Oops, my apology to eyewinder. You are correct. The stock piston 1977 onward, is 38 mm. I incorrectly measured the outer end of the piston, instead of the inner part, which is the true diameter of the piston.Wow, thank you all for the feedback. You've given me a lot to consider!
gggGary – I like your idea of one step at a time. It would be a really interesting learning process. If I were working with a more complete/functional brake system, I'd likely take the same approach. Thanks for the tip about the Yamaha dirt bike m/c too.
retiredgentleman – I was under the impression that all xs650 calipers were the same from 1977 onward... Measured my caliper piston again this evening and it is a hair over 38mm... very strange!
Paul Sutton – Your experience with the stock m/c and stainless lines perfectly illustrates my concern about that combination. Are you thinking of switching to a smaller m/c as well?
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