Bill,

The usual approach is to drill the hole almost to full depth and then swap the drill for a specially ground flat bottomed drill to create the bottom of the hole.

The skill lies in getting the geometry of the second drill end correct but it can be done in drills of this diameter. In my professional life we used to call on the services of a particular man in the toolroom to create flat bottomed holes in all manner of exotic aero engine materials. These were 0.050 inch diameter [ actually 3/64 inch] and about 1/2 inch deep. Skilled work indeed with sensitive drilling machines.

Regards Brian

…or, if appropriate, you might consider a through hole with an inserted plug – secured by your favourite method.

I normally use a milling cutter for flat bottom holes, but none of my 3/16 cutters are any where near long enough for a 1" deep hole

I have just realised what it was about this thread that had unsettled my tiny brain. I have followed it from the start hoping to find out what engine had a piston 3/16" diameter (I thought a Mamod would be larger than that).

The unsettling bit was trying to get my head round a hole with an 'outside' diameter of 3/16". All the holes I have come across have an inside diameter!

Ian P

(As to the hole in question, for me its a long series slot drill to clean up after normal drill, or investigate why the hole needs a flat bottom and whether it can be eliminated.)