According to Sheldon Brown, most medium range road derailleurs will handle up to a 30 tooth cog on the cassette.
I've decided that my 53 year old knees need lower gears on the occasional 15+% grades I run into so I thought I would convert my cassette (shimano) from 12-13-14-15-17-19-21-23-25 to 12-13-14-15-17-19-21-25-30 giving me a 20% lower bailout gear on the bottom end while keeping the close ratios in the gears I use most. (Yes, this is with a 52-42-30 triple in the front ) I found a 30 tooth cog at the local bike co-op and had no trouble tweaking the cassette.
Unfortunately, the tiagra rear derailleur drags on the 30 tooth cog. I've tightened the "B" screw in all the way and even pried the "B" screw flange back an additional 3/8 inch or so, but the derailleur doesn't raise enough. It will raise free by hand, but not by any reasonable use of the "B" screw even if I put a much longer one in.
I've also put a new chain on and adjusted the length to fit around big to big (bypassing derailleur) plus a full link and it didn't help.
Is it time to give up and buy an XT derailleur or is there something else to try?