I have an air grinder. Wife bought it for me as an xmas present. It's a nice heavy duty Canadian tire brand but is a single speed. I could probably add an in line valve to regulate air pressure and speed.
Its good though. I really like it. I bought some basic carbide cutter shapes to speed things up. I also have a Mr gasket sand paper roll kit,which is really nice. Cheap and lots of different things in it. Also cuts faster than I thought it would.
The biggest thing with porting is to ALWAYS keep the cutter moving and NEVER force it to cut. Let it do the work for you.
A 1/2 gallon or so of coffee works well too....
Here is an excellent read on the subject. Print it out,because this kind of info has a tendancy to cost money sooner than later.
http://www.mondellotwister.com/ArtHeadPort.htm
