Summary

UPDATE : for better resistance, I would suggest to use this diff instead - http://www.thingiverse.com/thing:1734496 (same smaller number of teeth => larger teeth => harder to break).

UPDATE : the pinion housing is now longer to provide better effort resistance.

This is a customizable blocked differential for RC cars. It's based on this library - http://www.thingiverse.com/thing:3575/#files

The STL files you have here are designed so that the large gear has 20 mm in diameter. Should you want a larger or a smaller diff, just alter the "SCALE_FACTOR" variable in the OpenScad source code and regenerate.

See a test here - https://www.youtube.com/watch?v=Fl0kDC48Tu8

This design uses 4 bearings (D8 x d5 x 2.5 mm). You can however customize the bearing size (check "BEARING_OUTER_DIAMETER" and "BEARING_WIDTH" variables).

This design uses 4 M3 screws to fix the gear and the pinion on the axle. The holes are 2.4 mm so that you can directly put the screws there, not nuts. You can change that (see "GEAR_FIX_SCREW_DIAMETER").

This design uses 5 mm precision rods. You can change that (see "GEAR_BORE_DIAMETER" variable).

You can generate an open axle (where you can see the gears inside) by calling "axle_top(open_top=1);" and you can generate closed axle (so that no dust gets inside) by calling "axle_top(open_top=0);".