While their mechanisms are almost as reliable as the Swiss watch, some maintenance is required to ensure that they perform as per specifications. The major threats to distributors are oil, dust / sand particulates. Notice how badly this one is fouled with oil leakage.
The insides of a distributor that is still in pristine condition, except for a slight wetting by engine oil penetration from outside of it's body. I called this "pristine" as the bottom looked dry, indicating that the distributor rotor shaft seal is in good condition. The shaft connects directly into the camshaft drive gear.
Another look at an aged distributor in pristine condition. This one is coming from a 1980 Mitsubishi Galant (4G32 engine).
The magnetic rotor, top actuating plate is removed to reveal a clean bottom, consisting of the centrifugal advance mechanism. When I tried to twist centrifugal advance rotor, it would not move.
It turns out that the shaft is slightly corroded, and the grease have dried and hardened, causing the centrifugal advance to be "locked" in position of zero advance.
This is how the bottom looks like after the cent. adv. rotor is removed. I had to pull it out using a bearing puller (sorry, not pictured).