John
Although I cannot speak about the XT, I had exactly this problem with my FJ1200 several years at about 75,000 miles. The fracture was confined to the part of the crankcase that houses the clutch, as a result of the chain bunching up on the front sprocket when it snapped at high speed (preferable to locking up the rear wheel!).
I stripped the engine down completely after my local bike shop told me about a competent alloy welder nearby. After stripping, I rebolted up the empty crankcase halves, being careful to tighten them up to the correct torque value. The cracks were then welded and I rebuilt the engine. At that mileage it would have been a false economy to put back any of the original bearings, seals, O-rings, gaskets etc so I replaced all of those, honed the bore, balanced the pistons, ground the valves, etc etc. A further 85,000 miles further on all is still well.
It's a good idea to use the workshop manual, even for stripping, as some steps have to be taken in a specific order. I didn't have any special tools but when there was something I couldn't do (like pulling gears or undoing the front sprocket) I took the engine along to my friendly bike shop to help.
I'm now rather particular about lubing and checking the chain ...
Regards,
Neil.
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