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Some are castle nuts or have the castle lock that goes over it and gets a cotter pin. Some nuts have the teflon ring. Some have a lock tab underneath.
I'm not exactly sure about that year and model.
I can tell you my 2001 Merc 25 has an all plastic nut. I can also tell you my '86 & '87 50 & 35 Mercs have the tab underneath you bend up to lock the nut.
