Idle Gear for Rotax Racing Engines. This gear connects the water pump gear to the water pump connection gear, and is a critical connection within the crank case.
This gear has a reduction gear set from 28 teeth to meet the 19 tooth water pump gear (driven off of the crankshaft) down to a 13 tooth gear on the reverse that mates with the water pump drive gear.
Each gear is sold separate. Note in cold operating temperatures these gears can fracture if engine is run too quickly too fast!
This is part number 15 in the diagram.