Well... I know a few of you told me I couldn't use a Comet 20 series on my 18 hp twin due to belt problems. I'm going to say your RIGHT! The problem I am running across is not that its destroying my belt, but rather it is not disengaging the system. I was doing a little research on comets spec pages, and the 20 series has a minimum engagement speed of 1200 rpms, and the 40 series min speed is 1600 rpms. I have been searching for an hour now for the idle speed of my briggs twin (18hp, model: 422437-0721-01), but no luck. I am assuming however that the idle speed is higher than 1200 rpms which is the reason the 20 series I have on it will not disengage. Does this sound reasonable? I guess I'm gonna have to shell out the cash for the 40 series.