I bought an 86 XJS without a spare. I see where it goes but how is it mounted or held in place? I am going to buy a new/old rim and tire.

 

There should be a large threaded steel rod with a hook shape on one end. The hook engages in the vertical bodywork under the fuel tank; the other threaded end of the rod pokes back to the rear and goes through one of the wheel stud holes in the rim. There is then a large female threaded black plastic knob plus an aluminium spacer, which hand turn onto the rod poking through the wheel, which holds the whole shebang in place!

I have not got a photo, but all the breakers will have the bits you need.

Greg