I'm looking for a new trigger for my Colt AR15. What triggers fit and what is the pin size for the Colt?

Colt has two pin sizes. The early ones, called the small pin, were .154 pins size. Colt also used a large pin size of .176 called the large pin. More then likely if it's older Colt it's the small pin.

There is a host of triggers that will fit, but as you noted the pin size of a Colt for the hammer and trigger is I believe something on the order of .172" vs mil spec of .153. You would have to specify with aftermarket supplier that it is Colt and they would supply the correct size pins. My numbers shown are only approx., but someone may have the exact numbers for you. Good luck.

It's reported that Colt went to the largre .176 pin to make it harder to use full auto parts. At about that same time they also quit milling out the inner walls of the lower receiver to also foil installing full auto parts.

You'll have to go by serial number range on a Colt to see when they did all this. I have a pre ban Colt HBAR that I bought in the 80's and it has the small trigger pins and the lower receiver has all the milling. That might give you some idea.

Colt pins a steel block in the receiver aft of the trigger group that blocks the insertion of the modular Timney trigger. I found out the hard way after I bought a Timney and it wouldn't install. Timney will modify your lower to accept it for a small fee but why go to the trouble when there are comparable products that don't require the extra work? In any other brand lower its a non-issue; just Colt.

That said, the trigger works well now that its in and Timney's service is excellent.

Giessele, pronounced Guys-Lee makes a superb trigger. I have one in my varminter.

There is also a shop in Illinois that can take it down to 1.5lbs single stage without any concerns.