My guess is that you had a fluke material "indgestion", like a pebble wedged itself in between the clutch bell and spur. A broken crank is very rare. Jeff was getting around 7.5, when I pitted him we didn't try to stretch it. He did comment that in the buggy it didn't get as good mileage as his more expensive Nova but that it didn't matter with a 150cc tank. Jeff drives VERY aggressively on the throttle so like the saying goes: your results may vary...