1983 380 SL
Almost, but not quite.
What I need is a counter bore for a TimeSert insert and not a counter bore for a standard 10 mm bolt. The cutter size of the counter bore is larger in relation to the size of the pilot than a standard counter bore by approximately the thickness of the wall of the TimeSert itself.
However, I believe I found the solution... I need a counter bore with interchangeable pilots. I went to the Global Industrial web site and ordered a 1/2" 3 flute counter bore and 2 interchangeable pilots, a 3/8" (.375) and a 13/32 (.406). The pilot in the counter bore supplied with the TimeSert kit measures .402 and it fits into the tapped holes with a little bit (very little bit) of play so I figure a .406 will fit better. I ordered the 3/8 (375) just in case the 13/32 is a bit too big. If one doesn't work the other one will.
So this will solve my 4 mm recess below the surface problem. I can counter bore as far as I need with the counter bore I ordered because it doesn't have a built-in stop. It will keep counter boring as long as I keep pressing it into the hole. Inasmuch as all I need is to increase the counter bore depth by 2 to 3 mm, a light pressure is all that is needed. I'll probably wrap some tape around the flutes of the counter bore at the 5 mm mark.
I also may have figured out why Mercedes wants the threads to start at least 4 mm below flush. It was actually AlterSchinken who filled in the missing information when he said (I think it was him) that 70% of the clamping force is exerted on the first 3 or 4 threads. At first I was thinking "so what" you'd still have to distort the metal for that to be a problem and how likely is that to happen?
Well, it's probably not likely for that to happen but it's not impossible either... and why? Because if you don't counter bore the threads then the first 3 or 4 threads will be sitting in the first 5/32 of the block metal but by starting the threads 4 mm below the surface you are spreading that 70% of force over twice the thickness of metal. Twice the metal is twice as strong (I guessed at that but it's stronger for sure).
Of course I'm only guessing that's why they counter bore 4 mm into the block... not because there is concern for the non-threaded portion of the bolt to bottom against the insert and not because the insert might pull up and bottom against the head... but for the simple reason that it gets twice as much metal to resist the force of not only the clamp pressure of the torque on the head bolts but the constant pressure of 4 explosions going off thousands of times a minute. Again... all guess work on my part