How do you tell whether or not a Colt has a small hole or large hole upper?  Also, are there any other little quirks with Colts that would impact what parts you can put on them?  Thanks.

re: large hole or small hole upper. This refers to the size of the front pivot pin.

The small hole receivers use a ¼" hole. The pivot pin and hole are the same size as the rear takedown pin. Large hole receivers have a 5/16" hole and front pivot pin. Most have a slotted screw head on both sides, though the later ones had it only on the left side. The right side had a "D" shaped head like a standard pin.

The only other thing is about the same time Colt switched to the standard (small) pivot pin they increased the size of the hammer/trigger pins. I thing just about everyone who makes upgraded triggers has them in both pin sizes. So be sure you order the one you need.

Some of the sear blocks Colt installed will interfere with some of the different upgraded trigger systems. If you have a Colt with a sear block and want a new trigger get a hold of the trigger manufacturer and ask if the type of sear block you have precludes use of their trigger.

Something I forgot to add. The standard sized (small) hole receivers use the same ¼" sized pin for the pivot pin and takedown pin. Large hole pins are large only for the pivot pin.

Both pins same = small hole

Front pin bigger/rear pin smaller = large hole

Is there a trick to getting the pivot pin off?  I have the large hole on my Colt with the slotted screwdriver heads on both sides.  They are on VERY tight.  I didn't want to mess something up, but have tried to get them off.  Any suggestions?  Once off, should I replace them with something else?  Thanks.

The threaded portion has a small piece of nylon embedded to act as a lock. Much like nylock lock nut.

Just use two screwdrivers that fit properly, stabilize the rifle and have at it. Standard right hand threads.

You can replace it if you like. I think DPMS probably has the best selection of replacement devices, but after 15yrs the nylon in my screw is rather loose and I can take mine apart rather easily. So it doesn't really bother me and I've never used anything else.

http://www.biggerhammer.net/ar15/uppers/