Hayabusa 3 Speed Grasstrack (Autograss) Gearbox

A direct replacement for the standard six speed gear set. First and second gear ratio’s are as standard, but third gear is 8% higher than second to allow for changes in track conditions and help to establish the optimum overall gearing.

The design incorporates some special features. The most important is 5 degree undercutting to prevent the engagement dogs from coming out of mesh under power. This is achieved by having the engagement faces dovetailed which effectively pulls the mating parts together under load. As only three gears are required the new gears are now wider for better durability. Gear changes are effected by the standard selector mechanism but using only the two selector forks acting on the output shaft. The input selector rod and fork should be removed as they are no longer required. Gear changes are made by two selector rings, thus reducing the inertia compared to the standard gears and hence allowing faster changes. The standard input shaft is retained but new gears, spacers and retaining clips are supplied for use in the three speed gearbox.

The output shaft is supplied assembled and requires only the fitting of support bearings. To overcome the problem of output shafts breaking, adjacent to the sprocket, the shaft is manufactured with a larger diameter to take standard Honda sprockets. This requires the standard spacer to be bored to suite the larger shaft.

The only other modification required is to stop the selector mechanism from changing past third gear. There are two ways of achieving this. The easiest is to remove the “pawl pin” from the selector drum to prevent the mechanism selecting fourth gear. The other way is to fit a pin in the selector drum track to prevent rotation past third gear.

The section below identifies which pin to remove.

If you are replacing a standard gear set which has continuously jumped out of second gear, we would strongly advise that you replace the selector drum and selector forks as a matter of course.

Superfinished in house as standard.