One of the cartridge plate bolts came loose and somehow worked it's way into the clutch basket. It then rattled around, smashing some fibre plates until it worked it's way out and locked the engine...
Just pulling the sidecover was an eyeopener! With bits of fibre all over, stripped scavange pump gears and a smashed timing chain guide... Stage8 anyone?
I'll just add that this isn't one of my quads and, as usual, the owner didn't seek any information until it was too late. Preventative maintenance may cost a little now but it saves a lot later.
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