BEWARE !!! Not only did the pitch change, but the mounting holes changed as well, and the 81-? master cylinder does not fit on a 79-80 body. I found this out 2 nights before my first race, the hard way ...
However, many years (and many beers) later, I don't recall just what it would take to do the conversion. I do know that we decided to backtrack and put the '80 back on, even though I had converted from drum to disk. It's never been a problem, as I added a proportioning valve to the back lines ...
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Dave Youngren
NER ITA RX7 #61