Porting Tools The easiest way to begin is by picking up a porting kit from Standard Abrasives that includes most of the abrasive materials necessary for head porting. Either an air or electric grinder capable of at least 10,000 rpm is required, and the abrasives can safely handle 18,000–20,000. Judson prefers using a Milwaukee 5196 electric unit. Additional necessities are a selection of carbide cutters, radius and telescoping gauges, a protractor, dial calipers, scribes, different–length mandrels, cartridge rolls for finishing, and lights with magnetic bases to help illuminate the ports. Port Volume A common method of determining properly sized cylinder heads for a given displacement and rpm range is port volume, but it’s not necessarily the most precise method. Port volume doesn’t mean anything," says Judson. "All that matters is cross section, because you have to compare like ports. You can’t compare 23–degrees heads to 18–degree heads since the longer runners in an 18–degree head means it has more port volume for any given cross section." Generally, port volume is just a substitute for measuring cross section, and the larger the cross section, the higher the rpm the motor must turn. Here’s the industry standard formula for determining the proper average cross section of a port: Port speed=piston speed × (bore area ÷ port average cross section)