If I were to copy a steamboat pulley pump, I would probably cast
just the large part and make the smaller part out of solid stock on
the lathe.  I have seen original pulleys that were bored out of true.
Making separate parts would make it easier to get it right and or
re-bore it and sleeve it if it was screwed up the first time.

The original pulley had four belts on the pump side and two on the
motor side.  This was necessary with the original leather vee belts
but is not necessary with rubber belts or urethane belts because they
do not slip as much.  If originality is not an issue, a lot of trouble
and cost could be saved with fewer belt ways.  A single pulley could be
used for the belt to the motor and double belts for the large pulley.

I have a perfect cast iron pulley that can be borrowed for use as a
pattern.  I want to keep the pump since I have the piano it goes in but
am using a remote pump so I can be without the pulley as long as I get
it back.

Best regards,
Spencer Chase
Garberville, Calif.
http://www.spencerserolls.com/