I'm pretty sure they are 5mm. On my 300SEL, the bottom one is tough to reach, I ended up using a universal joint with a 5mm hex bit socket to pull the bottom screw.

Other than that, it was straight forward.

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino