You can do it running but don't. It beats the snot out of the feeler gauge which will then be inaccurate and unless you have the fine touch of a safe-cracker, you can't be sure it's really right. Yes, I have done it (Fords,Chevys,etc), using the clip-on oil deflectors but it's still messy If you do it correctly, that means NOT running and with an indicator, you can do several valves at a time as both intake and exhaust on several cylinders will be on the seat/heel of the cam.