Andy wrote:It's certainly more common for the top to go first, but like I say, not impossible for a crank to fail.

Quite right Andy, I can speak from experience. When I stripped my motor down, the pistons and cylinders were ok, in respect to not ever having seized, albeit some general running wear. The centre crank bearing however was a slack as a bag of Knackers. The flywheel side bearing also had serious play in it too. If I'd continued to run the bike, one of those bearings would have collapsed / seized for sure way way before a piston grabbing due to running lean. My plugs were a nice dark tan brown.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30