So, I have a '69 6 blade clutch fan I'm thinking of using the next time around and my question is should I go with a non thermal type clutch? My thinking is that these spin around 1800 rpm max regardless of engine speed, so there wll be less drag and pull than a thermal type when it locks up? What do you guys think?

(The stock fan, electric fan or a flex fan are out of the question)

FYI: Always mic the shafts and hubs on w.p. re-building kits! I learned an expensive lesson!

I found a couple of thermal types that fit the Y-block pretty well - and bought one. Seems to work OK - not a lot of miles on it.

In their catalog look at part numbers #2747 (heavy Duty) and #2797 (Severe Duty). The #2747 will fit a 3/4 shaft and comes with a spacer for 5/8 shaft - the #2797 is for a 5/8. Their "normal" application seems to be for an 80"s Bronco with heavy duty cooling (AC).  

I used one because I wanted to utilize a smaller pulley to jack up the waterpump speed - and when I did, the fan was really wheeling. The inherent clutch slippage seems to deal with that nicely. I'm running a seven blade (aluminum) fan off a Pontiac.

The clutch - especially the "Severe Duty" takes some space - I don't know what you have available, but be aware.    

Its 1-5/8 from the rear face of the clutch fan hub - forward to the face the fan bolts to and 3-3/4 out over the nose of the clutch

As you can see - even with a 2-3/16 thick fan (measured lying down on a flat face) - the clutch is what takes the room.

t-birds have lots of room so its not a big deal - but the sedans will be different I'm sure.

You asked about thermal and non thermal fan clutches and stated that you will not consider a flex fan. Why not? I just replaced a thermal clutch fan on a Jaguar with a flex fan. The right thermal clutch fan is heavy and the metal fan is also heavy. On you Yblock even if the thermal fan never exceeds 1800 RPM the fan clutch STILL TURNS as FAST as ever  and this puts a sever strain on the water pump shaft. By using a flex fan and an aluminum spacer on my Jag I reduced the rotational mass by a hugh amount. Flex fans were developed to increase mileage and help emissions ( faster warm up). If your cooling system is in good shape and you have a fan shroud a flex fan is a good choice. You might want to re-think your decision. Pete

Don W.

Pete, Looks like a fex fan is my only option now. I'm running a generator so I don't have the ampereage for an electric fan and don't like the look of them anyway.

Paul, I installed the balanced, blueprinted, Isky cammed +.040 312 this spring so I'm safe. 6500 is my new redline, but will shift @ 6k when I'm "on it"!

My new radiator should be here the first of next week. Thinking of going with the large bearing truck pump with 3/4" shaft and drilling out the car pulley to match and spacing it forward to match the crank and gen. pulleys. I'm wondering if the truck impeller will pump too much water volume as it had deeper "blades"? It will be turning slower than the stock truck setup because of the larger pass. car pulley. Dose anyone have any thoughts on that?

I'd like some advice on which 18" flex fan to buy.  Number of blades, type, brand etc. I'll be running a 3 row copper/brass rad.

Thanks to all,

Philo

I think you would be better off with a fixed aluminum blade style fan - that had relatively low rotating inertia. You can find them with both six and seven blades. 

When I lived in Wyoming - my buddies with trucks that pulled trailers - told me the flex fans were worthless when you had to tach up the motor in second to climb a mountain. Just when you needed a lot of air flow - it was flat.

The fan came with a junk GM clutch I had to throw away.......

This started out because a factory medal fan at high engine RPM pulled the waterpump shaft forward and took out the radiator.

Now we are talking about how to keep a motorhome cool as we climb the Rockies and what happened to a Corvette when the fan clutch siezed.

If you have a high RPM engine and want to reduce the force on the waterpump bearings start by reducing the mass ( weight ) that is on the waterpump shaft. NO FAN-CLUTCH, NO IRON FAN. Look at a fixed aluminum multi-blade fan or a flex fan.

If cooling with a fan shroud is not an issue why try to solve a problem that does not exist.

OK I feel better. Pete

Glad you feel better. We can all use a little of that.

I wrote a long winded reply to your informative post this morning, but for some reason it didn't post..............The bottom line was; Where did you get the new pump you trusted and/or what brand was it?

Thanks!

Philo 

If I want to tow or haul something, I'll just hire you..............OK, Big Boy?

The OEM's are also "cost driven" - and they eventually went to higher allowable engine operating temps - and cost reductions they got from using electric fans. We don't have the luxury of rigging up high temp engines in our driveways - and we aren't a business buying thousands of electric fans assembled in Mexico. So.....the old technology direct drive fans and clutch driven versions will work well enough for our purposes. 

I agree with everything you say regarding manufactures putting thermal fan clutches on almost all their cars from the late 60`s until they switched to electric fans (and ATF in manual transmissions) to get the last mile per gallon out of   gasoline. And they do work well on systems that were designed to have them as OEM.

Philo was trying to avoid another fan lunching his radiator at high engine RPM. I think that until someone sells an improved Yblock water pump with bigger bearing or more efficient impeller design and larger pulley then his better choice to avoid a repeat is to reduce the rotating mass from the water pump by using no clutch fan and an aluminum fan ( fixed or flex) I don`t think fan noise is an issue and no mention was made of cooling issues, only wasted radiators.

If he buys that BIG TRUCK he mentioned then that's a different matter. Pete