Thanks guys for the suggestions. My w124 wagon has those rubber protector strips on the roof of the car, so I have never used my crossbars when transporting 10-20 2x4s from the lumber yard. But my w210 does not have anything on the roof other than the side rails of the MB roof rack. So I had planned to leave the cross bars on the car at all times, so that, without advanced planning, I could put something on the rack without it actually scratching the roof paint. I don't do a lot of highway driving these days, so the fuel economy hit wouldn't be big. I know they aren't supposed to actually hold a load, but they are actually pretty stout pieces of metal. I bet they could support 100-200 pounds if it was distributed like pieces of wood. But yes, for a kayak or bike rack, quality aftermarket crossbars would be necessary. Thanks again.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).