The gearbox breather is designed to let the gearbox vent without any risk of water finding its way into the gearbox. We found that it is becoming more popular to run a filter as a gearbox breather now instead of running a hose to an overflow bottle, but the problem is water would find its way into the gearbox when the motor was run in wet conditions. This breather is designed to make it near impossible for water to find its way into your engine.