Quoting the 1925 Duo-Art Service Manual:

  "A one-fourth H. P. motor turning 1150 R. P. M. is
  furnished with the six point pump.  The D. C. motor
  requires a 2-5/8 inch pulley and the A. C. motor
  requires a 2-1/2 inch pulley to develop the 70 or 72
  pulsations required for this type of pump."

It is virtually certain that by 1925, production of the six point
("steamboat") pump had been discontinued.  Oddly, then, the manual
fails to mention that later steamboat pumps were supplied with
identically appearing 1750 RPM motors.  The correct double pulley
diameter for the 1750 RPM motor is 1-3/4", this measurement having
been taken from two original pumps that I have.

It should be noted that the upright motor is mounted with its pulley at
the left; on the grand pump the motor is mounted with its pulley at the
right.  Unlike the later rotary pump, the steamboat pump may be run in
either direction, its main pulley being keyed to the camshaft.  However,
with the motor direction reversible, it is desirable that maximum belt
tension be on the side opposite the idler roller -- yet, with the idler
assembly rigidly mounted instead of being spring-loaded, this doesn't
make an awful lot of difference.  Indeed, my grand pump apparently
always has run "backwards" with respect to the idler.

I recall that one of the screws fastening the idler assembly to the
pump goes into end grain, making its hole easy to strip -- something
that can be corrected when rebuilding.

  "To change rotation of D. C. motors, reverse leads at brushbox."

  "To change rotation of A. C. motors, interchange two leads coming
  through bushing in cover."

-- 1925 Duo-Art Service Manual

Pictures to follow.

Jeffrey R. Wood