If the starter will not turn at all, turn on the battery master and the ignition (ignition "on" but NOT the full turn of the key to start the motor) and try shorting the two terminals at the back of the starter solenoid with a srewdriver or jump leads.
If the starter responds, then the problem is likely to be the starter solenoid, or possibly the ignition key positive feed to the starter solenoid is not reaching the solenoid. (It happened to me once... all that happened was that the wee lead from the ignition key to the solenoid had fallen off.... the wee lead is 12V positive, and when the ignition key is turned hard to the right, the wee 12V lead sends enough current to the solenoid to close the solenoid switch and command the starter to turn, fed by the enormous current from the big thick leads from the battery).
The solenoid is just a form of ignition key-triggered switch (a big heavy-duty relay). It is a big switch because of the huge currents that the starter demands. Such currents could never be carried by the ignition switch alone. It would overheat instantly.
If you put a screwdriver across the terminals at the back of the solenoid, you are by-passing the solenoid (simulating the solenoid closing in normal operation). Get ready for a big loud "click" and a big spark as the srewdriver takes the current load. Keep all hands clear of the flywheel, of course.
If the starter does not respond, then perhaps the starter windings have gone.
Make sure you are getting (nominal) 12 V at the solenoid terminal closest to the battery. Just use your multimeter between the solenoid terminal and the battery negative. Make sure your battery master switch is "on", of course, otherwise there would be no voltage arriving at the battery side of the solenoid.
Solenoids are normally reliable units. I have never known one to fail. Others have, I guess.
Stauffer, starters are normally reliable units. Investigate it before you buy another one, or the problem will just follow you.
Best...
Rockter.
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