I have a 1999 XK8 and the shop has lost my spare tire tie down bolt. Does anyone know the bolt size and thread.

 

6mm on my 2004. I suggest going to a U-pick junkyard and get a few from old nissans , toyitas etc.one is bound to fit

 

The 1997 - 2003 XK8 and the 1998 - 2003 XJ8 all use the same clamp bolt for the spare. Jagbits has a used one for $50.00 so I would start hitting wrecking yards. The clamp bolt threads into the tool kit clamp bolt. I have attached pics and part numbers for both.
The tire clamp bolt is listed as no longer available at the sites where I look for parts.