Can anyone guess what this little star nut fits? (That's a 1/4 - 20 thread.)
 

I thought for sure that my stating the threads are 1/4-20 would convey the message that this star nut is much smaller than a fork star nut, but since it obviously didn't, here's a photo of the little star nut (on the right) next to a fork star nut.
 

I'm disappointed. I just knew that Greg would figure this one out. Are you not reading the forum this week, Greg?

Quote from aikigreg on Mar 21^st, 2014, 9:35pm:

Bingo! Give that man a prize! It will replace the rubber expansion nut that RANS uses in their idlers. Your winnings are a couple of the nuts. You can pick them up at Rose's shop any time.  
 
For reference, the nut is McMaster-Carr part # 9419CA509. As it comes, it's not quite as tight a fit as it should be in the idler tube. It's intended for a .52 to .54 inch diameter, and the RANS tube is ~.56 inches, so it needs to be expanded just a bit. Do that by screwing a bolt a short ways in, placing the nut on a hard surface, then giving the end of the bolt a light rap with a hammer, as shown below. The two nuts Rose has have already been expanded.
 
I've changed the thread title so this can be easily found in the future.
 

I finally had a chance to tinker with this nut today. Even after being expanded, it still wasn't a tight enough fit to suit me. It would keep pulling out if you kept tightening the bolt. Removing the nut with a slide hammer is no problem if you change your mind about using it. What finally held it better was putting a flat washer on after the nut, but before everything else. Once the nut pulls out tight against the washer, it will dig in and tighten really well. How well it lasts over time remains to be seen. I'm trying it out on my F5, so we'll see how it does.