I have a 91 300se. I need to get the old alternator out and put the new one I bought in. However, I am having troubles taking one bolt off of it. I have tried everything and the bolt is almost stripped. It seems to be smaller the a 16mm but larger then a 15mm socket. Does anyone have any ideas on what size socket I can use in order to get this last bolt out. It is the only bolt that connects it to the engine.

Did somebody ram a 5/8" bolt in there?

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2007 C 230 Sport.

5/8 would be almost exactly the same as 16mm. Try an 11/16 wrench on it.

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1989 300 SEL that mostly works, but needs TLC

Hmmmm, 25.4 mm : 16 = 1.58 x 11 = 17.46 mm.
25.4 mm : 8 = 3.17 x 5 = 15.85 mm.

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2007 C 230 Sport.

Is that supposed to mean something?

Try this: 16/25.4 = 0.629 and 5/8 = 0.625

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1989 300 SEL that mostly works, but needs TLC

He said 15 mm is too small & 16 mm is too big.
Therefore he's looking for something between those two.
Your suggestion of 11/16 " is way too big.

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2007 C 230 Sport.

And so it is. I'm gonna blame it on the late hour, and not native stupidity. That's my story and I'm sticking to it.

That said, I re-read the original post and see that he's talking about a damaged bolt head. I've had great luck with these:
Bolt Out

They also work well on stripped allen-head bolts.

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1989 300 SEL that mostly works, but needs TLC