seems like yer a little confused. milling a head is a simnple maching process where a surface grinding machine take .030 inches (typically) off the bottom. it raises compression, but is not adjustable like an aftermarket head, so there is no choice of dome sizes. the advantage here is cost, its pretty cheap to have done. it will make your shee have more lowend torque, good for the holeshot. good luck with yer tuning.