[truckster]

There's a thick copper washer in the starter solenoid. When you turn the key, the voltage goes to the low-amperage side of the solenoid, creating an electromagnetic draw that closes the high-amperage side of the circuit. That contact occurs between the copper washer and two large contacts. After many cycles, carbon can build up on the copper and prevent the circuit from closing.

If you disassemble the solenoid, you can clean the copper disk and contacts with emery cloth and that should solve your problem.