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Because of the way the speedometer was designed with the trip odometer and the total odometer across from each other instead of over the top of each other, there is a lot more torque required to turn them causing them to quickly strip out the 2^nd and 3^rd plastic worms that are used in many of the GM speedometers.

 

Therefore, when rebuilding for this year speedometer, it is imperative that steel 2^nd and 3^rd worms are used. Also, when replacing the 2^nd worm on the ‘63/7 speedometers with steel, the magnet 1^st worm has to be replaced too because it is worn and will cause it to cease and snap the cable.