Recently a friend left me a Triumph Bonneville T100, he asked me to strip the engine. It had seized its big end and I promised that I would look at it when I had the time. I know we had still some Triumph engine parts on stock, including a crankshaft, among the boxes. At the time I couldn't imagine needing it for my friend's engine but it spurred me on to pull the engine down and check before selling it on.
I took off the top end to find it was the left hand big end that had seized. It's usually the first to fail as it's furthest from the oil supply. Oil feed is via the left-hand main bearing, a plain bush. If this bush wears badly, oil finds it easier to leak out of it than flow through to the big end.
I removed the timing gear cover to examine the bush. With the gears removed I found a strange thing: a gear tooth imprint in the end of the crank, indicating it had been shifting sideways enough to strike the camshaft idler gear. I never see this before and after a few telephone calls to some other mechanics it was time to split the cases and see what was happening... Unbelievable horror! The crankshaft had broken in two on the big end journal, the two halves being more or less held together by the con-rod. Wear polishing on the breaks showed, incredibly, that the engine must have continued to run for some time after the fracture. The zig-zag nature of the break promoted a ramp effect, creating a side thrust that pushed the broken half outwards, and causing the wear that led to the mysterious tooth marks on the end of the crank.
I'm not sure what caused the crankshaft to break – who knows what's happened to it in the past.