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Posted: 2/6/2006 4:47:44 AM EDT
| I have a rookie question. I have a pre-ban 6450 with a serial TA0. The carbine was from the 80's. It has a screw instead of a push pin in the front pivot. Can someone tell me if this is an off set? Did Colt make it this way for a reason? Are they supposed to be small or large pins? I am turning it into a SBR and need to know if a RO635 upper will fit on the lower with out any modifications? |
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You have a large hole lower receiver with a screw type takedown foerward pin and a small hole upper? Yes, there is a screw that has large diameter sholders for the lower receiver with a reduced center section for small hole upper receivers.( they are sort of an eccentric cam type design). Pretty sure Bushmaster has them, and so do most other part suppliers. Mine is Colt but it came with an upper I purchased a long time ago. They make adaptors that go the other way also.( small hole lower receivers and large hole uppers) Guess this sort of answers your question without pictures. |
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For reference see here: www.biggerhammer.net/ar15/uppers/ The easiest way to tell is simply to open your eyes and look. IF the front pivot pin hole is a LARGER diameter than the rear takedown pin hole, you have an old "large-pivot-pin" Colt. If it's the same size diameter (front pivot pin & rear takedown pin), then you have a small or standard-pivot-pin Colt which will mate with any current production upper. Mike ps - BTW many Colts (my CR6724 included) came with a screwed standard size (.250) front pivot pin, which you can replace with a beaded pivot pin sold by DPMS, if it worries ya. Personally I have no problem w/ the screwed pivot pin as ya don't remove it that often, or altleast I don't. 
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I guess both my upper and lower are large pivot pin. Is this normal for a pre-ban 6450 (1980's) ? Do I need an offset to add a small pivot pin upper and who makes one? Will this affect performance and function? What is my best course of action? I already purchased a RO635 upper to convert it into a SBR.    
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Looks to me like your upper and lower are large hole. As I said above, I think Bushmaster and others make a screw type pin that will adapt a large hole lower to a small hole upper. I have an old SP1 that is all large hole and I put on a Colt heavy 16" small hole upper on it. The upper came with one of the adaptor screws and it went together fine with no problems. Should work the same for you. Also as I said, the large pin/ small pin screw does have an offset so things will line up properly. I think model1, JT, Del-Ton, etc all offer the adaptors. Don't think RRA offers a screw for this but they might. |
| I actually had Ken from SAW tell me that it could cause issues. I was going to have him rebuild the entire carbine, but said that he could not guarentee the work with an off set. So this has me rethinking if I should just cut down the same upper that came with the AR and sell the RO635 upper I just got for my project. I'll try it out first to see how its works. Thanks for all your help GS1000SN. |
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Why don't you switch the barrels in the uppers once your Form 1 has been approved? I had a SP1 and that goofy cam pin made me crazy. If I opened the gun, the cam would rotate and the upper and lower wouldn't go together. I would have to take a screwdeiver and rotate the pin, to get the gun back together. The barrel switch would be easy. You could send your lower to American Manufacturing. They can fill the front pivot pin holes in and redrill them to the standard size. I think they charge $200 for that service. Good luck with your project. Scott |
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