Hi SB5 ,

(1) Use Dremel type tool with the right angle head .

(2) Make what is effectively a pilot drill out of silversteel . Make a short D bit type contersink with a hole down the middle . Bond in a good length of rod just smaller than stay hole . Insert into hole from inside boiler and grip pilot part on the outside and twiddle . Could be done under power but just turning using a small hand held chuck probably enough .

(3) There are commercially available tools :

(a) A hand powered tool witha spring loaded collapsing cutter . Can be pushed through hole easily and when inside blade springs out . Smaller ones are just pulled back out again when done .

(b) A hand powered tool with no proper name but sometimes called a 'skrawker' . This is just a thng like an L shaped dental tool with a single sided backwards facing chisel blade on end . Small enough to wiggle through hole and then just used as a scraper .

Notes :

In aerospace industry there are exotic methods for deburring internal holes and passageways . Most common is abrasive slurry pumping .

In case of stay holes in copper boilers don't go much past the basic deburring stage – certainly don't make a significant countersink .

Regards ,

Michael Williams .