1971 fan clutch, big block vs small block

[Ted K.]

In an effort to get a car running I have been trying to buy a fan clutch for my 71 LT-1 locally. None available, but a 70 big block fan clutch will bolt up. It's appearance is a little different, which I can live with, but the question is will there be a difference in cooling performance or anything else I didn't think of.
Thanks,
Ted

[Jack H.]

Re: 1971 fan clutch, big block vs small block

The fact that a '70 fan clutch 'bolts up' says your '71's water pump has been changed out...

The hub and its bolt spacing changed for '71-74 rendering earlier fan clutch's incompatible UNLESS they (or the replacement fan clutch) were of a 'universal' nature.

The 'universal' water pump design feature eight vs. four bolt holes, one pattern with 'narrow' spread to accept the early style fan clutch and the other set of four holes having a 'wide' spread to accept the later version of the fan clutch.

The 'univeral' fan clutches used 'slots' rather than discrete bolt holes so they could mount to either a narrow bolt pattern water pump hub or a wide bolt pattern hub...

[Joe L.]

Re: 1971 fan clutch, big block vs small block

The fact that a '70 fan clutch 'bolts up' says your '71's water pump has been changed out...

The hub and its bolt spacing changed for '71-74 rendering earlier fan clutch's incompatible UNLESS they (or the replacement fan clutch) were of a 'universal' nature.

The 'universal' water pump design feature eight vs. four bolt holes, one pattern with 'narrow' spread to accept the early style fan clutch and the other set of four holes having a 'wide' spread to accept the later version of the fan clutch.

The 'univeral' fan clutches used 'slots' rather than discrete bolt holes so they could mount to either a narrow bolt pattern water pump hub or a wide bolt pattern hub...

Jack and Ted-----

The waterpump pullies would also have had to have been changed if a 1970 fan clutch would bolt up to a 1971 waterpump, regardless of whether the pump had a "dual pattern" hub. Also, a 1971+ waterpump has a 3/4" pilot which will not work with the 5/8" pilot orifice in 1970 and earlier fan clutches.

[Paul O.]

Re: 1971 fan clutch, big block vs small block

Ted how early is your car my 71 LT-1 s/n 1914 I was 2nd owner in 1974 had a small hub on the water pump just like a 70. Not sure when the change occurred but some early 71's had the smaller/older design. Paul 18046

[Joe L.]

Re: 1971 fan clutch, big block vs small block

Ted how early is your car my 71 LT-1 s/n 1914 I was 2nd owner in 1974 had a small hub on the water pump just like a 70. Not sure when the change occurred but some early 71's had the smaller/older design. Paul 18046

Paul-----


I've heard of this before, but I've never been able to confirm it with any degree of certainty. A few things to consider:

1) I've never been able to find any reference to a "1st versus 2nd design" for any 1971 with respect to ANY of the involved components (i.e. waterpump, fan blade, pullies, fan clutch);

2) There was only one suffix code for non-ZR-1 1971 LT-1's. It would seem to me that there would have needed to be 2 suffix codes to account for a difference in waterpumps. Sometimes, merely a slight difference in an engine assembly resulted in a difference in suffix code, let alone a difference in waterpumps.

Still, I don't rule out the possibility that early LT-1's or base engine cars were equipped with the earlier design waterpump system. However, I've never been able to come up with any documentation to support it.

[Ted K.]

Re: 1971 fan clutch, big block vs small block

I am assuming it is considered an early design, it is a Nov build date. The pilot shaft on water pump is 3/4".
Does using a BB fan clutch on a small block impact the cooling, or are we saying if it fits use it?
Ted

[Joe L.]

Re: 1971 fan clutch, big block vs small block

I am assuming it is considered an early design, it is a Nov build date. The pilot shaft on water pump is 3/4".
Does using a BB fan clutch on a small block impact the cooling, or are we saying if it fits use it?
Ted

Ted----

A 3/4" pilot shaft indicates the pump I would expect to find on any 1971 Corvette. If it has a GM pump on it, I would expect the casting number to be 3991399 (original) or 330813 (replacement).

For 1971 and later, there were no fan clutches exclusive to big block or small block---the same clutches were used for both although there were often different clutches used for with C-60 and without C-60.

1965-70 Corvettes used fan clutches which were different for big block and small block. Primarily, the difference was the length of the shaft---shorter for big blocks and longer for small blocks. However, GM years ago replaced the small block fan clutches with the big block clutches for SERVICE of all 65-70 Corvette applications. So, this strongly implies that GM considers the big block clutch to be suitable for small block applications. However, as I mentioned, for 1971 and later configuration there was no such thing as a big block or small block fan clutch.

[Paul O.]

Re: 1971 fan clutch, big block vs small block

Joe as far as for my statement about early 71 may use the smaller hub I found this on my 71 and a few other early 71's in Cincinnati had 2 other corvettes that were earlier then mine we were about 600 number different. They were big blocks and one was original owners and all 3 of us were the small hubs. It was also strange that my driven speedo gear was a large head 70 design for a 4:11 rear, early on I had to replace the gear and they ordered for a 71 and the gear diameter was to small.

About the water pump again I heard that the shaft was same for big or small block they just pressed the shaft in less for the small block.

Paul 18046

Adding for Ted I just saying this is what I found for my car and a few others that I know of with this difference when and if there is a change point I can't say.

[Joe L.]

Re: 1971 fan clutch, big block vs small block

Joe as far as for my statement about early 71 may use the smaller hub I found this on my 71 and a few other early 71's in Cincinnati had 2 other corvettes that were earlier then mine we were about 600 number different. They were big blocks and one was original owners and all 3 of us were the small hubs. It was also strange that my driven speedo gear was a large head 70 design for a 4:11 rear, early on I had to replace the gear and they ordered for a 71 and the gear diameter was to small.

About the water pump again I heard that the shaft was same for big or small block they just pressed the shaft in less for the small block.

Paul 18046

Adding for Ted I just saying this is what I found for my car and a few others that I know of with this difference when and if there is a change point I can't say.

Paul-----


Regarding the speedometer DRIVEN gears, through 1970 BOTH the "small" and "large" OD DRIVEN gears were used depending upon the installed DRIVE gear. Generally, the "small" OD driven gears were used with lower numerical rear gear ratios and the "large" OD driven gears were used with higher numerical rear gear ratios. For 1971 and later, only the "small" OD driven gears were used. However, these gears were essentially the same as the "small" OD driven gears used for 1970 and earlier.

The small block and big block bearing/shaft assemblies were not the same, either for 65-70 or for 71-74.

1965-70 small block= 5/8" pilot, 5/8" through-the-bearing shaft, 5/8" impeller shaft

1965-70 big block= 5/8" pilot, 3/4" through-the-bearing shaft, 5/8" impeller shaft

1971-82 small block= 3/4" pilot, 3/4" through-the-bearing shaft; 5/8" impeller shaft

1971-74 big block= 3/4" pilot, 3/4" through-the-bearing shaft, 5/8" impeller shaft.

Even though the 71-82 small block and 71-74 big block appear to have the same shaft configuration, they are not the same piece. The lengths of the shafts are different and this can not be achieved by simply pressing the bearings into the pump castings by differing amounts.

[Paul O.]

Re: 1971 fan clutch, big block vs small block

Joe as always thanks for all the info really helps working through all the little details it really make a difference when looking at all these parts. Paul