I would CAREFULLY clean the tops of the pistons but leave about 4mm area arount the outer diameter of the piston alone. You don't want to disturb the carbon seal arount the edge of the piston. Any gouges or scrapes on the pistons or head can cause a hot spot. This can cause pre-detonation or "pinging". Please DO NOT use rotary a wire brush. If you want to use a wire brush, use a brass bristle wire brush, tooth brush sized.
Please let us know how it turns out... I will probably PM you to find out all the ins and outs. I bought my wife a '94 E320 wagon and It needs the same thing. Hope all goes well CHris

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.