AR Rear Takedown Pin Detent Spring Upgrade

Fixing the AR Rear Takedown Pin Detent Spring

or....

Best AR mod since Eugene Stoner invented the platform

The most commonly lost parts of the AR rifle platform are the takedown detent springs. Installing the buffer tube to the lower receiver requires coordinating everything while holding the rear takedown pin detent spring from flying out or getting crushed/pinched by the sling adapter plate (or buttstock).

Here's the permanent fix:

Tap the detent spring hole for a #4 x 1/8th" set screw. This probably should be the case not only with new lower receivers but should've been implemented from near the beginning of the platform. But it wasn't and still isn't. But the fix is easy.

The rear takedown pin detent spring hole is already roughly the correct size to tap for a #4-40 setscrew. The Colt specification for the lower receiver rear takedown detent spring hole is 0.093" +/- 0.004". The recommended drill bit size for a #4-40 setscrew tap is #43 which is 0.089".

Receiver drawing view AA

Chuck a 4-40 tap in a cordless drill and slowly tap the hole. Go slow and reverse the drill often to clear the aluminum (or polymer) chips.

Detail of tapped hole for takedown pin detent spring retainer

The detent spring should be trimmed the length of the new setscrew; 1/8" or 3/16", whichever length you use.

Detail of takedown detent spring

Use a 4-40 x 1/8" or 4-40 x 3/16" NC setscrew. There is no real need to use thread compound because the setscrew has nowhere to go. But if you prefer, you can add a tiny dab of blue thread locker.