What's the best way to adjust the A/C belt pulley? I put it on using hand tension while someone else tightened the bolt, but it's squealing a little on takeoff from idle.

Is there a special tool or trick?

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2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey

SOLD:
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino