I have to jack up the 1991 560sel to check the brake rotors and pads for possible replacement. I'm planning on utilizing a hydraulic floor jack under the jack points, but once the car is off the ground where do I place the jack stands?

For the front, place the jackstands on the unibody frame. For the rear, place them under the rear subframe mount.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

yes, but can't they cause the frame rails to bend?

You can either put them on the sub frame mount, frame rail or on the pads sit on the rocker panel Keep the jack under pressure as a secondary precaution.

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With best regards

Al