Starter motor sparks

Situation is old rusty bulkhead (for sizing and testing only) engine and gearbox on a new rolling chassis in process of rebuild......

Hi all, 2M100 starter from a petrol series 3. It did turn a few times ok out of the car. When connected in position the battery terminal on the back of the starter twisted and when we tried to turn it over nothing happened.
Removed and opened starter to find the wires twisted off as expected I resoldered them back. After re-assembly and reconnecting battery all I get is sparks. I didn't see any insulation at the terminal ends.
Any idea what is the problem? I have ordered new brushes to replace later .
cheers Peter
ps the test was done on the bench

Update..... I wrapped some insulation around the stud where it passes through the starter body and it worked ok.
That will have to do until I can investigate further or someone comes up with a better solution.

cheers Peter

How about a piece of nylon air line, that is available in 6/8/10/12mm inside diameter with a 1mm wall thickness.
Slipped over the stud.
Tough stuff too.

Hi Flossie, thanks for the input. I have since found some electrical sleeving of various sizes and colours in my home wiring box.
I don't know if it is shrinkabe or not but it stretches over the stud ok and into the starter housing.
I am wondering why it shorted, when there was no insulation fibre washers etc to begin with.

cheers Peter

Could it have been rubber that turned to dust?