Take the nut off the starter post and remove the wire. then snug up the other nut on the post (don't go crazy or it will break the plastic insulator that the post is mounted in) then reinstall the wire and first nut. there are 4 brushes in most starter motors. 2 are wired to the post and 2 to ground. when the nut gets loose (no idea why they will spontaneously loosen) you will either end up with a weak connection to both brushes, or total loss of connection to one set.... then the other set tries to do all the work hence the heat
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