I wouldnt worry much about it.Very few automotive alternators have had keys in shaft for the last 20 years or so,tighten the nut properly and it wiil stay.If you are concerned about it put a little Blue loctite on the threads before you put the nut on. My$.02 Don

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Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles