"326" Water Pump Shaft Failure

[Joe C.]

Is this a fairly common, or rare occurrence?

The pump is an original GM "326" pump for a 1965 327. It was rebuilt using all the best parts including a new billet steel hub, F.A.G. bearing/shaft and seal, and a cast impeller. The fan is an original to the engine, the fan clutch is a restored original style "Eaton", and the pump pulley is a reproduction with deep grooves such as used with the SHP engines.

The shaft sheared off cleanly, right at the bearing. I had been running at the track, and had just shifted at 7500 RPM.

Thanks in advance.
Joe

[Peter J.]

Re: "326" Water Pump Shaft Failure

Wow Joe, I had never seen that kind of failure. Did it do a lot of peripheral damage under the hood?

[Joe C.]

Re: "326" Water Pump Shaft Failure

Wow Joe, I had never seen that kind of failure. Did it do a lot of peripheral damage under the hood?

"By the grace of God", no.
Nobody killed or injured!
Minor damage to the DeWitt's fins.......no leaks.
Scratched paint on fan shroud and engine cradle.
Abrasion and deformation of outer 1/4" of 3 of the 5 fan blades. Fixable......no gross bending.
Belts, hood, pulley, fan clutch unscathed.

[Timothy B.]

Re: "326" Water Pump Shaft Failure

Joe,

I had one break on me while helping a friend work on his engine. Come to find out he had installed the 3/4" shaft diameter fan clutch on a 1/2" waterpump shaft. The eccentric motion broke the shaft.

Could you have a balancing problem with the combination of parts.

[Joe C.]

Re: "326" Water Pump Shaft Failure

Joe,

I had one break on me while helping a friend work on his engine. Come to find out he had installed the 3/4" shaft diameter fan clutch on a 1/2" waterpump shaft. The eccentric motion broke the shaft.

Could you have a balancing problem with the combination of parts.

The pulley is a reproduction for 1965 327/365 w/o C60
The fan is a 5 blade original from a 1965 Corvette 327 w/o C60.
The clutch is an original Eaton type, Corvette long shaft, newly rebuilt by F. O. in FLA.
Belts are 3V, cloth wrapped reproductions.
There were never any vibration problems.
Clutch works as it should, and is fully "disengaged" most of the time. Engine maintains 160-170 coolant temp at all times, even on 95 degree days, whether sustained idling, sustained WOT operation, or sustained highway cruising.

[Edward J.]

Re: "326" Water Pump Shaft Failure

Joe, Remove the water pump and do some investigating maybe something jammed the impeller. the last pic you posted the shaft looked a little dark where it snapped.(heat?) Maybe a balance issue?

[Joe C.]

Re: "326" Water Pump Shaft Failure

Joe, Remove the water pump and do some investigating maybe something jammed the impeller. the last pic you posted the shaft looked a little dark where it snapped.(heat?) Maybe a balance issue?

Thanks Ed, great minds think alike.
That's the first thing I did last week when this happened...........pulled the pump cover to examine.
Impeller was free, not seized.
There is (was) no trash in the system.
There was no sign of distress anywhere on the cast iron impeller, nor on the pump housing.

[Ronald L.]

Re: "326" Water Pump Shaft Failure

I'd have to see it under magnification to confirm, but you do look to have a nick or corrosion, that would be the stress initiator and then the rest happens pretty quick. This is a risk in "re-"using original bolts too.

[Dick W.]

Re: "326" Water Pump Shaft Failure

I'd have to see it under magnification to confirm, but you do look to have a nick or corrosion, that would be the stress initiator and then the rest happens pretty quick. This is a risk in "re-"using original bolts too.

That is as reasonable explanation as I have seen. It seems that the shaft has either a step just outside of where it broke, or has a bad groove worn in it. Which is it??

[Joe L.]

Re: "326" Water Pump Shaft Failure

Is this a fairly common, or rare occurrence?

The pump is an original GM "326" pump for a 1965 327. It was rebuilt using all the best parts including a new billet steel hub, F.A.G. bearing/shaft and seal, and a cast impeller. The fan is an original to the engine, the fan clutch is a restored original style "Eaton", and the pump pulley is a reproduction with deep grooves such as used with the SHP engines.

The shaft sheared off cleanly, right at the bearing. I had been running at the track, and had just shifted at 7500 RPM.

