Hi I'm thinking of doing some cyclocross and I wanted to change my gearing to give myself more options. I already have a Shimano Tourney rear mech ( 6/7 spd ) and was hoping this would stretch to work with a 9 spd cassette?
I have always wondered about this and just compared the cage width and pulley play of an xtr m971 and a bottom-of-the-range Tourney I have, I can't tell any significant difference, if anything the xtr's top pulley has a little more side-to-side play.
Since chain inner widths (which determines pulley width profile) aren't any different and cable pull ratios are well known to be identical, I can't see any reason why it wouldn't work perfectly. Certainly no harm should arise by trying, imho.
ps To be sure, I have just checked the late great Sheldon's page and it confirms as much.
The wheels are a bit thicker at the bushing on pre 9 speed mechs so the cage is just that bit wider but not by a lot , we have repaired dead 9 speed mechs with missing pulleys by taking 7/8 speed jockey wheels and putting them on the 9 speed mech - you have to use the bolts from the 7 or 8 speed mech though as the 9 speed jockey wheel bolts are most certainly shorter.
Where you will have to try it and see though is when it comes to the age of the mech - 7 speed mechs can struggle getting at the very largest sprockets on 8 and 9 speed cassettes as they lack the range at the knuckle, they just don't quite swing far enough.
I tried two STX-RC mechs on an 8 speed set up, seemingly identical mechs but the slightly older and tattier one of the two did not have the range for the 8 speed cassette at all.