Originally Posted by rufus
my 7 speed 105 derailleur from 1989 shifts 9 speed just fine. the derailleur should have plenty of travel, unless it's worn out.
if it can shift 9 speeds in friction, it ought to be able to do it indexed. the derailleur can cover the range from small to big cog, and the indexing is determined by the shifters, not the derailleur.
You can't index with pre-9-speed derailleurs except DuraAce. They changed the ratios of everything up to DuraAce at that time.