He tried to ride fast and the engine seized. I’ll assume the barrel wasn’t already cracked, because that should have been obviously noisy.
Fuel starvation is common but it’s unlucky if that causes a seizure without some missing/spluttering first.
Too-tight rebores are also common, but they usually seize at moderate speed on a long hill, not very long after the rebore.
If the oil supply fails, I don’t actually know what goes wrong first on a Triumph: pistons or bearings. Generally, if the lubrication system is working, it keeps working.
I’d venture to say those 9:1 pistons are marginally a bit high compression for a road Tri 650 nowadays. If the ignition timing is a bit off or the main jet a bit smaller than ideal, or the petrol octane rating a bit low, you could have an acute attack of detonation that you don’t hear at high speed.
The piston top is well coked up, which raises the compression ratio, causes hot spots and signifies oil in the combustion chamber. Oil reduces the knock resistance of the fuel.
Mine once seized because the top came loose on the Monobloc.
