My mechanical tachometer or speedometer is seized (cannot be rotated).

A mechanical speedometer or tachometer that is seized (cannot be rotated) at its input end is caused by excessive pressure of the flexible shaft core pressing against the magnet shaft.

This is revealed by a flaring of the magnet shaft hole and is not the fault of the instrument.

To avoid this condition use a shorter core or a flexible shaft which employs a compensating tip at the lower (drive) end. The latter design enables the core to "float" in the casing.