Thanks in advance.
Joe

Joe-----


As I have said on previous occasions, the 5/8" shaft water pump used on all small blocks from 1955 through 1970 is marginal, at best. Yes, I know they worked ok for the vast majority of small block powered cars that used them but they are marginal, at best. Besides being somewhat weak, they are usually not too long-lived. 50,000 miles for one of these is good service.

Keep this in mind: Chevrolet did not go to a 3/4" shaft waterpump for all 1971 and later small blocks because a 5/8" shaft pump worked just as good.

[Ronald L.]

Re: "326" Water Pump Shaft Failure

I cannot tell from the photos, would rather have the parts in hand.

But--
Either a step or a groove from some event, whether it was a flawed re-build, or?? would be the source or initiation point.

What you see is that this spun for a while the smooth flat area gradually working towards the center and then when he red lined it, catastrophic failure resulted - the mountain peaked area. There would not be the propensity to have too much damage as the fan was spinning about its' center of gravity and only banged on things a little when the earth's gravity took over. This is unlike a clutch pressure plate which is like the carousel which spinning tries to throw the objects outward.

I cannot rule out a corrosion initiation site, but as before the groove or notch I see is enough to do this, and quickly.

[Joe C.]

Re: "326" Water Pump Shaft Failure

That is as reasonable explanation as I have seen. It seems that the shaft has either a step just outside of where it broke, or has a bad groove worn in it. Which is it??

The groove that you are seeing is the filleted groove wherein the ball bearings ride on the shaft. There is no inner race, as the shaft itself is hardened. This creates a minor diameter which is the natural weak point, and the usual place where failure occurs. If that groove had no fillets, then the failure rate would be much higher. The fillets minimize the stress risers which would occur if that was a simple step with sharp corners.

Since posting here, I have seen photos of other occurrences of shaft failure. The similarity between all of them is striking!

I cannot tell from the photos, would rather have the parts in hand.

But--
Either a step or a groove from some event, whether it was a flawed re-build, or?? would be the source or initiation point.

What you see is that this spun for a while the smooth flat area gradually working towards the center and then when he red lined it, catastrophic failure resulted - the mountain peaked area. There would not be the propensity to have too much damage as the fan was spinning about its' center of gravity and only banged on things a little when the earth's gravity took over. This is unlike a clutch pressure plate which is like the carousel which spinning tries to throw the objects outward.

I cannot rule out a corrosion initiation site, but as before the groove or notch I see is enough to do this, and quickly.

See explanation of groove, above.

Redline: a recommended safety limit; the fastest, farthest, or highest point or degree considered safe.
The "redline" for this engine as built in 1965, was set at 6500.
The "redline" for this modified engine, as it exists now, is about 8000. Its power peak occurs at about 6500 RPM.

[Dick W.]

Re: "326" Water Pump Shaft Failure

If the shaft was not radiused at the step, it would set up a serious stress riser and lead to failure.

[Joe C.]

Re: "326" Water Pump Shaft Failure

Joe-----


As I have said on previous occasions, the 5/8" shaft water pump used on all small blocks from 1955 through 1970 is marginal, at best. Yes, I know they worked ok for the vast majority of small block powered cars that used them but they are marginal, at best. Besides being somewhat weak, they are usually not too long-lived. 50,000 miles for one of these is good service.

Keep this in mind: Chevrolet did not go to a 3/4" shaft waterpump for all 1971 and later small blocks because a 5/8" shaft pump worked just as good.

Joe,

I am looking into the rumor that F.A.G. makes a bearing/shaft assy for this application that uses needle rather than ball bearings. I heard this from Arthur Gould. I have asked my pump guy, Howard Stuart (who supplied me with the failed shaft/bearing) to look into it.

The use of this style of bearing would spread the stresses out over a longer distance on the shaft. The greater benefit, is that the "minor diameter" used as the inner race, would be substantially larger, and much closer to the 5/8" major diameter.

[Clem Z.]

Re: "326" Water Pump Shaft Failure

back in the day we used 5/8" shaft water pumps on race engines and never had that type of failure. i wounder if it was a case if they use a 3/4" shaft bearing and grind the end of the shaft for a 5/8" pulley and they leave a stress riser by leaving a sharp edge at the intersection of the two different diameters