This may be one that is best done "experimentally." If you stick with the stock dimension (whatever it is) then you might find yourself with the need to do the "collar" trick on the front (under the fan) to keep your lateral motion below the recommended 0.010.
The game goes like this:
Press on
Assemble water pump from the rear bushing backwards (i.e. impeller in place on shaft with thrust washer etc, etc, just as you took it apart.)
CHECK to see how much free play you have (don't forget the gasket!)
Rinse and repeat to get the 0.010 or slightly less.
I say slightly less as this will make your water pump less likely to "pump" out the gland, be it a modern seal or the original graphite wick.
When you get your distance set, then drill and pin the impeller in place to prevent a heartache later as the fan "propellors" the shaft into the backside of your radiator and wipes a big circular indent into your radiator fins. (see the mode of failure here?)
With now 80 years of wear on that "bridge" inside your engine head, it's unlikely the original shaft dimension will be anything near what you need. And you can buy "extended" shafts to make up distance should an original fan shaft result in not enough out front to take the fan hub on it's taper.
(There are other solutions to the receding head bridge issue including drilled/tapped/bolted adjustable pump shafts, building up with weld, etc. All grist for discussion by those who have needed to do.)
Good luck with this. Pin that impeller - a lot of repop shafts are a smidge too small in diameter and result in insufficient "press/adhesion."
Joe K
