We have had an ongoing issue (not at our track but at others) with 4002 motors getting such a big short it heats the endbell hardware up such it melts itself to destruction (spring).
For the first time it happened today at our track.
A visitor from another track who isn't a motor fiddler was driving his car and stopped for a while. When he went to go again 'poof' smoke arose from the back of his car and it was all over.
The positive brush got flung out by the spring releasing its tension once the screw pulled free of the melted endbell and fused itself to the body - it was that hot! Neither brushes showed an abnormal wear pattern.
We put another motor in and got him going and I kept the parts I could for review.
A quick autopsy tonight revealed nothing more than a dirty comm (oil?) and some bridging of one comm segment to another.
Thoughts on why? It was running strong before this happened...