Putting together a lower over the weekend, I'm having some problem with the front pivot pin. The pin dry fits fine. But once installed with spring and detent, it's very hard to move. I had to use a hammer to punch it in and out, and not just a gentle bang but real force hammering. Looks to be a lot of spring pressure on the pivot pin.

So I removed everything and took some measurements. I measured the channel depth to be about 1.0225". I inserted a punch to the bottom of the hole, mark the edge that's exposed, then measure the length from tip of the punch to the marking. Can someone confirm whether this depth is in spec or not?

With the spring inserted, there's about 0.0705" of spring protruding. That seems a lot. From online installation videos, I see the spring is usually flush and not protruding out. I'm using the Armaspec enhanced pivot/takedown pin set which comes with its own springs and detents, so I'm not using a wrong spring. And I believe Armaspec is generally good stuff.

Anyway, I feel it's either the spring too long or channel that holds the spring and detent is too shallow. If it's the spring, I'll try to see if I have any spare springs around. If it's the channel that is too shallow, I may just clip a few coils off the spring. But I hope to get some confirmation here before I do irreversible changes. Thanks.

Sounds like spring bind to me I would trim it flush and take my chances. Worst case you need a new spring it maybe for the rear pin try switching them.

One reason for the variation in how much a correct pivot spring sticks out or is flush is because the mil spec depth hole is not as deep as some commercial ones are drilled. On a mil spec lower the spring normally sticks out.

Check the channel of the pivot pin with a magnifying glass. Check for any burrs. Try a standard detent and spring. Do you have another lower you can rob the parts from?

Checked the channel, looks smooth, no burr. Also found several spare springs, so I can play with them a bit. Only question I have left is, what's the mil-spec depth for this pivot pin channel? This is just for the OCD side of me, I want to make sure the channel depth is not totally out of spec.

It sounds like the detent is seating too deeply into the recess in the pin.  Being it is the front pin I would probably let it ride, but you could massage the transition from the recess to the channel so the pin starts easier.  Or, you could also grind more of a point on the detent a little for the same result.  But, it is the pivot pin and I seldom pop those except for multiple uppers that share lowers.

The depth on a mil spec is measured from the center of the pivot pin to the bottom of the hole. It is 1.125" +/- .010". You can't really measure off the edge you mentioned since it has its own +/- allowance from the center of the pivot hole

Good info to know. Thanks very much.

Lube up the channel and work it back and forth a bunch. The detent will lap in a bit.

Have had the same issue with that pin on builds.

What I found as the fix is to pull out the pin, then take out the detent. With an emory board, buff the edges of the tip of the detent that rides in the channel until you feel no edges at the detent’s tip with your fingernail.

Reinstall the detent and the pin then add a drop of oil and you should be GTG.