Well, that was the size for a while. Yamaha has now gone back to using the very early size which was only 5mm thick. The ends of the cam don't protrude very much out of the engine. A thinner seal seats in deeper and may get better and more purchase on that stubby end. Mike's sells what they call an improved seal but many report issues with it. It is a triple lip type but is also 7mm thick so those extra lips often do no good because they don't get onto the end of the cam.