Having sorted the conversion for the 13" wheels & got the plates made I have had to put the Disc Brake project on hold for a few weeks. My little conservatory had become a store room for a whole load of Fiat brake parts with a drawing board in the middle, did not impress my wife

I also had a test pair of adaptor plates made to suit the smallest standard Fiat disc (227mms) but fitting the disc requires either taking a skim off the outside of the hub flange or a couple of thou " from the inner shoulder of the disc. Tiny bit of work but I cant get near a lathe at the moment so have been unable to assemble the set up.
When I get back on it , probably in september , the next phase will be a set up for the 12" wheels. I think that I have solved all the problems & I am confident that the final design will be better than the expensive "bought in" set up that I am running on my car at the moment but will require the most machining / specialist work.
Now to answer your question, yes I did a little looking into rear conversions & just missed out on getting a pair of rear calipers from a Croma which I think use 227 mms discs. So I am interested in the conversion just for the challenge but it is last on the agenda. Another problem is getting hold of the parts to try out different set ups. I put out an appeal on a couple of Fiat forums for parts , even if they are worn out and I got a big zero response. To get to where I am now I had to buy Cinquecento , Seicento , Panda , early and later Punto calipers & associated parts , carriers , pads , fixings etc.
I probably need to have a sale before I get more parts in

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I would be interested in any info & perhaps a photocopy of your plates for future reference though [-/