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they are used in the upper counter gear the gear that connects the 90 degree bevel to the lower diff gear on the 3mm shaft. you need 4 of them.

yes 2 servos and a Y cable or you need a radio that can be setup for 4ws.

the gears work fine in PLA just the small bevel gears should be made in nylon.

the assembly of the CB03 axles is basically the same as the CB02 you just assemble vertical instead of horizontal. the pics above are enough to figure out how it goes. the axles can only assemble one way. due to the diff off-center placement.