I have a Colt ar15 with the front take down pin being a two piece screwed into each other set up. If I want to swap between a “normal” upper I understand I have to get an adapter part which i don’t even know what it oooks like ir where to get it. If I use that new pin for a “normal” upper but then want to go back to the original uppper I have to use the original two piece pin?
Can someone explain this to me?
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https://www.m4carbine.net/showthread.php?45998-Converting-a-Colt-AR15-A2-to-M4-flat-top
No idea how to hotlink here. JSE Surplus has the Colt offset bushings and adapter pins you need.
Tig
I’m assuming that adapter is for regular upper but if I want the old one back on I have to use what it came with right?
The part adapts the front lug on the upper receiver to a std lower.
So you need two pins depending on which upper you use, one pin doesn’t allows you to use both types of uppers is what I am trying ask but I don’t think people are understanding me.
You need two different pins. Google “colt big hole lower” . If you use the two links I shared you only have to use one pin. I have a colt big hole lower with a Daniel defense big hole upper.
If not you need this:
Using the above linked part will get you what you need.
I think we were on 2 different wavelengths, same topic. 
Glockycharms:
The offset pin is used to adapt a small hole (normal) upper to a large hole Colt lower. It will work, but you will have to turn the bolt (the long part of the offset pin) until it slides into the pivot hole. Once aligned, it should go in easily.
I have a bit of experience with these, and you should use blue (removable) loctite on the bolt threads. For some reason, the nut (the short part of the offset pin) wants to unscrew itself while shooting. On many occasions, I have had to go looking for the nut after it fell out. So, blue loctite is recommended.
If you have the money, and you plan to use the offset pins a lot, buying 2 or 3 might not be a bad idea.