Faster Sprockets

Aren't all engine clutch sprockets the same?  Not if you look closely. There are important differences.

First we have to talk about the bearing.  The bearing is composed of rollers, a cage to align and separate the rollers and a hardened steel shell to hold everything together.  The shell is the critical part because of the way it is manufactured.  It starts as a flat piece of steel that is stretched into a cylindrical shape.  After the cage and rollers are inserted the edges of the shell are bent for containment.  Unfortunately it is impossible to bend steel into a perfect cylinder.  Roller bearings are not round!  See for yourself.  Press a bearing out of a sprocket and measure its diameter in several different places.  But there's a solution.  The shell is very thin and will conform to the shape of the much beefier sprocket.  If the sprocket bore is round, the bearing becomes round also when pressed into place.

Small sprockets like the chains that connect them must be hard and strong.  It is very difficult to cut sprockets from a hardened steel bar, therefore sprockets are machined from annealed (soft) steel then hardened.   Steel becomes hard when it is heated to a high temperature (1500 degrees plus or minus according to the alloy) then dropped into a tank of water or oil.  Heat treating solves one problem but creates another.  Thermal shock from the rapid cooling causes steel to warp slightly.  The sprocket bore is no longer round or straight.  If a bearing is pressed into this sprocket, it will not be round and straight either.  Furthermore, the interference fit between bearing and sprocket is inferior and the bearing will probably fall out during the race.

There are two solutions to this problem.  One right.  One wrong.  The wrong solution is to machine the sprocket with a snap ring groove to contain the sprocket.  Wrong because it does not solve the bore inaccuracy problem.  The bearing stays in place but does not turn freely.  The drag generates heat that wastes the engine's power. The right solution is to re-cut the bearing bore after heat treatment.  Simply hone the sprocket  in the same way one would hone an engine's cylinder.  Most manufacturers skip this step to save money.  Big mistake.  For pistons and bearings alike, a precise geometric shape is the key to speed and acceleration.

Three indicators of racing quality sprockets:

1. I. D. Finish - "Cross-hatching" on the sprocket's inside diameter indicate it has been  "blueprinted". 
2. Uniform Inside Diameter - Measurements at all points should be within a few ten-thousandths of an inch.
3. Width - 0.750 inch.   Snap ring take up additional space.   Anything wider is a "Fun Kart" sprocket.

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