I posted a topic several months ago regarding this subject. Kurt S. and Jonesy were kind enough to reply. However, I am still confused and need further assistance.

Anyone out there with a 396-375 hp with a short water pump?

My main question is what is the diameter of the crank pully?

I have a water pump pully that is about 7-1/8" in diameter and have been told by Kurt S. that the water pump pully should be 7.1" I think I have the right water pump pully but it is not stamped with the any numbers. The correct number should be 3906656.

I also have a two groove crank pully with the number 3874414 stamped into it. Kurt S says this is for a '68 BB w/ A/C. The diameter of this pully is 7-3/8". Kurt says the correct pully would be a 3869978 three groove pully.

At his point I am not as concerned about the correct numbers as I am about the correct diameter.

I thought I would be able to use the above mentioned crank pully with my 7-1/8" water pump pully. Here is the problem, they will not fit into their respective areas without interfearing with each other.

I measured about 6-7/8" between the centerline of the crank and the centerline of the water pump pully. As you can see, if I take the radius of my two pullys and add them together I come up with a centerline distance of 7-1/4". Should my crank pully be smaller? Maybe the water pump pully is incorrect?


If the crank pully is smaller than the water pump pully then the water pump will be turning slower than engine speed.

I do not want to turn the water pump too slow and overheat the engine.

Any help would be much appreciated.

Kel

Try the Camaro Research Group -CRG http://www.camaros.org
They just posted first gen Camaro pulley data there in downloadable PDF format.
David

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The older I get, the faster I was!

Thanks David. Lots of good information there. Still, they did not have the diameter listed for the crank on a 396-375 hp. I think it is probably going to be about 6-3/4" by looking at all the other pully information. It seems whenever there is a water pump pully at 7.1" the crank pully in 6-3/4". I have a chrome 6-3/4" crank pully and it fits just about right with the 7.1" water pump pully installed.

Can anyone here who has an L78 do a measurment check on their crank pully diameter?

Thanks

looking at both pulleys for my 68 l78. water pump pulley is a two groove. part no. is 3906656 cv. outside lip to lip on backside is just a tad over 7 inches. i have a three groove cast iron pulley, no. 3869978 bo. front side measurement, edge to edge is almost 6.75 inches. hope this helps. mike

Thanks Mike.

Yes your measurments certainly do help.

I suspected the crank pully was 6-3/4" and thanks for confirming it.

Looks like you have the correct pullys for the '68 L78 with power steering.

I have a waterpump pully that is dimensionaly correct but it is not stamped. I do not have the correct crank pully. I do have a one groove chrome one that is funtionally correct though, and will use it until I locate the correct one ata swap met.

I know this is a very old thread. But I am searching through them all because I am going through the same thing now. I figure why not tap all the info from the guys that have blazed the trail...

I am not the most knowledgeable person by any means (including spelling). I noticed an incorrect statement in the post below and thought I might chime in for general clarification purposes...

If one installs a smaller water pump pulley, the water pump will actually turn faster, not slower than it would with the larger pulley.

OK, the "professor" will put down the chalk now... :p

I posted a topic several months ago regarding this subject. Kurt S. and Jonesy were kind enough to reply. However, I am still confused and need further assistance.

Anyone out there with a 396-375 hp with a short water pump?

My main question is what is the diameter of the crank pully?

I have a water pump pully that is about 7-1/8" in diameter and have been told by Kurt S. that the water pump pully should be 7.1" I think I have the right water pump pully but it is not stamped with the any numbers. The correct number should be 3906656.

I also have a two groove crank pully with the number 3874414 stamped into it. Kurt S says this is for a '68 BB w/ A/C. The diameter of this pully is 7-3/8". Kurt says the correct pully would be a 3869978 three groove pully.

At his point I am not as concerned about the correct numbers as I am about the correct diameter.

I thought I would be able to use the above mentioned crank pully with my 7-1/8" water pump pully. Here is the problem, they will not fit into their respective areas without interfearing with each other.

I measured about 6-7/8" between the centerline of the crank and the centerline of the water pump pully. As you can see, if I take the radius of my two pullys and add them together I come up with a centerline distance of 7-1/4". Should my crank pully be smaller? Maybe the water pump pully is incorrect?


If the crank pully is smaller than the water pump pully then the water pump will be turning slower than engine speed.

I do not want to turn the water pump too slow and overheat the engine.

Any help would be much appreciated.

Kel

I went with the correct deep 3-groove crank pulley and had to go with a smaller water pump pulley because the'correct' ones overlapped each other. My water pump is overdriven slightly due to the smaller pulley and I ran it for 20minutes during cam break-in in 105 degree heat in Phoenix and it never went above 190. :thumbsup: