Back in the day, we used to Heli-Coil all the head bolt holes whenever the cylinder heads were removed. The reason was simple. Nobody wanted to take the chance of stripping the threads during re-assembly (this happened frequently).

To make things easier, MB provided us with some special tools to perform this repair. One of them is a drilling jig (#117-589-02-23-00), which bolts on to the mating surface of the engine block. This jig was to assure that the original threads could be quickly and accurately drilled out. I'm sure that most MB dealers (like mine) still have this jig hanging on the wall of their tool room, although we don't use it much anymore.

MB also offered a Heli-Coil kit specifically for this job. It included a tap with a pilot shaft, a full depth tap, and enough Heli-Coils to do the job.

Once the heads were removed, it took the better part of a day to drill, tap, and install the Heli-Coils (there are thirty-eight in all), so like Steve says, it adds a considerable amount of labor to the job, and when I say labor, I mean labor. Anyone who has done this job will tell you that it's exhausting (especially on the wrists and forearms).

These days, many MB's that require this repair aren't worth the expense involved, so many owners look for cheaper alternatives (like used engines).