" The bendix is strictly a mechanical unit that relies on rotation of the starter."
All car starters I’ve seen use magnetic force and a lever to move the pinion into the flywheel. I have, however, seen small engine starters that just use rotation to do this.
Is the starter being discussed a common old style Ford starter with a shoe on top or a Delco style starter with a solenoid on top? The old Ford style used the shoe to pull the bendix into the flywheel. When the solenoid energized the starter field windings the shoe was pulled down by the magnetic pull of the adjacent field coil. If jumping the solenoid causes the starter to spin without the engine turning the most likely problem is a failed bendix although a stripped starter ring on the flywheel might be the problem but that is less common. If the starter has the solenoid on top as well as the one on the firewall there might be a problem with the wiring from the external solenoid to the starter mounted solenoid.
I think a picture of the engine and starter for identification would help others in diagnoses. There are a number of configurations, knowing what you are working on would help.
This is from the 1978 MOTOR Auto Repair Manual showing a connection from the solenoid to the neutral safety switch.
I’m wondering if there is some confusion here about the OP’s starter motor/solenoid configuration. I have two vehicles, a 1970’s Ford truck, and a 90’s Corolla. The starter solenoid configuration is much different between the two. On the truck the starter solenoid is a separate gadget, located near the fire wall in the engine compartment. On the Corolla, the starter solenoid is part of the starter motor itself. The truck solenoid has four terminals, 2 thick, 2 thin. The Corolla starter solenoid sports two terminals, one thick, and one thin. Plus it has a sort of hidden thick terminal, for debugging purposes, used to test if the solenoid contacts inside are on the fritz.
OP, is your vehicle configured more like the truck, or more like the Corolla?
Some Ford products use the remote solenoid and a Delco type starter solenoid also. Those with the redundant solenoid can be improperly wired so that the starter armature will spin without the bendix being moved into the flywheel.
It certainly depends on the application/configuration. I am curious of what sort of “kit car” this is. The most common old kits were VW based but seem to be rare today. Of the Specialty car manufactures, the Gazelle is built on a chevette chassis, Zimmer used Ford chassis.
If it’s an Excaliber or Zimmer I’ll be over there with cash.
Well on our old John Deere we just wrapped a rope around the crank pulley and pulled when the starter didn’t do it.