t.leslie will likely give you measurement guidelines for V8s. Don't know of any measurement guidelines for inline 6s. If you have access to water manometer you could check pressure inside cowling. New blades will push 8" with hot oil and rated speed in both V8s and inline 6s. I have had pressure as low as 4" but cooling becomes a problem then when it gets a little warm out. I would say anything below 6" would be pushing it if you get a hot day. Worn blades will start to make a screaching noise in air intake area for it starts slicing air poorly. Also good to replace blade early rather than late for it will likely be getting more out of balance which can cause more problems. We replace the drive rubbers and bushing that aligns that area on V8s to prevent problems that can come up if put together worn. Watch the area that bushing wears on for we will have that built up and turned down if it isn't going to hold things straight. Good time to clean heads very well if doing the job. look very close to see air passages are clean all the way threw. May need to soak up with pressure washer and keep at it until its good and clean. If you get coolers wet with water then keep at them also until mud is out. Most of our cooling problems is dirt or mud in hard to see passages or within cooler where you can't see. Good luck.