The lower unit on my 8hp Tohatsu was looking pretty rough. The original owner apparently did not maintain any type of sacrificial anode on the engine, and when I bought the boat the lower unit was fairly badly pitted on the outside. I cleaned things up with a wire brush, installed a zinc, and applied new zinc anti fouling paint, and this was good enough for a couple years. Last spring while commissioning I noticed that the corrosion and pitting seemed to be getting a little worse and I decided that I would have to repair it somehow before launching the next (this) season, plus I was due for a water pump change this year anyway so the lower unit would need to be dropped anyway. Then last fall after hauling out I did the winterization for myself for the first time. Preciously I had let the yard winterize the engine since they gave storage discounts for having them do some yard work and it worked out cheaper to let them winterize the engine than pay the higher storage rate. This year they changed the policy so I winterized everything myself, and I had noticed that there was a bit of water in the lower unit oil. The yard had never said anything to me before about seeing water in the oil, and there was none there when I had last changed the oil myself in 2009 after buying the boat. Needless to say this was the year for lower unit maintenance.
After a lot of thought and a little input from some other people I decided that my plan was to soda or sand blast the lower unit, then fill in the corrosion pits with Marine Tex epoxy putty, sand everything fair and repaint. Welding was not considered as I know that to weld an aluminum lower unit casing without any corrosion is extremely difficult, and add the additional challenge of the corrosion and the chances are quite high if not certain that it would cause more damage than the corrosion ever had, and I would likely be searching for a new lower unit (probably to be attached to a new engine too). Early march I went to the boat, dropped the lower unit and brought it home to get started.
I tried soda blasting the lower unit with my own small setup consisting of a 6gal 150PSI compressor and a hand gun, but that proved to be insufficient both for the size of the job and for the tenacity of the corrosion, so after 2 afternoons working at it having gone through nearly 10 pounds of soda and being only marginally 25% done, I took the lower unit to a local sand blasting place where they did the whole thing it for $50. I’m certain my neighbors appreciated that they didn’t need to listen to my compressor any more for this project.
What I found when I picked the lower unit up was on the verge of horrifying. Not because of anything the shop did wrong, but because of the degree the corrosion and pitting had progressed to underneath the paint that remained. One small spot actually burst a hold through the metal, but thankfully that spot was about 1/2″ aft of the gearcase itself, and was part of the exhaust output and open to the water normally.
As you can see there is a lot of missing material here, and had I let this go for another year or two, It might have destroyed the gearcase beyond repair.
Farther up on the exhaust leg there is additional pitting, but nothing so severe as that around the gearcase.
The Petit rep at the Defender spring sale talked me into purchasing Petit Easy Fair over Marine Tex claiming the two products are very very similar, with the only significant difference being the Easy Fair coming in a caulk gun tube for easy and precise measuring. It says it’s designed to adhere to Aluminum, and is rated for permanent immersion below the water line, so it should work well, However after using it I can tell you it is definitely not as strong or hard as Marine Tex once cured, and will be much more susceptible to chipping from an impact. The Petit seems to cure about to the hardness of good chalk. I don’t believe it will be a problem, but I know that I’m going to be second guessing the decision to not use Marine Tex for this for a couple of years while I wait to see how well this repair holds up over time.
After a quick wipe and a rinse with Acetone to clean the surface, I applied the epoxy filling to every pit with a mixing stick, working the epoxy into the hole making sure there was no air bubbles. Despite the fact it looks warm and sunny, it was in the low 50’s when I did this, which gave me plenty of pot life for working with the epoxy. Once it had cured to the touch I flipped it over and did the other side, then left it inside over night to cure in the warmer temperature.
While working with the lower unit on it’s side I discovered I have an oil leak from the shift linkage, which made things a little challenging since it would leak onto the edge of the surface I needed to work on if I wasn’t careful. This now explains where the water came from when I changed the oil last fall. New shift linkage seals needed added to the replacement impeller order, and I’m happy that it’s a cheap easy fix for the leak.
After letting the epoxy set I sanded down the excess with my Dremal Multi-Max, which is quite possibly my favorite tool for sanding. I found that there were some spots that needed a 2nd application of epoxy since I didn’t completely fill the hole to the point where it was level with the material around it. I mixed up another batch of epoxy and filled the remaining holes. Here is what they looked like when I finished with the sanding.
Once the surface was sanded with 150 grit paper and feeling acceptably smooth I cleaned everything and rinsed / wiped the surfaces down with acetone to remove any grease and oil left on the surface, and sprayed the lower unit with zinc chromate primer, followed by about 6 coats of Tohatsu Light Gray Metallic engine paint. The can didn’t give any indication about drying time except it was a “Quick-Drying Formula”, and said you can recoat anytime. I think I should have waited a little longer between coats as it took over a week before it fully hardened, and not before I managed to mess up the finish in a couple places. Considering it will be coated in antifouling paint I’m not really concerned with how good it looks, as long as it protects the lower unit from further corrosion.
To fix the leaking shift linkage I had discovered, I ordered a new set of O-Rings and seal bushing from boats.net (the old bushing was pretty discolored but probably fine, yet for $2.82 why not replace it at the same time as the seals and not have any question). Part numbers for the seal bushing and O-Rings are 332-66032-0, 369-66031-0, and 369-66021-1. When I pulled the old unit out I saw immediately why it was leaking. First it has been installed without cleaning out dirt and crud from the base and was not able to get fully seated down in the base (evidenced by the fact the ring that holds it in place had been bent a little to allow it to fit without being fully seated), and then the O-Ring around the linkage was completely rotted away. It looks like it was replaced previously with the wrong type of rubber, and it had just simply dissolved over time in the lower unit oil. I hope to see a dry lower unit when I change the oil next winter.
Along with the impeller, the water pump housing gaskets, and the shift linkage seals, I also ordered a couple of the OEM anode’s (part number 338602182M) from boats.net at the same time, and when I installed the new shift linkage seals, I also replaced the water pump and gaskets, as well as the new zinc, so everything should be all set and ready to go back in the water for another season.