The XT 9 speed cassette has the 6 largest sprockets mounted on an aluminium carrier. To use a 40t (or 42t) range extender cog, you need to remove one of the lower gears. With a standard XT 9 speed cassette, none of the lower (smaller) sprockets are suitable for removal because the three lowest sprockets are all specially designed to work as a group.
To get this working, the 17t (smallest sprocket of the 6 mounted on the carrier) needs to be removed and the aluminium carrier needs to be machined down.
Here's what I did:
- used an angle grinder to grind down the retaining pins holding the 17t in place;
- pried the 17t off;
- punched the pins out of the carrier;
- machined down the carrier such that a single 16t cog would fit flush against the carrier with the correct cog spacing.
Once this was done, it was then an easy matter to add the 40t cog on the top end and a 16t cog to replace both the pried off (and discarded) 17t and the loose 15t.