Countershaft Sprocket Thickness

1999 YZ400F. Bought a Primary Drive chain & sprocket kit, steel sprockets and x-ring chain, all stock size. I noticed that the countershaft (front) sprocket in the PD kit is thinner than the worn sprocket I'm taking off. The different thickness is not huge but it is noticeable to the eyeball. Is this something to be concerned about? Should I get a thicker cs sprocket (OEM or other?) , or should I just fugeddaboudit and run the PD? I can provide pictures and measurements if it will help. Thx

I've been running the Primary Drive stuff for quite some time now on my 426and wouldn't worry about a different thickness. The old one you took off may have been aluminum (thicker) where the PD stuff is steel (thinner). Maybe the narrow width shaves a few grams to put it on par with aluminum?

The inside width of a 520 chain is spec'd at .250" (6.35mm). Sprockets for use with a 520 should be around .225" (5.7mm). It is important for them to be as wide as reasonably possible for several reasons:

  • Wider sprockets wear longer
  • The wider contact area distributes the load more evenly over the width of the chain roller and pin
  • The wider sprocket stabilizes the chain laterally

Made of the same materials and treated to the same standards, the narrower sprocket will not last as long.

I can be a real idiot sometimes... the PD steel countershaft sprocket is not thinner than the old sprocket (which was steel) -- the thicknesses are the same at the output shaft splines. What threw me is the fact that the sides of the teeth on the PD sprocket are narrower at the tip. Not when looking at the side of the sprocket, but looking at the edge view, the teeth taper in more at the tips.

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