Alan Albrecht, Milagro, #751 has the Euro version Yanmar (3GM30FYEU) and writes: Johnson Pump, USA does not sell a rebuild kit for our pump version F4B-903. Here are the correct impeller numbers (both Johnson and Yanmar) for this model as well as the individual part numbers required for a rebuild of the pump.
1. Impeller = Johnson Number 09810B or Yanmar 128990-42200
2. O-Ring = Included with Johnson Impeller kit above or Yanmar X0506523-01 (The O ring was out of stock but any automotive right size O ring should work)
3. Cam Johnson 01-452888
4. Cam screw Johnson 01-46794-03
5. Shaft Johnson 01-45313
6. Bearing Johnson 01-45207-3 (This is actually a bushing similar to what you have on an automotive water pump. You will need a universal bearing press to change this out.)
7. Yanmar cooling water pipe 128690-49010 (This is the formed hose between pump outlet and the heat exchanger you might as well change it while you are at it)
8. Glycerin, Vaseline or lube to lubricate the inside of the pump until it refills with water.
Other useful numbers:
9. End Cover Johnson 01-42388-2
10. End Cover Screws 0.0141.500
11. Replacement F4B-903 Pump Johnson Number 10-24165-2 (it is around $400 USD)
You may want to consider installing an automotive radiator flush and fill kit on the pump inlet hose to help prime the pump at start up. Items 3 and 4 are optional for a pump rebuild if the originals are at all usable. If you have other problems e-mail Johnson Pump directly they were very responsive and helpful: