I am looking to purchase a pre ban rifle, but most of what I see are Colt large hole setups.  Years ago I sold my SP1 pre ban because I was having difficulty finding the upper I wanted to put on it.  Has there been any new things that make the small hole fit on the large hole lowers?  Can you open up the small hole or put bushings in the colt lowers to match the upper small hole?  Sorry if this has been cover before, but I did look breifly for topic that covered this

Thanks,

Robert  

You can find the offset pin set for adpating a small pin upper to the large pin (COLT) lower. There is also available the reverse offset pin to adpat the large pin (COLT) upper to the small pin lowers. To me, the great advantage with the large pin lowers is that using the offset pins to lock the small pin uppers. It locks both halves rock solid.

What are the dimensions of the "large hole" and "small hole"?

Large hole typically refers to the front pivot and is .312.  Small hole is .250.  You will also see large and small fire control pins for Colts.  These are .154 and .169.  .154 is the standard mil-spec size and is often preferred.  However, it is no problem to find parts in either size, as well as adaptor pins and bushings for the front pivot.

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

Thank you, Fox.
I was about to start another thread, but maybe you can help with this too:  what other things differer between Colt and other manufacturers?  I understand the sight heights and buffer tube diameters are different.

May I jump in here too?  I'm putting a "large pin" upper (SP1) on a RRA "small pin" lower.
I found the reverse offset pin @ Bushmaster's site: $15.95, but they want another $6.50 for shipping (for something that could be mailed in an envelope for 35 cents) Nuts!
Anyone know where I can find these bushings elsewhere -- maybe for less $$?
Thanks.

Have you called Bushmaster?  I once had the same situation with KelTec and they dropped part in an envelope instead of charging me $6.

OK, for the 40the, or maybe the 50th time [:D], DON"T BUY THE BUSHMASTER ADAPTER PIN! It, and most of the others that you'll find, will drive you nuts every time you open the action. It will rotate when you open the action, but it won't rotate back when you close the action, meaning that you'll have to drag out your screwdrivers and reset the thing.

The only adapter piece worth a damn is the one sold by CDNN [url]www.cdnninvestments,com[/url].

Part #TAR2, $6.99.  CDNN also has a minimum shipping charge, but you can order spare mags, or anything else off their list, for the same charge, so peruse the site to see what else you can use.

The CDNN piece, once set, will stay in place without adjustment. It rotates when you open the action, and rotates bck when you close it.


(Added)  Whoops, just noticed that the reference to Bushie was for the adapter pieces to make a large hole upper fit a small hole lower, and for that you do need the sleeve pieces that (I think) only Bushie sells; not the adapter pin.

The adapter pin is for small hole UPPERS to big hole LOWERS.

Sorry about that [:D]

If after doing this you are having feeding problems such as the bolt overiding the round in the magazine You will need to buy the tensioning pin and use it with the center spacer/bushing to pull the upper and lower tighter together.