I have been running my 1970 Duster 340,
which now has a solid cam, pretty hard lately
(it sat for 7 years and I guess I’m trying to make
up for that.) Unfortunately, I was winding third
gear tight last night, and I noticed a bit of vibration,
then a moderate “boom” noise, then the
engine got quieter and the vibration stopped.
My first though was a stone trapped in a tire
tread, but then the temperature gauge began
to climb and the ammeter showed discharge. I
pulled over, opened the hood, and I found that
the fan, complete with the clutch and pulley,
and come off and eaten halfway through the
radiator. I am now a very unhappy camper. Yes,
the water pump shaft snapped.
Obviously I know I need all the stuff to
be replaced, but my question is: How can I
prevent this from happening again?

Until 1977, all SB water pumps had a ½˝ diameter
shaft and bearing. Responding to disasters
such as yours, a 5⁄8˝ shaft pump was released in
1978 for Police use. As the years rolled on, this
became the standard smallblock pump, the
Mopar P/N was 4728533, it can be identified by
the scallops, not holes, in the bearing support
nose (see photo, below). These are widely available
new or reman, I suggest all new to insure
that you aren’t screwed (again) by metal fatigue.
Rock Auto’s P/N is AW7103.
I suggest the cop pump on any SB build.
Unfortunately, to use this on a pre-’70 SB, you’ll
need all the later (’70-up) front cover components
and brackets, and even the ’70-up style radiator.

Standard smallblock (’70-91) water pumps can be identified by the series of holes in the bearing
support snout area (arrows). These pumps have a small bearing and a ½˝ shaft that fails. The premium
pump has a scalloped bearing support, and a 5⁄8˝ shaft that is much more durable. Both have ½˝ fan
and pulley pilots and are 100% interchangeable. There’s no reason to use the weak-sister variety. Ever.
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