I use an electric die grinder, and a speed controller to tailor the speed of the carbide burr to the job. After I wore out several air die grinders and a compressor or two porting and polishing race engine cylinder heads, I decided that a 1/4" shank carbide burr in an electric grinder was the way to go. Sandpaper "cartridge rolls" to do the final shaping.

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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!