The following pictures are gear holding tools and the damage they cause.
The gear holding tool stresses the nylon gear causing it to fail. The picture below is a Bendix D3000 series dual magneto
distibutor block and gears with missing gear teeth. The teeth, the tool stressed, broke off. The result was a complete failure
of the magneto. Notice that the broken teeth were located where the magneto would be installed at its timing position.