Next, place the brass detent from the lower parts kit onto the spring. You'll find that it centers itself fairly well on the spring. Incidentally, just like with the detent springs, some manufacturers do supply a third detent as a spare in case you lose one by launching it. Note, for later in the article, the pointy tips of the detent. This will be relevant later.
Carefully press straight down on the detent, using the front pivot pin to push it in. If you're not careful, you're very likely to have the detent slip past or go sideways, and send the detent and spring after it flying up into the air. Some folks choose to do this step inside of a large, clear plastic bag in order to contain any fleeing parts.
Once you've gotten the pivot pin down into place, exert a bit of sideways pressure so that it locks into the large loop in the front of the receiver. You may notice it requiring some extra force to install; this is normal. Remember how I said the pointy tips would be relevant later? The force required now is fairly great because the point is making very firm contact with the hole in the pivot pin.
Pop the pin home, then push on the other end of the pin so that it slides back the other way again. Your goal here is to work the pivot pin back and forth enough that it rounds off the pointy end of the detent, thus allowing you to operate the pin with your fingers. This will not diminish the accuracy or integrity of the weapon.
-- SeanNewton - 18 Nov 2008