Another method, which I have used, is to find a hardened bolt with a head of the correct size (22mm or 7/8in, for the current application). Screw down and tighten real hard three nuts on the bolt. Then put the head of the bolt into the allen cavity and turn it counter clockwise with an open-end wrench on the nut nearest the bolt head. Use a hardened bolt dso as not to break off the bolt head in the allen cavity with attendant problems getting it out.

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Douglas
1959 M-B 220S cabriolet