Starter hums will not turn engine over.

[Fred K-OR]

May also be a bad starter switch.

[ryanheacox]

Originally Posted by Terry, NJ

It's a little hard to "diagnose at a distance" however, if it won't engage with the flywheel, you an bet on the the problem being with the Bendix. When you say, "it worked perectly when you had it out of the car and tried it, did the Bendix move on it's shaft to engage the flywheel and return? Is it free? Because of a bad experience I had many years ago with grease , oil and a stuck Bendix, I prefer to oil it and wipe ALL the oil off till it's almost dry, or even dry. It's the most natural thing to lubricate! It seems to cry out for grease or oil. Don't do it! Even light oils can stop it from sliding. Generally speaking, a Bendix doesn't require any maintenance beyond checking the 3/8 bolts that secure it to the shaft. These are sometimes a problem.
Terry

I'm going to third this idea. When reassembling my bendix I naturally assumed that it needed lube so I smeared on some lithium grease... bad idea. Mine still engages in cold temps after bliping the starter a few times but it will come out in the spring and get cleaned up.

[TerryO]

https://www.fordbarn.com/forum/showthread.php?t=257702