Repairing Plastic Cannon Pinion on ca. 1960s Cuckoo Clock

Here is a German one-day musical cuckoo clock from about the 1960s. It ran and cuckooed and the music played fine. But the clock would stop once per hour, always with the minute hand pointing to 9 on the dial. When I moved the minute hand forward, I could feel it turn hard when moving it past 9.


I removed the movement and found that the plastic cannon pinion had a crack. I’ve seen this particular movement many times. Some have a normal brass cannon pinion, but some have a plastic one.

The cannon pinion is the black plastic gear in the center (with the two lifting arms).
The minute wheel is the gear to the left of the cannon pinion.

I repaired the cannon pinion:

  1. Glue the crack with Gorilla® Super Glue and close the crack by clamping the pinion. Clamp it overnight.
  2. File out the center of the pinion until it is an almost free fit on the metal spline on the center arbor.
  3. Secure the pinion to the arbor with Gorilla® Super Glue.

The cannon pinion doesn’t have to deliver much torque, so I expect this repair to last. It split because  it was driven tightly onto a metal spline.

Repair job 6224.

See more photos.

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One comment

  1. Thanks for posting all this great information! About the plastic gear cracking… This is something that I’ve studied a lot about and it turns out that while it was a quick and inexpensive method of fabricating this type of pinion the fatal flaw wasn’t reveled until a decade or so later. As the plasticizers evaporate and UV and ozone cause the molecules of plastic to cross link the plastic both shrinks and becomes less ductile. This is what causes the crack, and depending on the material, every one of the the parts they made will eventually crack. I’ve remedied this problem in microscopes by having a brass gear produced to replace the plastic one. I currently have a Bausch & Lomb microscope with a similar problem. I think I might try some epoxy adhesive rather than cutting a new gear…

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