Reviving an old post here, but while I was researching about my own water pump problem on my 3YM20 I saw this post and would like to give an update.
My engine is around 3years old or at least as old as when they first introduced the YM series, and early this year the water pump had a small leak, then it became worse, so much so that the engine bed got flooded after an hour of cruising. After a week, the bearings ceased.
No option but to fix the pump. After Disassembly, the most important things noted were that the bearings did cease probably from saltwater from the leak.. nope they were not SS bearings, and that the water seal was in bad shape distorted, and lastly the shaft had a shallow gouge, from where I don't know!
Repair was a bit straightforward, at least the local yanmar dealer was kind enough to give a copy of the exploded illustration of the pump, including part numbers, so was able to order from them the following:
X03431760 Ball Bearing
X03431748 Ball Bearing
X02173432 O ring
X02173476 O ring (big)
X02233010 Rip Seal
Actually the most important parts were only the 2 bearings and the Rip Seal, the others were ordered just in case, the small O ring and the plate apparently are there to provents leaks from getting to the Orings, yeah right.
Disassembly was with the help from mechanic friends using soft brass and plastic tools to prevent damaging the SS and other parts, you also need a pliers that removes ring locks.
I then sanded the gouge on the shaft, be careful to use fine grit sand paper though, i used 1000 and 1200 grit, mounting the shaft on an electric drill, be careful when using this setup, can still cause some injury.. I should know.
I also ordered a replacement impeller, with the impeller came a tube of Glycol gel that apparently is the only thing that should be used on those rubber, so I used it on the rip seal as well.
So there, if you have a 2 to 3 year old 2 or 3YM model engine, please check your water pump for leaks, and next time you order yanmar parts, you should get a rip seal as well, we don't know when your's will break.
Oh and I chatted with some engine guys in Japan, and they said, yes this is quite a common problem with the waterpump.
I still love my 3YM20, but I am not happy with how they designed the waterpump as well as whoever decided to delete the manual emergency crank feature, whoever decided to take that off the design is not a sailor at all and should keel hauled!