Anyone Can build a gear. VERY few can build a good one. A lot fewer still can meet the demands of gears in a redrive, especially on engines that have torsional resonance issues (i.e. two cycle singles and twins and four cycle up to 4 cylinders). Did a bunch of gearbox testing for one of the very best gear makers on Earth, and still had failures (in medium duty trucks). BTW: had lunch with a friend who used to build RR turbines. What he/they went through in machining parts was far beyond what your wildest dreams/nightmares could imagine.