It's not Just a plastic washer it also centers the post. Without the propper washer the post could move and short again.
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This statement confuses me.
That black wire is not a ground, it is the charge wire from the alternator to the car then ultimately to the battery.
I assume it was grounding the output of the alternator to the alternator housing the way it was attached which would also ground the entire car wiring system causing the fusable link to blow.
BTW a fuse CAN work in place of a fusable link BUT a fusable link can handle a lot of excess current for a short amount of time without damage. Typical fuses blow fast, not what you want in that cir
Plus what amp fuse did you put in?
I would get a propper fusable link