Here is a new old stock Johnson antique outboard motor water pump piston.  Johnson part number is # 13-13.  This will fit all the

Model A and Model A-25 made from 1922 to 1927. Includes the bronze spring that appears to be slightly used.  The wearing surface

of this piston gets worn and grooved with age.  This affects the pumps ability to cool properly. Replacing the piston and spring often doubles the cooling.  Look at the photos and email with questions.  Thanks for looking.