If you have any lattitude I recall optimal is 1/3 in. But do some checking on my recall. Other considerations are the thermostat design being used and the accuracy of the gage.
Update. Finally figured out that I had a big block fan shroud (22 inch) and an 18 inch fan which was the original fan but not original shroud. I listened to TimR's suggestion to replace the fan with a 20 inch fan and now it's like night and day how the cooling system responds. I even had to put my 180 degree thermostat back in to keep it from running too cool. It was running 160 while cruising. Everything I did improved the system but nothing helped as much as this fan to shroud matchup. Thanks to all especially TimR!:beer
Glad to help :beer Those 5720's work awesome. inexpensive, well made, no worries about 40+ year old cracked blades or loose rivets, now go enjoy the car!!! :3gears:
Just another note. They quit making the 5720 fan a couple of years ago so I had a heck of a time finding one. I did notice that Summit has what looks to be a similar fan made by Durale No 17320. The bolt pattern I'm not sure about though.
