Colt large pin upper to modern lower--?the bushing's eccentric??

Alberic · Mar 24, 2017

Hi guys,

So I have a friend with an old (80's vintage) Colt large pin upper, wants to mate it to a modern lower.
OK fine, go buy the bushing. Done.
It's eccentric. The small pin thru hole is off center. Is it supposed to be? When put together, the upper rattles on the lower like nothing I've ever seen. All non-custom ARs rattle a little, but this is insane. (It's off by ?.020"? Maybe more.)

So my questions are these:
A) is the blinking thing supposed to be eccentric?
B) if so, how do you time the bushing's rotation? (What's the goal setpoint?)

Or (C) do I just machine one that's actually concentric, and start over?

Thanks,
Brian

Colt45 · Mar 24, 2017

Colt receivers from that vintage had the pivot pin on a different axis from the Mil standard.
Install the bushing and let it "slip" until the lower receiver pivot pin aligns. Be carefull or you can crack the receivers and/or oval out the holes.

Alberic · Mar 25, 2017

Thanks, I figured they'd probably banjaxed the locations, just to make it that much harder. Gurr...
Cynics wouldn't be so annoying if they weren't right so often.

So let it swivel home to minimum gap (solid lockup), then loc-tite it. Got it.
I've tried yanking the barrel, but the nut's on there so hard I was worried about trashing the upper if I kept yanking on it. Even with Kroil.

Thanks,
Brian

ccw1911 · Mar 29, 2017

There is also an pin made with an eccentric to adapt the two but the bushing is much handier once you get it in the right place. The way I've always handled too tight barrel nuts is to lock the barrel in the vice using aluminum vice jaws that fit the barrel closely. Someone is making a steel "arbor" that sticks thru the upper and engages the barrel extension now, kinda pricey and does the same thing as my old way. I have a clam shell upper holder and the jig the uses the two take down pin holes to hold the upper but both seem a little shakey when you have a barrel nut that has been way over torqued.

72bwhite · Mar 29, 2017

you can just cut the barrel nut off

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Colt large pin upper to modern lower--?the bushing's eccentric??

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