Most hardware stores carry both Metric and Standard Screw Checker gauges. The gauges I use are one’s made by “Ruelle Industries-6650 Highland Rd.-Waterford, MI 48327-(313)666-4433″. They are heavy duty and measure approximately 3″ X 5 3/8”. The gauges are made of .072″ thick metal and are tapped for various thread and screw diameters. The various screw sizes make it great for measuring various unknown thread types and especially useful when custom building. They can be ordered or purchased at well stocked hardware stores too.
Note: When working on your instrument be careful using electric components and use procedures and safety recommended by manufacturers. Always wear safety glasses when soldering or cutting wires. Keep in mind the value of your instrument and use a qualified repairman when doing your work. Good luck….