I was having trouble with by 69 bb overheating. The mechanic looked at it and determined the clutch fan was not locking-up when the engine started to heat-up. He replaced the clutch fan with a new one. I still have a problem with over heating. When I check the clutch fan when the engine is hot (and turned OFF), I can easily turn the fan. I thought the correct function of the clutch is when it becomes warm it starts to lock-up - is that not true??

No, it dosent work like that, The heat makes the fan spin faster. Once its stoped running you cant feel it (locked up) . Do you have the shroud and a good stat? Does it get hot only when idleing for a long time?

I have a fresh frame-off restoration - bb with factory a/c. Shroud is believed to be correct, with a fresh 4 core rad. Condensor was checked by a rad shop as well. The car will over-heat at hi-way speeds (guage into the red), and around town if I use the A/C. When I stopped at home and shut off the engine (guage indicating in the red), the clutch fan could easily be spun by hand, even though it is brand new.

There are 2 types of clutch... 1 is a non-thermal clutch...2 is a thermal clutch and they do the same thing as far as cooling. The non-thermal clutch is what i have and at low rpms it locks up and you can feel the flow with the hood open. The thermal clutch has to get up to temperature before it will lock up and to check if it is working correctly, simply when the car gets up to temp put a piece of paper in front of the grill to see if it is pulling air, you can also try this before it gets up to temp.

Find out if your mechanic installed a thermal or a non-thermal clutch.

thanks I'll find out.

And when it locks up, if you're standing next to the car, you should know instantly. Ya know,,,maybe you should consider calling Summit for a clutch. How well do you know and trust your mechanic? How many blades on your fan? You need seven.

thanks I'll find out.After you find out and check it, the way i described above. If infarct it dose work then, T-Stat, water pump, or an air pocket in the system.
Also timing and carb tunning will have a roll. If your vacuum advance isn't working then theres your problem, If its running to lean this could be another problem.
Don't forget about blockedge which i don't think so.

If it heats up on the highway, then I think you have a blockage somewhere or an air flow issue. With a big block and A/C, you definately need a 7 blade fan. The fan should sit 1/2 in and 1/2 out of the shroud. All my thermal style fan clutches, when the engine was hot and you shut it off and tried to spin the fan, you should a lot of resistance. All mine were like that. Also, in the morning after the car sat over night. if you try to spin the clutch, you should encounter resistance because all the wax inside the clutch has settled. So until it spins for a minute and frees up, it's going to act like it would if the engine was hot, but that will only last for a minute or so. If the engine is luke warm, the fan should spin freely. If the engine is hot, you should feel resistance. This is with a thermal style fan clutch. I am not sure about the non thermal. I think they operate at full output at low RPM's and the faster you go, the less they spin. They rely more on air flow at high RPM. At low RPM, they are engaged all the time.

Skip has a good point. If the timing is too retarded, it can cause overheating. What is the timing set at?

Wow, ten posts and we've written an encyclopedia, everything you ever need to know about coo,,,,,,

thanks for the replies guys. I have a 7 blade fan, and it sits 1/2 into the shroud. The mechanic I used builds performance engines, and in fact built mine (approx. 400 hp max.) When we started it to brake-in the cam, it started to over heat slightly. after break-in he said the clutch fan was shot, and he replaced it. I never asked what type, although I remember seeing a spring on the front of the clutch assembly - if that helps id the type? The mechanic adjusted the timing - I think he said 30 degrees on full advance, I think.

I talked with some other mechanic friends today, and they all said that jobber clutch fans are garbage and most often fail - maybe mine fits the bill, cause I know it doesn't ever sound like it's working.
I also have a new T stat, and a rebuilt water pump.

The spring indicates a thermal cluth. But if you are over heating at hwy speed your problem is NOT air flow from the fan. You have air blockage,coolant flow, or engine tune issue.