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How to adjust the accuracy of a mechanical watch

For modern watch movement, there is a "Fast , Slow" handle.

Mechanical Clock - for easy viewing the parts which are similar to the mechanical watch movement
Clock Balance Spring Clock Balance Wheel

The Balance Spring pass through a slot which is the ""Fast , Slow" handle"

After removing the balance spring, you can clearly see the slot (red ellipse).

If there is a "Fast , Slow" handle, we do not have to remove the pin in order to adjust the Effective Length of the balance spring.

Assume all factors are unchanged, if the Effective Length of the balance spring is extended, the watch runs slower. If the Effective Length of the balance spring is shortened, the watch runs faster.

Balance Spring Effective Length

Balance Spring Effective Length = A to B

In page 1, the Unadjusted Movement does not have the "Fast , Slow" handle; therefore, B is the same as C which is the place of the Pin. So, removing the pin is required for adjusting the accuracy of the watch. Pull out the pin is easy; however, put it back is nightmare !

Mechanical Watch Movement

On a modern watch movement, you can find the "Fast , Slow" handle next to the end of the balance spring C which is locked in place with a screw rather than a Pin.

If you have to adjust the accuracy of a watch, you can easily turn the handle to the left or the right according to the indications : + faster , - slower.


The "Fast , Slow" handle make the task easier, but you are recommended to try and learn on a useless movement first because the Balance Spring is the weakest part of a movement.

P 1 , 2

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