My borer simply drilled the piston with two 1.5mm holes either side of the piston so that they would lubricate the area either side of the exhaust port- basically they're roughly in line with the ring pegs and approx 2cms down from the top edge of the piston (close to the underside of the gudgeon pin). The holes on the outside of the piston have been countersunk by about 3mm.
At the moment no problems................................................

Chris