I had this happen on my 82 300d. The bolt holding the mount was I think a 7mm allen hex. I think I turned it the wrong way first, rounding the hole. The fix was easy. I used a the largest titainium drill bit that would fit in the allen hole and turn without binding. It went through that hardened bolt like butter, realeased the pressure on the threads and came out by hand. I took the motor mount off and put a thread insert in it.

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82 300d 194
75 240d sold
Gulfstar 41 4-107 Perkins Westerbeke 4.4 Gen
Puegot Bicycle Legs
AYP Lawn Tractor Briggs 12.5