Dont know what your doing wrong but I have removed allot of MB allens and not had a problem and I do use the impact wrench on flex couplings. I did have to use an "easy out" on a motor mount allen that someone had managed to "round out"
Make sure the allen is cleaned out well before inserting the tool.

__________________


1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"