The metal gearsets I have experience with are the ones from Mato. They supply the metal gears that replace the nylon ones. These were not preassembled metal gear boxes. I had the gear boxs out, so this is when I ran them using my drill. It was probably for over an hour. Use some tooth paste that has Baking soda in it. This will accelerate wear, and washes out easily with warm water. I work them until they free wheel with little resistance and definetly no cogging!!!!!.