| Hi Alan, Interesting one.
Many years agao I bought an impact screwdriver in the states to bring home and use on my Jap motorcycles. Worth their weight in gold in the days of overtightened crosshead screws!!
Anyway I lent it to a couple of the crew who were taking up engine room floor plates and they consequently batterred it to death with a big hammer! I took it apart and discovered that the only real damage was in the ball bearing carrier cups which had split. I therefore needed to make a couple of cups from mild steel, approx 8mm OD and about 1mm thick and about 5mm deep.
The best way I could think of starting was to drill the internal bore but I needed a flat bottom and we didn't have any milling bits in those days.
Back to your point, I got a standard twist drill and ground the tip with the normal two cutting faces but with an included angle of somewhere in the region of 175 deg. Surprisingly it worked perfectly. I actually started by grinding the end completely flat then lifting the cutting edges by a couple of degrees each.
In effect, for your job, you are not even cutting the whole face, only a shoulder so I would have no doubt that a suitably ground twist drill would cut the counterbore that you are after.
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“Dirty British coaster with a salt-caked smoke stack, Butting through the Channel in the mad March days" | 
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