So when I get that plastic piece, I simply cannot put that onto the metal piece. Why cannot I just do that?

What is a motor dremel? Will a screw hold that plastic in place for an extended period of time?

Do I have to remove the mushroomed piece of metal and get a new piece of metal to attach the sliding jaw?

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)