Originally Posted by
grinningfool
I guess my biggest concern was that the 6 speed chain wouldn't fit through the cage and wouldn't fit on the cogs. I assumed the 9 speed derailleur was made for a narrower chain, so wouldn't operate properly with a wider chain.
Thanks for all the help.
For all practical purposes there are no 6-speed chains around these days, at least not decent ones. The widest chains you usually find are 7/8-speed and they work fine with a 6-speed cassette/freewheel and any "8-speed" rear derailleur will obviously have no problem with them. They should also work fine in a 9-speed era rd too.
I just tried feeding a section of Shimano IG90 8-speed chain through a Shimano 105 9-speed rd and a Campy Chorus 10-speed rd and there seems to be plenty of clearance with both.