My 1985 F150 wont crank at the key, but when I arc the solenoid it starts.

There may be a relay involved , What shops did you take it to , ice cream shops ? It just seems odd that the " mechanics " couldn't find the problem and you end up here .

Power from the crank circuit from the ignition switch goes to the clutch switch, when you push the clutch the switch closes and send that power to the starter solenoid’s S terminal located on the passenger fender, that power will close the solenoid and porcine battery power to the starter. You may need an wiring diagram for the starter circuit or even the steering column. If you can’t find it online then the public library in the reference section might be your best bet. Or buy a shop manual.

The clutch must be depressed for the engine to crank. There's a clutch safety switch that will prevent the engine from cranking if the clutch pedal is not pushed down. It's the same as a neutral safety switch on an automatic transmission vehicle. For testing purposes, you can hook the two wires that plug into the switch together. Make sure you have the transmission in neutral so the vehicle doesn't lunge forward or backward if/when the engine cranks during the test. If it does crank, the switch is bad. Hope that helps! Jim