Primary Clutch Spider Shims: Which one to use? A good question and asked by many with no standard answer. This is a tuning part that racers and tuners test with to obtain the best performance for their application. Engagement requirements, belt grip for horsepower levels, and type of riding/traction are all things to be considered. There are also a few other variables like belt compound, flyweights, and springs that can alter the effect.
Shims are of a stepped design and the thickness of the two steps change the way the clutch reacts: Using a thicker large diameter “untucks” the flyweight at engagement which will increase the leverage/belt grip and often lowers the engagement. The smaller diameter part of the shim increases belt side gap and also therefore does effect the leverage position of the flyweight at point of belt contact. There is no set in stone answer for the amount of change; test, test, test.