Ammeter heating up.........

The above is "exactly why." WORSE on some of the newer "stuff" the housing for the cluster is PLASTIC

There is nothing, really, holding the studs to the meter guts except the nuts. So if something works loose, it makes heat, "melts" (softens) the plastic, and gets worse

One way ---if you insist on keeping the ammeter, and I used to like them too-- is to silver braze (solder) the studs to the shunt. Then with that "hard" connected, you can "double nut" the studs and the two nuts will maintain the connection.

Lucky you found that, before the smoke all fell out. We would have to call the EPA LOL