Installing the pivot pin on an AR-15 is a simple process on this rifle. Some Colt receivers, however, use a screw rather than a detent to secure this pin. Start the assembly by lubricating the pivot pin with oil,
and apply some grease to the spring and detent.
This job is almost impossible without a pivot pin installation tool like this one by Model 1.
To use, simply insert the tool through the pivot pin bosses on the front of the lower, and line up the hole in the tool with the detent hole in the lower.
Now insert the spring,
then the detent.
Depress the detent with a small punch until the detent is just completely inside the detent hole.
Then rotate the tool carefully to capture the detent with the tool.
The detent rides in this groove in the pivot pin.
Insert the pivot pin, pushing the installation tool out as you go.
Now you can rotate the pivot pin until the detent snaps into the groove.
The pin is now properly installed and captured by the detent.