Originally Posted by
san_antone_rose
It is a RD-1055 — the old manual says max difference of 13, lower/upper limit of 12/28T.
Ok!
And a total capacity (difference between front chainrings plus difference between smallest and largest cog on cassette) of 28. You could choose any 12, 13, 14-28 7 speed cassette and put it on there without issue. Keep the "corncob" cassette for racing.