Since I was too stupid to figure out how to disassemble my own spare tire retaining bolt on first look, I'm presenting the following for your entertainment and edification:

If you've added wider wheels or tires to your car, and used the same size as a full-size spare, you may have run into a problem trying to use the OEM retaining bolt for your "new" spare.

In my case (2001 A6 2.7T going from 5-spoke 16" to S6 Avus 17"x8"), there were two problems: the bolt was too short, and the cap was too large.

Side view of OEM bolt as-shipped:



End view of OEM bolt as-shipped:



The second problem is the most easily fixed - simply pull the "cap" off the "wheel" portion of the bolt:


Yep, no catches, no threads, just plain old friction fit. Stumped me for an embarassingly long time....just pull ;)

Ok, now to make the bolt longer.... You've got to do a little work. The centering ring (stepped cylinder) has to be removed first. Two barbed plastic tabs hold it to the "wheel", press those inward to release and slide the ring off:





Now you've got the centering ring - we'll reuse that, so set it aside. Note the (poor picture of) two plastic tabs you released earlier:



There's still a cap over the bolt head to deal with, and it's held by 4 of those plastic barbs. These can now be easily reached and released by pressing inward:



Now you should have the "wheel" with bolt and the bolt cap (don't lose that either):



All you need to do now is push the OEM bolt out through the top of the "wheel" and you're almost home:



The Audi bolt is a standard metric M8 bolt (length varies by model). I found my OEM bolt to be about 1" too short, so 130mm was my guesstimation. I don't live in a metropolis, so none of my local hardware or auto parts folks had a 130mm M8. Luckily I stumbled across a local storefront for Fastenal. They didn't have a fully-threaded bold anywhere in stock, but luckily a nearby warehouse had the partially threaded version - Fastenal Part #38633 in stock. A couple of days and a couple of bucks later, I had my new bolt (shown with Audi bolt for comparison):



Re-assembly is the reverse of the above steps, though I left the "cap" off to allow the trunk "floor" to close over the spare.