So I just finished taking the SRAM PG950 cassette off my bike for the first time. As I went to lay it on the bench, I was surprised that it didn't stay in one piece. Instead, the small bolt that runs through all the big cogs fell out and all 9 gears slid past each other in a small jumble on the bench.

I'm talking here about the little bolt that's about 2mm in diameter, maybe 25mm long, and passes through the purpose-made holes in the gears and spacers. As it turns out, this bolt--whose job, it appears, is to hold the cluster together--was not actually screwed into anything. And as far as I can tell, there is nothing in the assembly that's threaded so that the bolt couldscrew into it. As it is, the bolt just passes through 6 cogs and 6 spacers, and the threaded end is just sitting there. In its current state, it's just a relatively unnecessary, and loose, guidepin.

So, I reassembled it the way it was, figuring that if it hadn't hurt anything after 1700 miles it wasn't going to start now. But what's the story here? What should that little bolt thread into? And should I just remove it and be done with it?

just remove it. It's what "holds" together the "OEM" cassettes, but they always fall apart anyway.

Actually, I seem to have answered my own question. I figured out that the bolt actually reaches to the 8th cog. (I had remembered wrong before.) And the hole in that gear is actually threaded to receive the bolt. But because I'm a little bothered by the fact that it worked itself loose once already, I decided to remove it.

Hey Aerodave.
Did you have any trouble reassembling the cassette without the mini bolt? And does it work okay now?
I am having a similiar problem with being unable to reassemble a SRAM 8 speed cassette and get the lock ring to secure.

Won't it all merge once you have it, one at a time, re-assembled on the hub? I was wondering about that, too. I wanted to swap-out some of the colored spacers into a color-combination. Silly me.

I had this same issue on Friday. I was taking the cassette from an old bike to put on the new wheels for my most recent upgrade.

The pin was loose and I was quite surprised when the sprokets came off one by one. All went back together OK but the pin was left loose as it had nothing to tighten into

It's there to keep the cassette together for easier assembly, and once sprockets have been lined up on the body its job is done.

My other SRAM PG-970 (I have 2) does not have this tiny bolt/screw/whatever. Just pull the spindle it comes mounted on in the box, and it become a pile of loose cogs and spacers.

I heard the mini bolt actually has a nut on the end, but my friends was missing when I took off his cassette.

https://www.sheldonbrown.com/free-k7.html

Update. My friend was putting the sprockets and spacers back together and used "the bolt to nowhere", starting from the large cog to the small cog where it threaded in holding all the spacers and sprockets together. From there he just dropped the entire assembled cassette onto the ratchet body. I have a shimano cassette and it has three small bolts, but the sram 8 speed cassette only had one.