Water Pump
 

I have a 1968 Corvette 327/350. I have changed the water pump three times in the last 6 months. I don't believe I am tightening the belt too much. In addition, I get an engine/transmission vibration at 3000 RPMs in all four gears. It seems when the pump fails (Pump leaks), the vibration goes away and the engine runs without vibration after 3000 RPMs. Any suggestions.

Make sure your pulleys are not bent. Could cause the pump issues and also the vibration.
What condition is the front balancer?

I purchased the car last winter. It appears the car was restored. The pulley looks new. How can I tell the condition of the front balancer?

disconnect all your belts, and run your engine. according to your diagnosis, the engine should run without vibration. add 1 belt at a time until your vibration is back.. that is where your problem is.

^

Yep.

Start with no belts. If you still have a shake check the balancer, if not; 1 belt at a time.

Thanks guys. Logical approach!! I'll keep you posted.

Also check the size of the water pump shaft. There are two different size shafts maybe more. If you have the wrong size shaft as compared to your fan clutch there will be nothing to index the clutch other than the bolt holes which is not enough. I ran across this on my 68 and vibrations went away with the right combo. I believe the correct size shaft for a 68 is 3/4" and other GM cars were 5/8" Yea some pumps came with a small brass adapter the put the large clutch on the small pump but that was not always a good fix!

Can you explain in more detail what you mean by "there will be nothing to index the clutch other than the bolt holes which is not enough"

He means that if you're supposed to have 3/4" shaft, and the shaft you have is 5/8", then its too small and the hole in the clutch is too big so it's a sloppy fit, meaning the clutch is never perfectly centered.

If this is the case, then the imbalance of not centering the clutch (indexing it on the pilot shaft) will quickly wear your water pump.

I checked the old water pump shaft and the hole in the fan clutch. They are both 5/8 inch. So, that appears to be ok. After doing some research, I figured out that the fan clutch is not stock. The stock fan clutch has four holes that mate to the water pump. Instead, on mine, where the fan clutch holes should be, each hole is in the shape of a U, I guess to make it more universal to other water pumps. However, since both the center hole on the clutch is 5/8 and the shaft on the water pump is 5/8 then in theory it should not wobble. Once I install the new pump, I will run the engine without any belts.

What hole size is in the Pulley for the water pump? If it is 3/4" then the 5/8" shaft will not center the pulley. If you have a water pump with 5/8" shaft. Then do you have a water pump that is too long or short for a corvette application? Most Corvette's have a water pump based on year and not all short pumps are the same.

Just my 2 cents

It's possible the fan clutch itself is bad internally....or a bent fan blade. You're on the right path.

JIM

Making any Progress ?
 

How is your water pump thing going?

Gents,

Today, I removed all of the belts checked the Harmonic Balancer; it looked fine. I installed a new water pump. I started the engine without the power steering or water pump belts. There was no vibration. I ran the engine with the water pump and belt but no fan clutch; still no vibration. When I installed the fan clutch the vibration was evident. I removed the fan clutch, and the vibration was once again gone. I have a new stock fan clutch on order.

I will keep you guys posted. Thanks for all of your advice.

ensure the replacement fan clutch has same size pilot hole ID as OD of forward end wp shaft.

Gents,

Just installed an OEM fan clutch. It solved the problem. I can get the RPMs up to 4000 (3000RPMs used to vibrate the drive train) with no vibration. The new fan clutch is smaller in diameter and has a shorter shaft. I threw the old clutch in the scrap metal container.

Thanks for everyone's advice!!

Glad you got it figured out! A relatively easy fix too!