I bought a Zenit EM recently for parts, to strip it for the light meter. Butt the meter Cell didnt fit the camera I wanted to use it for, so I decided to put back the body together.
It suprised me to see a gear which lost 2 teeth! Its the gear with the spindle that flips up the mirror. More suprising was that it was made of plastic. The same type of grey'ish plastic used for toy soldiers.
This is the first time I found a plastic gear in decent KMZ camera. Maybe it was a test or a matter of cost. However, its one of the most important and essential parts!