Owing to a strange mechanical failure, I snapped the tension screw on my XO rear derailleur. After inquiring at four bike shops and being told at each "I don't know, man, you can check with SRAM..." on a whim I checked the hardware store.
Voila! A metric M4 bolt 25mm in length solved the prob. A 20mm would also work. The bolt head takes a 4mm hex wench (not the 3mm of the SRAM part), and the bolt head just *barely* clears the cutout on the derailleur... but it does work, no prob.
Note - with a long enough bolt, the bolt-head clearance issue makes no diff.
Mine's a 2006 XO. Don't know if anything's changed since then.
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Thread: XO rear derailleur tension screw