I haven't removed a star head from the selector drum on an XT, but I have on a couple of RD350LCs over the years and the selector drum is pretty much of the same design. On an LC the star head is held in place by a cross-headed set screw and cup washer, is it the same on your drum?

If it is, the screw is usually extremely hard to turn/remove. Drill the head off first, remove the cup washer and then pull off the star. You can then grip the exposed threads of the set screw and turn it out. Obviously you will need to obtain a replacement screw and possibly cup washer (easily obtainable from a fastener shop). Make sure you use Loctite when refitting the screw.

Interestingly all the separate parts of a 350LC selector drum (including bearing and star head) are available individually, unlike the XT.

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