Is It Possible To Convert Small Pin Ar15 Upper To Large Pin???

J

jschance

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Has anyone on here ever attempted to convert a small pin upper receiver to a large pin configuration? I know that the pins are not only different dimensions but slightly different locations. It would seem that with the correct machining set up, it would be possible to rebore the pin hole to accommodate the larger pin diameter and different location.

I've got a 'pre-ban' Colt lower that I need to build an upper for. I know there are offset pins available, but that seems like an awkward way to fix the problem.

Thanks.
 
Do you have a mill? Preferably one with DROs? If so, you should be able to get the lower set so you could get X-Y coordinates off the centers of the holes, then replace the lower with the upper. Use the proper sized center cutting end mill and re-occupy the hole positions to make the new holes - a drill bit will just wander around and mess up the upper.
 
Can you school me on this?
I assume you mean the front takedown pin? Are the old Colt takedown pins larger diameter? I knew some of the older Colts had larger trigger pins.
Defens- I'm confused about what you said. How would you replace the lower with the upper in the mill vice and get the holes in the right place. Especially if they are offset?
R
 
If I know what you are thinking about, I believe you will find there is not really enough material in the lug to take the stress of the incidental drop opening of the breakdown. You can more or less run a reamer through it, but it will be too weak and probably crack. I would proceed with caution.
 
The Colt SP-1's had a larger diameter front pin in a different location to make it a "civilian" model. In order to put an SP1 upper on a new lower I had to make an offset bushing for the upper. I take it your lower is an SP-1 with the large diameter pin hole?
 
Yes, I've got a mill but it does not have DRO's (It's an old Enco Bench Mill, but it serves my purposes).

No, you can't just ream the hole out because the small pin upper receiver has a different center than the large pin lower, if you tried to ream it out you'd end up with a oversized hole with a lot of extra play. I figured it would be an exercise where you would use an end mill indicated into the correct dimensional location to open up the small hinge pin hole to accommodate the large lower pivot pin.
 
Assuming there is enough material on the upper lug could you:
mate the upper and lower and clamp the assembly in the mill. Center the mill on the pivot pin hole in the lower. Drill through the upper lug with the appropriate end mill without enlarging the hole in the lower.
Seems pretty straightforward but maybe I don't understand all the issues.
R
 
As long as the holes are in the same location (Or close), I see no reason why you couldn't make 4 bushings. OD to fit in the lower and ID for the smaller pins to fit through. Make them flanged to keep them from falling in. If the holes are in a slightly different place, make the bushings eccentric (offset it in the 4 jaw) then rotate the bushing toll everything fits. Just an idea, take it for what it's worth! (Not much :))
 
Why not just get a new upper there cheap enough?
 
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