A lot of the quills I have taken out show signs of more wear on one side than the other suggesting lack of perfect concentricity between the main shaft hole and the hole in the timing cover. However, that does not seem to cause big end failure. What does cause that on every original design Vincent or Alpha big end I have worked on it the annular groove at the timing end of the crank pin. Dirt in the oil centrifuges out of the oil and fills up this groove until such time as the oil cannot flow into the big end and it fails. It is possible that the combination of modern oils and paper filters might have solved this problem but I am not sure.