It should have only taken me about a half hour per side to change the pads on my rear disc brakes, but it ended up taking about an hour for one side without accomplishing much, because the in-board (piston side) pads, which I bought about a month ago had a clip for a bigger caliper piston. Everything else was correct, but the 3 pronged clip on the backside of the pad was clearly too wide to fit. Unfortunately, I didn't notice this difference until I had already put lubricant and stop-squeak on the pad.

The existing pads had plenty of life left, I just wanted to put new ceramics on. I ended up putting the original ones back on after unsuccessfully attempting to modify the clip.

When I bought the pads, I don't recall there being any choices based on caliper. So, my question is, should I be surprised that the rear brakes have a smaller caliper than the front, or should I expect them to be the same? Is it possible that the parts person gave me front pads rather than the rears that I requested? I've done very few brake pad replacements in my life and this is the first on my Jag - is the rear caliper always smaller than the front? What should I tell the parts store about my particular Jag so that they know what style I need?

Any feedback would be appreciated!

 

The rears are always smaller than the front. The front brakes do most of the braking work on the car due to weight transfer or something like that when you step on the brakes.

Sounds like the parts store gave you the wrong set for the rears.
Vector

 

I had a similar problem - two successive suppliers supplied the wrong pads - I still have a (useless) set that was not worth the cost of returning .
The XJ8 rear pistons are smaller than the previous X300 ones - many parts suppliers appear not to recognise this and will continue to supply the wrong parts.
The correct spring prong arrangement are those with one large top prong and two smaller ones at the sides, I think the bore width is 38mm but don't quote me on that !
Your supplier has supplied for the wrong model, rather than the wrong end, as the front calipers have 60mm pistons and the pads are a lot bigger and have 3 equally sized prongs.
Hope you get there in the end.

AndyP