My Kyosho Lazer suffers with that problem of sand grains getting stuck in the spur gear, this then damages the pinion gear. However the Losi 22-4 is completely closed - out of the 3 times I have raced it, I have never seen any grain of sand get into the spur gear and the drive train is completely clear of debris.
Also on the 22-4, the motor is screwed to a plate in order to set the gear mesh. This plate reduces the likely hood of debris from entering the drive train via motor vents - I think this is the problem he refers to with 2wd cars, debris can easily enter.
Cheers
Simon
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