AR Rear Takedown Pin Detent Spring Upgrade

 Fixing the AR Rear Takedown Pin Detent Spring

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. 

Installing set screw

No more spring launches!

Completed upgrade

To save some searching, here are links to the tap and setscrews;¹

Century Drill and Tool 95302 4-40 NC Tap and 43 Drill Combo Pack

4-40 x 1/8" Socket Set Screws Cup Point Stainless Steel 10 Pack with .050 Hex Key Wrench (1/8" Length

¹ Disclaimer:If you buy through the links Amazon might pay me a few cents.

American Fascists - How to Spot Them and Stop Them

 

Army Talk Orientation Fact Sheet #64, Native Fascists; How to Spot Them and How to Stop Them is a pamphlet printed and distributed by the United States War Department as an educational tool for use by soldiers to help them understand the reasons for fighting in World War 2. It was printed in 1944.

81 years later, it serves as a stark reminder of what fascism is, how it works and how to defeat it.