Which SB water pump should I use? (pictures)

Long story short, I figured I needed to change my waterpump as I didn't think the old one was moving water very effectively, as I've always had cooling issues, even with a new radiator, and now the mercedes fan.

I just got the old one out, and as you can see in the pictures below, I think it may be the better choice. There is less clearance between the vanes and the front of the pump, and less clearance to the timing cover than the new pump. Both pumps have 8 vanes.

My pulley measures ~7" diameter to the outside edges. How big is the small pulley? I should probably head to the junkyard to see of I can find one. I rarely go over 3500 rpms, so I don't think high RPM cavitation will be an issue.

Sooo, what say ye moparts? New pump? Old pump? Old pump & smaller pulley?

To throw a wrench in the whole process, I plan to replace this engine with a magnum 360 hopefully this summer, so I didn't want to drop $100 on a new water pump.

Thanks for the thoughts.

First, the old pump:

And the new pump:

IMHO,use the new pump

you can see where the vanes on the old pump are rubbing on the pump housing

I bet that if before you removed it with fan and pully in place you could move the shft by hand..the bearings and seal are on the way out when it does that.

the 6 vane pump goes with a small a/c pully

the 8 vane pump goes with the larger non a/c pully

6 vane spins faster to move x amount of water at idle
8 vane spins slower to move same x amount of water at idle

small pully on a 8 vane pump can cause it to cavitate at sustained high rpm

you can use a HD thermoclutch and a 7 blade fan for max cooling on either combo,but need to be the right ratio on the pullys

any play on the impeller to shaft?

if it is tight then put the old one back.

may be you have feets problem, sucking the lower hose shut at speed?

The lower hose has a spring in it, so it shouldn't be sucking closed.

I just put the old pump in my vice, and can now feel just a tiny bit of play in the shaft. Soo, new pump it is.

I did have a 7 blade fixed fan on it, and now have the electric mercedes fan. At idle with both set ups, it will maintain itself, but wouldn't actively and quickly cool itself off.

Quote:

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small pully on a 8 vane pump can cause it to cavitate at sustained high rpm

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On the highway it usually runs around 2700 rpm. It's had a couple 4k romps, but few and far between. I wouldn't think that would be high enough to cause cavitation? This is a work truck, so it rarely sees anything a drag car would consider normal. It's also old and tired, and I don't want to blow it up