That job isn't a DIYer. You need a drilling jig($400) to drill the holes at the correct angle! Also you need a special starting tap to guide the tapping process.
Doesn't matter if you use "Timeserts" or "heli-coils" you still need the drilling jig.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)