I ve seen 7 main bearing John Deere s snap a crank and still happily clatter away.
Heard stories of big cube MM s with big strokes flex the crank enough to touch the inside of the crankcase.
Met another fellow that ran an old diesel super stock built on a 401 ford. Spun it fast enough that the crank would flex and grab the rod by the sides. Wasn t fully counterweighted.
The issue with the 3 vs 5 main bearing 4 cylinder or 4 vs 7 main 6 cylinder is not the main bearing itself, but crankshaft flex. Which can show up in the mains.
Take a piece of channel iron and support it at each end and place weight at two places equal from the center, then place a center support and notice how much better it supports the weight.
If you were to increase the mean cylinder pressure high enough and barring any other failures you would eventually break the crank through fatigue. Providing the piston and rods are up to the task as they usually fail first in most cases.
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