Starter Stuck When Engine Very Hot

The starter motor in my mongrel Mk I with not turn the engine one iota when the motor is very hot which can happen when the car runs in city traffic for some time, and then the engine is turned off. In this case the water temperature gauge pegs at some maximum point. The water temperature also gets at about 100 ^0C when the car climbs a hill as fast as it can. Then the gauge also pegs after the motor is turned off. In all of these cases the radiator does not boil and lose fluid.

The motor is a 3.8 early E-type with an early MK I 3 head using 2 SU Carbs. I have no idea what the starter is.

As soon as the temperature cools to about 80 ^0C or so, the starter again turns the engine over at a merry rate.

So what is the mechanism for this untoward behavior?

It could be various things.

When it’s hot does it engage and start to turn but stall, or does the solenoid just click but the motor fails to turn?

Start with the easy things:

Is it the battery? (try jumping to another car while the Mark 1 is hot).
Is it the earths? (Check the battery to body earth point, and the body to engine strap.)
Is it the power wiring? (Disconnect battery and snug up all the large wires.)
Is it the external solenoid? (iirc the Mark I had an external solenoid with a manual pushbutton.)

It could be insulation breakdown in the starter itself.

My experience is that about 80% of the time the problem isn’t the starter itself.

It does not start to turn at all.

I have not heard a solenoid click.

If you have the traditional Lucas starter solenoid they usually have a rubber button on the end that allows the solenoid to be operated manually. Try pushing the button when engine is hot and see if the starter will spin.