Home Forums Hunting & Shooting AR and Tactical Rifles Large Pin lower/ Small Pin upper?

Has anyone ever tried to convert a small pin upper by drilling out the hole to the large pin diameter? I know the holes are are offset but have a friend who is a machinist and it seems like the trick would be to devise a holding fixture to line everything up.

I picked up a small pin Colt flattop upper to put on my large pin Colt service rifle lower and am tired of messing with the offset pin adapter.

Mine's just the opposite, i.e., a Colt lower and a Bushmaster upper. I just use the pin adapter to join them.

The whole setup is offset so drilling would be a bad idea. There are conversion pins out there but the ones I liked were from Colt and had a flat side that prevented the pin from rotating.

Aren't pins available for all upper lower combos.... I sure as hell would not drill...

My experience with the offset pin so far is that it tends to rotate and binds up if you try to crack open the lower.

I would certainly not try to drill something like this myself, but my machinist buddy is an avid long range competitor who has machined a couple of pre64 70 action clones from blocks of steel, so with his skills it might be doable.

Seems like more effort to me than it would be worth when a stripped lower can be had for around 80.00.

That's why I recommended the Colt pin with the flat on the side.

It's your money.

True, but the large pin lower is set up with a Milazzo two stage trigger I really like, and the upper is the matching dark grey vs more common black.

I'll likely just suffer with the offset pin I have or look for the Colt one TWR mentioned in his post.

If your buddy can machine a model 70 I'd go talk to him about it, rather than us!

I might be somewhat concerned about needing to re-anodize the pin holes for wear but other than that I'd have him look at it. I suspect he might be able to figure it out plus machine new pins to boot.

[quote=rost495]If your buddy can machine a model 70 I'd go talk to him about it, rather than us!

Point taken, but I don't underestimate the knowledge base available here...

I had thought about the anodizing issue, but it doesn't seem like there would be much to cause wear.

He'll be looking the lower and uppers over next weekend, we talked about it a little yesterday when I was giving him a 1911 mainspring housing he is blueing this week.

Thanks everyone for your input.

You would think not much wear there, but those pins deal with recoil a bit every shot... no anodize is softer than anodize. Little bits of hardening sometimes make a difference.

Shot the modified upper today and all seems well. We'll see how the long term durability is. . .

We set it up in my friends turret mill Wednesday evening after fabrication of a piece of scrap bar stock to create a piece that went in the Mag well and into the charger handle slot.With the upper and lower clamped and leveled, a boring bar was centered on the front lower hole within .001" and the upper pin bored out a little at a time until the factory pin was a snug fit. The modified upper to lower fit is very snug without any need for an accuwedge at the rear. About 1.5 hours of set up and 5 minutes of milling.