complete Stealth upper at some point.. I tried to put the Aero upper and Larue lower together and it wouldn't work.. the LT side rear takedown ear of the Aero upper is catching above the rear take down pin and putting a light scratch inside the back of the lower near the takedown hole. Now if I just put it together and try to put the pins in, the rear will go in but not the front, it is off a slight bit.

I tried to mix and match different complete uppers and complete lowers to see which one is the problem:

- BCM upper with Larue lower- no issues
- Aero upper with BCM lower- no issues but a little tight putting together at the front take down when trying to assemble
- Larue lower with PSA upper- no issues
- Aero upper with PSA lower - no issues
- Aero upper with stripped PSA lower-- no issues

Something is out of spec, I just don't know what it is.. I don't have photos but I can get them tomorrow


I mean these parts aren't going together as complete guns... but WTF?

[ViperTwoSix]

Since both the upper and lower work independently with other uppers and lowers, it could be a case of tolerance stacking. The takedown ear of the Aero M4E1 upper and the wall of the Larue lower could both be on the high end of the tolerance spectrum making them ever so slightly too tight in that spot for smooth compatibility. Or, you could have a similar tolerance stacking issue at the front takedown (hence the tightness with the BCM lower).

[Gatorgrizz27]

I agree it’s a tolerance stacking issue. I had a similar one lately. PSA lower, Rainier upper, rear pin basically would have had to be beaten into place with a hammer. I never fully pushed it through. PSA lower, PSA upper, no issues. Rainier upper, Runner Runner lower, no issues.

If you’ve got a Larue upper coming, I wouldn’t sweat it. No point in filing/forcing/ruining something to run it temporarily.

[mpom]

Is there a nylon tipped screw in the lower that may not be backed out sufficiently, the kind that is there to eliminate play between upper and lower?

Mark

[Stickman]

As far as I know, Larue has never manufactured their own lower. However, regardless of who makes the upper or lower, tolerance stacking is something that happens at times. I would agree to check the tension screw, after that, you can mark and check the interference or just call LT if it doesn't fit their upper.

[pag23]

I agree it’s a tolerance stacking issue. I had a similar one lately. PSA lower, Rainier upper, rear pin basically would have had to be beaten into place with a hammer. I never fully pushed it through. PSA lower, PSA upper, no issues. Rainier upper, Runner Runner lower, no issues.

If you’ve got a Larue upper coming, I wouldn’t sweat it. No point in filing/forcing/ruining something to run it temporarily.

yup I was almost to that point with the hammer smack..lol

It was weird that when mating the Aero upper to the Larue lower, and then pushing the rear pin in on assembly, the front one doesn't want to go in, it binds up right at the take down hole...

[pag23]

As far as I know, Larue has never manufactured their own lower. However, regardless of who makes the upper or lower, tolerance stacking is something that happens at times. I would agree to check the tension screw, after that, you can mark and check the interference or just call LT if it doesn't fit their upper.

There is no take down tension screw in the lower.. It is a Larue LT-15 billet. Unless I am missing something...

Here is a photo

64981

I appreciate the help everyone and once the upper arrives and if there is an issue I will call Larue.

There is a little bit of scratching from the Aero upper to the inside of the Larue lower from my multiple attempts to figure this out..

[SeriousStudent]

I have had a similar thing with a lower from a very reputable manufacturer. It worked with one of their uppers, but no other upper I own.

It was a great carbine, shot well, and is still in my safe.