Been meaning to look into this for some time, just needed an old starter to experiment with. Thanks to Jon Bovey, I have one and here is what I found.
Most of the problems I have been hearing are about the starter working when cold and then either intermittent operation or not working at all when hot.
I took an ohmmeter and connected from the soleniod terminal ( the one from the ignition sw.) and ground. Reading 0.6 ohms. Wow, this is low. This would draw 20 amps to operate. Too high I'm thinking. So, I took a heat gun and heated up the solinoid to about 180 degrees- now 0.4 ohms. Worse!!
For you non electricians out there, a 0.6 ohm load will draw 20 amps and 0.4 ohm load will draw 30 amps. I believe this circuit was designed for a much lower load. I=V/r, basic DC theory. Battery voltage =12 volts, resistance = 0.6 and 0.4.
I disassembled the solenoid and cleaned it up. Blew it dry with compressed air and let sit to completely dry out. Measured again, and now I have 1.9 ohms cold. Don't have my heat gun to heat it up but you can see the improvement. 1.9 ohm load = 6.3 amps.
There was a lot of gunk around the connections and inside the cap of the solenoid. While I was in there, cleaned up the battery to motor contacts which were pitted and dirty also.
So, my theory is: a lot of our starter problems come from dirt and oil mixing and getting into the solenoid, causing high current draw and the system is not able to supply that much, so no worky, or intermittent.
There are two small connections on the solenoid cap that are soldered and you need to unsolder these to get the cap off. Wish I had a camera, but they are not allowed here at work and I don't have a computer at home.
I had a 70 1.7 back in the mid 70's that had this heat soak problem and here is what i did to overcome it. It would only happen rarely so I got a push to start switch from the parts store and mounted it in the engine tin right above the starter and ran a wire from the starter bat terminal up to one side of the switch and the other wire from the switch down to the solenoid where the ignition start wire is. There are two spade connectors there on the solenoid. Make one wonder why!! This set up worked just fine for years. If it wouldn't start with the ignition switch, I put it in neutral, key to on , pop engine cover, and push the switch. Car starts, close cover and off I go.
Hope this has been a help to some of you out there.
Tom

`Interesting information Tom. Glad I could be a part of this. So I gaver you one starter and then gave another to Kelty later that day. I guess it was a good thing I had a few laying around