Agreed there are differences from X to Js mainly the combustion chamber size from what i remember. I do think when people say they "flow the same" they are talking CFM in stock form. I would believe there are differences in the casting but have no evidence. Most just say they "flow the same". On another note what are you guys using to get castings clean? is it shot peening? I am also wondering who does this work at home vs a machine shop? I am interested in a Black and decker style valve seat refinishing kit . They are roughly 4-500 dollars, does anyone here refinish valve seats at home? or is it a machine shop only process? Alot of machine shops are drying up in my area so I am thinking it may be good to start rebuilding OEM style heads at home part time, who knows how it would pan out. May be a retirement gig like sharpening lawnmower blades LOL! For the record I took machine shop in tech school, I just never worked at a machine shop. There was more money to be made flat rate at the time as a line tech. I have a vague knowledge of how to check heads for wear etc and how to repair it. I say vague because I went to tech school almost 30 years ago LOL!