Installing heli-coils with the head on is not a good idea UNLESS you isolate the area & have a shop vac to suck away the debris as you drill & tap.
WE use Time-serts instead of heli-coils as they "hold" better. A 8mm kit comes with all of the needed tools except the drill motor.
Make sure to use NEW bolts 8 x 70 MM that are 10.9 hardness.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
42 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)