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My bike came with an Eagle NX cassette which sits on the standard Shimano freehub body.
Because the NX cassette is 615g I'm swapping it out for the GX Eagle one which is 450g.
I have bought the xD Freehub body for my wheel.
Questions:
1. Do you need to use any lube / grease on the xD Freehub or the GX Eagle cassette when installing the cassette? If so oil or grease and how much and where?
2. What torque to tighten down the cassette onto the xD Freehub? (I have a nice long torque wrench and a cassette lockring tool - I'm assuming it's the same tool as a Shimano one?)
First time for me with a SRAM drive train. Don't want to muck it up. Thank you.
Definitely grease/anti seize the cassette/freehub interface, iirc torque setting is 40nm but it’s def in the manual.
Thank you, perfect.
I'll download the GX manual to check the torque but that sounds about right.
Always got away with the torque by hand with Shimano cassettes but I remember reading on here that the xD system can break or get jammed if it isn't torqued correctly.
Lube it up!
40Nm sounds and feels like a lot, I went for a little less and its all been fine. Make sure your spline tool has long splines for full engagement. People have had splines damaged and the casette stuck on the freehub
Long splines as in these two tools I have? Or longer?

The cassette should be prelubed I think?
Just a quick final post on this to wrap it up. Thanks for the answers.
The SRAM Cassette Manual PDF does indeed include all of the information. I should have looked on there first.
Available here: SRAM GX Eagle Cassette - Webpage with Manual PDF Download
Summary:
1. The splines on the cassette tool must be 7-8mm (both of my tools are)
2. Grease up the xD body threads
3. Torque to 40Nm
4. Go Ride!
Man doesn't read instructions, shocker...
I really don't know why IKEA is such a big successful company...