Originally Posted by
ridelikeaturtle
I've got an RX100 rear derailleur on a bike on the stand, and with friction dt shifters, it happily spans an 8-speed cassette.
<edit> I'm pretty sure I took this rd off a 7-speed bike, but sure it could be an 8-speed rd.
I don't know the whole story, but while I consider RX100 a 7-speed group, I think the RD geometry got updated to span 8-speed by the end.* I've got one of these A551 RDs on my bike:
VeloBase.com - Component: Shimano RD-A551-GS, RX100
I'll echo the advice above -- get a 12-28 cassette (discontinued 13-28 can still be found here and there, if you don't need a screaming 53/12 top gear), brand-new chain, and you should be in business.
* If I had to speculate, it would be that since RX100 was basically 105 with a different finish, perhaps the RD had no choice but to become 8-speed when the 105SC group was updated to 8-speed.
