Here is what you will need to complete the job:
1. Remove the cassette from the wheel using a chain whip and cassette lock ring tool. XX1/X01 cassettes are different from typical cassettes in that they thread directly on to the freehub body instead of being held in place with a lockring. Because of this difference the entire cassette will move away from the freehub as you remove it.
2. Use a large flat-blade screw driver to remove the old 42T cog from the cassette. Position the screwdriver blade next to one of the press fit pins between the 36T and 42T cogs. Twist the screw driver to gently pry the 42T cog away from the 36T cog. Twist just enough to shift the pin by about 1/3 of the engagement length.
Repeat this process for all 18 press fit engagement points around the ring. Target to take three times around the cassette before the 42T cog is completely disengaged.
3. Apply grease to the spline interface on the GCX, the threads on the XD driver of the freehub, and at the interface of the steel cassette body and the internal sleeve. This will ensure the seats properly on the XD driver freehub body when you install the cassette.
Optional: Take advantage of the opportunity to get some grease into the interface between the cassette body and internal sleeve since this interface has potential to creak on XX1/X01 cassettes.
4. One of the press fit tabs on the cassette body is wider than the rest. You will need to locate this tab to properly align the cassette to the GCX. It may be helpful to mark it with a permanent marker to make it easy to identify.
5. There is a triangle laser marked on the GCX that denotes the large press fit hole. Align this hole with the wide tab on the cassette body.
6. Use a large pliers to squeeze the tabs of the cassette into the holes on the GCX. VERY IMPORTANT: Place a couple layers of duct tape over the tips of the pliers to prevent it from scratching your GCX.
Squeeze the press fit tabs about 1/3 of the engagement length into the hole. Repeat this process for all 18 tabs around the cassette. Target to have the tabs fully engaged after three revolutions around the cassette.
7. Install the cassette back on your wheel. Take care to ensure the cassette is fully seated on the splines of the freehub body.
8. Congratulate yourself on a job well done and head out the door for a ride!
GCX 44T Gear Charts
29 Inch Wheel
26 Inch Wheel
27.5 Inch Wheel