That's also known as "pocket porting". It can be done with a hand-turned reamer with an angle of 70 degrees or so, or by carefully hand-shaping the port with a cone-shaped stone in a die grinder. If you cut too deep, you end up with a water-cooled combustion chamber!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!