cassettes are usually cheaper than new cranks/rings as long as you don't have to change RD. Do you know whether you've got a mid or short cage RD?
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"