The bits for cast iron and aluminum are different - coarse for aluminum, fine for iron.

Not sure what you mean by "double cut".

You'll need an assortment of cutters as well as different lengths.

Use a good carbide bit and they last - should be no problem doing a set of heads with a single set of bits. Use cheapies and you won't get a single port done.

Electric grinders spin about 25,000 RPM. The problem is they get HOT. Also, I've found the bearings go out of them really fast because of the side loads and speed.

Air grinders are a bit slower and stay cool BUT you need a serious compressor to keep them going.

I don't do any jobs as big a heads any more so I stick with the air grinders