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'66 Sprint GT, '67 Duetto, '70 BMW 2800CS
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That slightly cruddy piston at the top can be polished nicely. You only need the cylinder surface to be smooth.
Robert: That's true for drum brake cylinders, where the seal is on the od of the piston and contacts along the id of the cylinder. But disk brake calipers are the reverse: the seal is on the id of the cylinder and contacts along the od of the piston.

Therefore, the finish of a disk brake cylinder (aka "caliper") is irrelevant, as long as the groove for the seal is OK. The finish of the piston is what is critical.

Having said all that, the one piston shown in rooony's photo in post #2 doesn't look too bad. Assuming the other three aren't any worse, some light polishing should be sufficient. As you noted, the bad rust on the piston in his photo is on the area that the seals don't contact.
 
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