Last time I had to remove a rounded oil drain plug bolt, I followed Gilly's advice and used a sharp chisel.

I carefully created a good dent in the direction of removal and just continued to whack. Within a several minutes it started turning.

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)