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Thanks! I've only ever used plastic 3D printed gears with this project which have held up surprisingly well so I'm not sure what metal gear would work best. If you do find a metal gear though, you could possibly print a very tight insert to convert the center axle hole into a hex hole that would fit over the rear axle nut. I'm sorry I can't help you more but if you ever do find a good metal gear, would you be able to send me a link so I can include it in the post for others with the same question? Thanks again!