Antikythera mechanism
There were 82 parts in total. With the parts found it is not possible to get the mechanism with all displays and dials fully operational. It is also not possible to know how many parts have been lost. In total, 30 gears were found. One gear is probably part of a planetary display, that has been lost. To make the reconstructed displays work, another eight gears would be necessary, but they have been lost. There are two crown gears, all the other gears found are normal gears (that require a special alignment of the axes). The gear was a primitive one, and the transmission was not uniform. The mechanism was repaired at least once, which shows that it must have been used often.