Sometimes on cars that aren't sealed like a 22-4, it's better to have a cut out under the spur gear as there is less chance that a stone will jam in & buggier the spur gear up, instead it gets pushed out of the hole.
On my old Yokomo BX, the rear diff is quite open, but it would stay cleaner for longer like that then when I sealed it up a bit, the dirt would collect instead of passing through.
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Model Junction, Boughton Raceway.
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