Ar Top End Interchangeability

[ranger]

I recently purchased a used AR15 top end for my current Olympic Arms AR15. The "new" AR15 top end will not fit on my current Olympic Arms AR15 bottom end. The "new" AR15 top end will physically fit on the bottom end but I cannot get both pins into the housings. I can get one pin in but I cannot push the second pin in and I do not want to force it in. Any comments or advice welcomed.

Thanks, Scott

[uscbigdawg]

Offset pins are available form Brownell's. Sounds like you have a Colt upper and Oly lower.

Rich

[stingerjg]

If you could give us a mesurement of your pins on the lower and inside dia. of the holes on the upper it would help us point you in the right direction. Also what pin wont go in, the front or the rear?

You may need an offset bushing or reciever adapter. Or your pins could just be really tight. When i do builds on new recevers and/or uppers, i use a brass punch and a hammer to remove the pins for at least 100-200 rds.

bushing/adapter link

:edited to add link:

Edited December 3, 2006 by stingerjg

[ranger]

OK - I'll get out the micrometer tomorrow and measure the pins, etc. Thanks for the feedback.

[George]

Again, which pin won't go through? If it's the front one, you most likely have an offset and need an offset pin. Not all lowers/uppers are a fit without an adapter pin.

[ranger]

The pins measure .249. I can get the front pivot pin in - I cannot get the take down pin to start in the replacement top end.

[George]

If the front pin fits (the front one is the "Pivot" pin, the rear is the "Catch/Latch, or Takedown" pin), then the rear should also. It may need to be gently tapped in with a plastic mallet/punch.

[Patrick Sweeney]

Or, you may have to admit defeat and not use your new upper. Yes, I know it sucks, but what with all the makers machining uppers and lowers, there are going to be the occasional mis-matches that just won't fit.

It may be the new upper that is "off" and it may be your lower. In any case, you may be able to talk to whoever built/sold it and see if you can try various others until you find one that fits on your lower.

I gave up a long time ago trying to have a "one lower and X uppers" kind of AR. If I think I really, really need a different upper I just build a lower to go with it.

[Dan Sierpina]

Is the upper an Olympic also? One thing I had issues with when putting an Olympic upper on a Colt lower, was the radius at the rear of the upper needed a little filing before the takedown pin would go through the slot. You might want to check that.