Join Date: Aug 2005
Location: Pittsburgh, PA
Bikes: '''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
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A 28T cog will have a radius about 0.08" (2mm) larger than a 27T so if you have, or can adjust for, at least 2 mm clearance between the upper pulley and the current 27T cog, the 28 should work.
Generally, posters have reported that even a 30T cog works with most Shimano road rear derailleurs on most frames but, as operator noted, a close coupled derailleur hanger can keep it from working on some frames.