The nylon plastic gears for driving the balance shaft can be replaced by metal gears from the Rotax Max engine (part number 234435).

Steel gears have a longer service life, a higher weight and a significantly higher mass moment of inertia. The plastic gears dampen the noise slightly, but overall the difference is hardly noticeable.

Steel and plastic balance shaft gears

According to the repair manuals of the Rotax 122 and 123 engines, the (plastic) balance shaft gears should be replaced every 15,000 km. Worn balance shaft gears sound like a reverse gear. If the gears break, the fragments can also affect other engine parts.

The plastic gears have 27 teeth, the metal gears have 50 teeth.

Installation of steel balance shaft gears

The original gear wheels are used on the Rotax 122 in such a way that all markings point to each other (on the Rotax 123 only possible on OT, see here). Unfortunately the markings on the metal gears do not fit here.

The markings on the outer side of the gears have to be changed as follows:

  • Crankshaft: Move the punch mark by 16 teeth clockwise
  • Balance shaft: Move both punch marks clockwise by 17 teeth
Position of the moved punch marks

With the shifted markings, the gears can be inserted exactly like the original gears line-to-line or the point between the two points.

Both gears are identical to the original gears, one side of each gear has the markings for one shaft.

If you place the plastic gearwheels on the metal gearwheels to transfer the markings of the plastic gearwheels with the same position of the spline hub, two teeth will be directly opposite each other.

Originally I had tried to determine the installation position this way, but the gears cannot be mounted exactly like the plastic gears with the markings inside.

Mounted metal balance shaft gears

A vibration measurement shows that the vibrations are lower when the tooth of the gear on the balance shaft is above the tooth of the gear on the crankshaft.

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