The crankshaft moves up and down within the limits of the bearings and thrust washers. As it does so, the carbon seal must slide on the shaft in order to stay in contact with the sealing surface of the lower bearing. The quad ring is better suited for this sliding action, whereas an o-ring will tend to roll before starting to slide or may not slide at all, causing the carbon to momentarily lose contact with the bearing. At least that is my understanding. What I don’t understand is why the made the change on the Big Twins and not the 22 cu. in. 18/20hp motors. Whatever, I personally choose to use the quad ring across the board.
