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Is the piston on b7 rear calipers any bigger than on a b5 rear calipers or is there only a mechanical advantage gained by using a bigger disc. I know they will need a bigger bolt to mount them. Brian

there is a bigger swept area on the b7 disc so the caliper must have a bigger piston. stick with the same bolt size as on the b5 caliper and elongate the holes on the b7 carrier as you need to bring the carrier out a few mm so it doesnt foul the disc and then you get the pad centred to the disc, if you bolt it straight on the carrier will touch the disc and you will get around 4mm of untouched disc on the outside

i would intend using a b7 rear disc as i have a set of them. what i'm wondering is, is it worth the effort and cost of new calipers for very little gain.

the b7 s certainly look better than the b5s, there are different schools of thought as to how much work they do . heres a pic of the b7s when i took them off my car to upgrade to the ap,s

and a b5 disc