[Paul Precht]

A 915 head never milled is frequently about 84 CCs, theoretically a .062" cut would bring it to 74CCs. A 906 head never milled is frequently about 88 CCs, theoretically a .058" cut would bring it to 74 CCs so your valve to piston clearance would be similar but could go either way depending on the casting. A nice set of 915 heads will cost 500 and up, 906 heads are like grass, they are everywhere and can be had for nothing sometimes.