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After the delay in getting parts ("I don't think 'in stock' means what you think it means..."), I got my parts on Friday.
I ordered the following from Sound Tractor, the local Kubota dealer
-- 15534-73030 (water pump for D850 engine / B1550 tractor)
-- 15676-73430 (gasket)
-- 15852-73340 (water pipe fitting for top of pump)
-- 15531-73350 (hose between pump and thermostat housing)
Total cost about $150, for OEM Kubota parts.
I ended up not needing the last two items; the pipe fitting because the pump came with that already installed, the hose because my water heater feed/return uses those two fittings.
Pump was a perfect replacement for what came off the 1985 M25 motor.
Installation was easy, even for me (I have approximately zero skills with motors). About the only hiccup was that I forgot to fully drain the coolant using the petcock low on the block, so when the lower hose came off I spilled coolant into the sump under the engine. Oh well.
On recommendation from KennethK, I used a thin layer of Permatex "water pump gasket maker" on each side of the paper gasket. Probably superfluous, but what the heck. The surface of the pump mount on the block was not perfectly smooth, I dressed it as well as I could with a bronze brush to make sure I had a bright surface, and from there I figured the Permatex would deal with any imperfections. The instructions for the Permatex were to spread a thin layer, tighten fasteners finger tight until the compound oozes out slightly all around, let it sit for an hour, then tighten the fasteners to spec and leave it for 24 hours before introducing fluids.
That last part was hard - I really wanted this thing *running* again.
I couldn't find a torque spec for the water pump bolts. Not sure how critical it is to have the exact right torque...?
Somewhere along the way I also replaced the coolant-bleed petcock on top of the thermostat housing. The old one was not opening, happily the nearby chandlery had one and it was an easy replacement.
Having time on my hands, I also replaced heat-exchanger zinc, cleaned the end-caps, checked the transmission fluid, a few other things.
And I also installed a coolant expansion/overflow tank. Boat didn't have one - there was a piece of tubing coming out from under the pressure-cap, but the other end was just laying down in the sump under the motor. Got a tank from the auto-parts store for 7 bucks, hung it on the bulkhead just aft of the manifold and connected up the tubing.
Along the way, I discovered a delight at the bottom of the motor. Someone (Christian? Prior owner? Not sure, but I have a strange and powerful love for whoever it was) installed an elbow in place of the oil-pan plug, and attached a couple of feet of red rubber hose to it, with a plug at the end. I discovered the end of it zip-tied into an interior corner of the galley cabinet. Took it down and unplugged it, and oil started draining out.... no more pumping oil out through the dipstick hole with a tiny tiny hose!
Sunday afternoon, back to the boat. Bolted on the pulley wheel, installed and tightened the belt, added a bunch of coolant, and started it up. Ran smooth, no leaks, and most importantly no horrible screeching noise like the last time it had been run.
Shut it down, topped up coolant, doubled-up docklines, restarted it and ran it - in forward - at about 2000 RPM for 15 minutes. Came up to temp nicely and stayed there, no weird noises, no leaks. Bled air out of the petcock, filled the new coolant overflow tank halfway, and called it good. Next up is to take it out and run the motor under load for a couple of hours, just to make sure all is well.
On to the next thing....
Edited to add:
As far as the antifreeze, Prestone, like I used, is yellow.
I ended up using the Prestone 50/50 pre-mix. It said "good for any small engine, compatible with any color antifreeze.
The guy at the Kubota place told me to be sure I got antifreeze that was "diesel-rated"... but then wanted to sell me the exact same Prestone stuff. I asked around at the auto-parts store, they said the only antifreeze that is "diesel rated" is designed for high-mileage/high-compression truck motors, and it's not needed for small motors like ours.
Anyway, I drained it, filled it with new Prestone, and I figure I'll drain and refill it again after a couple of hours of run-time.... probably overkill, but a relatively cheap way for me to make sure I "know" what is in it and what kind of shape it is in.
