Installing the bolt stop of an AR-15 requires attention to detail because the roll pin is tight-fitting and in a difficult location to reach. Be patient. The bolt stopper release assembly is made of four parts: the release, the roll pin, the plunger, and a spring.
Oil the roll pin first then lightly grease the spring and plunger.
The action block will secure the lower during this assembly.
Place the spring in its recess followed by the plunger then the bolt stop.
Now install the hammer block. Stand the receiver on end and secure it in a vise. Everything is held in place with this roll pin and the leading edge should be well-beveled.
Due to the location of the pinhole, an extra-long roll pin starter will help prevent marring of the finish.
A little masking tape is also a good idea, should your punch slip off the pin. Start the pin with the starter punch then drive it the rest of the way home with an extended drive punch.
The pin should protrude slightly from both ends. Now you need to check for function. Insert a magazine and make sure that the top of the follower engages the lip of the bolt stop, forcing it upward. This is what stops the forward movement of the bolt when the magazine is empty.