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Why do we need to replace distrib. cap, rotor, and wires?

What happens to each of these 3 that makes them wear out?
How do spark plug wires wear out?
How does the rotor wear out?
How does the cap wear out?

High voltage corona deteriorates the plastics.

They’ll end up shorting out, and if that happens, your engine will run really crappy or not run at all until you replace them.

Wires - the plastic or rubber insulating materials get hardened by heat, and coated with dirt. Eventually the electricity isn’t conducted fully and/or properly. The electricity can jump from one wire to another adjacent wire causing misfires. Depending on the “core” material of the wire some materials deteriorate more and faster than others. Result is misfires again. Finally the boots (or caps) at the plug end and distributor cap end get old and do not seal out the water and moisture in the air effectively - misfires result again.

The rotor takes a beating. It spins inside the cap without actually touching the contact points on the distributor cap. Meaning electricity arcs (or jumps) from the rotor to the cap. Each arc leaves a small pit, like a golf divot. Enough pits and the tip of the rotor gets worn, covered with carbon and debris, and resistance to the electricity flow increases.

The cap is at the receiving end of the arcing so inside the cap there are mini pits on each of the contacts for each cylinder. Also heat deteriorates the plastic material in the cap and some caps can crack which changes the flow of current. As the cap wears and resistance at the contact points goes up the electricity looks for a less resistive place to go and starts arcing where it shouldn’t and this leave carbon tracks in the plastic making the errand current flow even easier.

Maybe this makes sense of the deterioration of secondary ignition components

http://www.highvoltageconnection.com/articles/corona.pdf

it’s the best that I could find.

Spark plugs deteriorate in a few ways, deposits can eventually cause the plug to short (not common now with unleaded gas, that’s why plug change intervals are now much longer than they used to be), the gap eventually becomes larger as the sparks slowly vaporize a small amount of the metal each cycle, and the sharp edges of the electrodes are worn off, requiring a higher voltage to fire.

Great answer Turbo. Thanks. I guess I never thought to think it’s a corona fireworks show inside that dizzy cap.