I had a bit of play in the crank pulley (up and down .5mm), it kept seizing, and was rumbling badly -

Prevention

I expect this was because of water ingress into the crankcase from the previous owner who probably forgot to shut off the water intake before stopping the engine - allowing water to seep through the exhaust into the cylinder, or possibly wave action/condensation coming up past the exhaust  goosneck?

Anyway it needed doing, and fortunately I had a couple of spare engines to strip before the real thing.