Anyone ever seen a short selector spring before? This was from a PSA kit I got in the mail yesterday. The spring measured .076 with 20 coils. It was way too short, and it only put enough tension on the detent to keep the selector from falling out of the lower. It would not lock in safe or fire, and freely rotated. Another PSA build lower I did had a spring that was 0.94 with 25 coils and I had an extra colt spring, and it was 0.96 with 28 coils. I thought it was the pistol grip at first, but after trying an extra I had lying around I realized that wasn't the case.

Here is a pic. The bottom one is the bad one, compared to the top one with is the other PSA one that works fine.

They had put them in the "reliabolt" too. Not sure if they changed it or not, probably not.

I would say you are lucky it was that easy to spot.

That looks like an extractor spring in the bolt.

I ordered two LPKs from PSA in late Feb and got them in early March.  Tonight I started building a lower and noticed the detent wasn't catching on the selector as it should.  After confirming it wasn't a grip that was drilled to deep, I took out the selector spring from another AR and came to same conclusion the OP did - the selector spring in PSA's lower parts kit is too short.  The two I have from PSA are in the middle.  The selector spring on the left came out of a Bushmaster.  And for reference, the two springs on the right are the take down pin detent springs.

https://lh5.googleusercontent.com/-iXqxPQWBivE/VRDA2s2WYTI/AAAAAAAAPUw/aRzzuqzzHhA/s800/20150323_212319.jpg

Those look awfully small in OD and awfully thick on the wire diameter to be selector detent springs..........

I had my doubts, but that's that last spring left to install.  :)

It freely drops into several A2 grips, MOE grip, and other grips I have lying around.

I'm going to see if I can find a ball bearing that I can drop into the grip to add some height.

https://lh5.googleusercontent.com/-dfcLQM9xCJM/VRDJjZb3FQI/AAAAAAAAPVI/fdVJOfxPMSE/s800/20150323_221723.jpg