I had a customer approach me to make a small gear for a really old piece of farm equipment. He had the old gear in his hand but the teeth were very badly worn. He also had a smaller gear that he said matched the gear on the machine perfectly but it was to small and the number of teeth were wrong The small gear was 12 dp, I don't remember how many teeth. The gear he wanted meshes with a very large internal tooth ring gear (30") The gear he wanted and I measured was 1.75" 18 tooth and I assume 12dp because the smaller gear was 12dp But the math doesn't work a 18 tooth 12dp gear should be 1.667" What am I doing wrong? Did they use to make gears different?