There are, indeed, plastic gears in the tach with very thin gear fluid. It is pretty simple in construction. There was a time when the tach shaft slipped out of one of the gears, and I was able to push it back in and reassemble. Alignment was done with the needle in place and pushing it counter-clockwise to slip it in the home position.
Of course, this is not possible with all cars like the Honda Prelude I worked on. Once the shaft slipped out, it was broke.
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