Hi,

If this a Bosch Starter the problem is inside the solenoid.
The coil plunger that get pulled back and is short stroking internally.
It is a manufacturing assembly tolerance problem as in that it's just a "hares breath" away from not having more than enough movement to fully push the contacts further together inside when made.

You can test the solenoid with an ohmmeter attached across the contact studs.
Put the solenoid in a vise and then assemble the plunger into the coil body.
Push the plunger back against a spring until you hear or see a continuity signal.
You want to be able to continue to push the plunger deeper after hearing or seeing that continuity signal for another 1/16" or up too about .100 of an inch. 2 to 3 mm for metric folks!

There is a spring that loads the contact bar against the terminal studs the wires fit on.

It goes like this,
After a long time of use, the contacts plates, down within the closed up body, dig a burn pit hole into either the bar or off the faces of the studs. A little of each adds up.
Eventually, there is enough space created between them, that the application of an internal spring cannot get them together tight enough or solid enough, to make a good circuit.
I know this from actually destroying a coil body as they are crimped sealed.

The contact bars are not that hurt at all but just can't get themselves together.
The whole thing goes back again to lack of pushing on a rod under the plungers core.

That rod is hit by the same rod you see sticking out with the slot in it but mounted in that solid chunk of steel! The rod I'm talking about has a slot that fits over a lever that pulls the bendix gear drive into the flywheel.

The rod with the slot needs to be pushed farther down into the steel body about 1/16 of an inch.
The rod has a slight press fit but can be push with your vise jaws.
Only trouble is, it's held onto the plunger body by a tiny tack weld.
Grind away the tack weld just enough to push the rod in farther and tack in back together and you are done.
Basically there is just enough length left on the slotted rod to get it there, they just didn't use it.
Call it an oversight, undersight or plain short sighted, might work as well! (:)

In my case, to learn about the issue, it was far cheaper to get a whole stater from the wrecking yard for $35 and remove the solenoid.
I made my modification and put that solenoid back on same starter that the old solenoid came from. This proves that the rest of the starter was working, even though, I clean some brush dust out and checked the brush length. Over 250k, same starter at the time, plus 15K more now!

These starters will last a very long long time if the engine was kept in good tune during its life!
I did mine over two years ago and the car was a used 1992 and it was unmolested!
I know this because that big armed Swede, everyone complains about, put in in there from the factory! (:)

If you have other starters laying around or just the solenoids, in a wait mode, I suggest you test them in a vise with a meter.
Make sure you have extra stroke after the contacts make up to each other!

Hope that sheds some light for someone in the future.
Maybe I need to make this post a thread of its own!
You tell me?

Phil