My colt lower has a large hole pivot pin and Im sort of a noobie to AR's so I accidently bought a small hole A2 upper. I bought a cam type pivot pin from sherluk but I wasnt happy with it. It would loosen on me while shooting. So I broke down today and decided to just fix it.

I started out removing all the components of the upper and then sandblasting with fine glass bead. The existing finish wasnt exactly great in the first place. Then I turned down a .255 aluminum pin so there would be a couple thousandths press fit. Then I heated up the upper a tad and pressed the aluminum pin in. Then I tig welded the .255 pin to the hole filling in the chamfers. I also added material the bottom of the lug(newby term).


Then I thought of a way to hold the upper in place on the mill by taking a 1" aluminum bar and tapping two 1/4-20 holes in it to accommodate my angle plate. The 1" bar fit perfectly in the upper and provided ample clamping force for milling the lug down.


Then I mounted the upper on the angle plate and indicated the bore of the upper straight.


I measured the gap on the lower where the upper lug rests with gauge blocks. .498 felt snug so I shot for that number on the mill knowing I could polish it later to suit. I put the work on the mill and milled down the aluminum weld until I barely cut into the existing material. Then I went ahead and side milled the bottom of the lug leaving about .050 more material than it originally had.


Then I flipped the upper over to mill down the last remaining side. After milling I deburred the new lug and checked upper to lower fit. Then while holding the two together firmly and having the rear push pin in-place I took a transfer punch and made a center mark for the hole. There are better more accurate ways to locate the hole but this is the fastest way and I got lucky with it

Then I went to the drill press and drilled out the hole. After drilling I checked fit and it was slightly tight so I polished the hole until I got the desired fit. Then I radiused the exterior of the lug to allow the upper to fully open with the pivot pin in. I used a belt sander to radius the edge until the upper would fold over without rubbing the lower. The end result was very pleasing as I will no longer have to screw around with that cam pin.

IT took me about 3 hours to take my complete upper apart and convert it to a large hole. I might have saved some time if I hadnt used red locktight on my free float tube

The next step is to put it back together and refinish it. IM probably going to experiment though with some paint rather than send it out. Im also going to make a new gas block and flute the barrel.

Sorry for the crappy pics fellas.

WOW !

That's great.
I can't believe I never thought of that.
That's an excellent idea.

Wanna do one for me?

I'd love one just like that, also a flat-top upper.

Thanks Derek. Thinking about it now I think I should have measured the hole spread on my lower and just picked up rear hole on the upper and plunged the front hole in with an end mill.

I should say thanks again to all the guys who gave me pointers on tigging aluminum in one of my older threads[beer]

Thanks for the very interesting thread and photos.  If/When you decide to do work on the side, I'm sure there will be lots of customers lining up.   Including me!

Good ol' American ingenuity strikes again. That thing looks sweet. From an amateur machinist, I tip my hat, when you get that upper refinished, you won't be able to see the change. Great.

About the finish, check out these links for info on how to do it yourself.

[url]http://www.caswellplating.com/kits/aluminum.htm?source=overture[/url]

[url]http://www.turborick.com/anodize2.html[/url]

The fact is, I dont think I could do one cheap enough. You can buy a stripped upper for like $100.

Thanks for the link Tac40. I thought AR's were parkerized or something and not anodized. If anodizing would look good Ive got a friend that works at another machine shop who works with a guy that does anodizing ay home. He told me he could get annodizing free.

Good job!

Great job!!!
I have always wanted to do that since the I wanted to convert my uppers to flattops!! My only question after the tig welding, do you need to re-heat treat the upper??

Thanks,
Stuart

Maybe, but not an A3. Colt made a few, but they are very hard to come by. Daniel Defense makes one now and sells it for $225 with forward assist and ejection port cover.

[img]http://www.biggerhammer.net/ar15/uppers/a1_vs_a2_frontlug.jpg[/img]

[img]http://www.biggerhammer.net/ar15/uppers/aruppercomparison.jpg[/img]

A small portion of where I welded might have softened but most of it was cool enough to carry by the handle after I had welded it.

Im thinking about milling off the carry handle and making a larger handle that will connect to the 4 sided weaver rail free float tube(kinda like the G36). What do you guys think?

[img]http://www.boomspeed.com/kisara/KisaraAR36.jpg[/img]

Hell yes Kisara! Thats exacly what I had in mind. Thanks for the informative pics M4Madness.