Outboard Lower Unit Crack

Cracked Lower Unit Running Cracked Lower Unit Running

We have an old 3 cylinder Yamaha two-stroke on the 15' Boston Whaler and the normally reliable beast recently stranded us for the very first time before magically fixing itself. It has been running perfectly ever since. I think it has an intermittent magneto problem, but can't prove it until it cooperates by malfunctioning.


In a half-hearted attempt to force such a malfunction, we recently took the boat up to Nav-A-Gator, some distance up the Peace River. I say half-hearted because we did ensure that we had a current towing insurance card along. The boat ran perfectly once again and when we got home, I ran it on the fresh water flusher for a bit. The picture at right shows what I noticed.


Judging by the corrosion around the crack, it has been there for a little while, I just haven't noticed it. It's pretty hard to miss when you put on the flushing ears, but I did manage to miss it and only noticed when the engine was running and it was spitting water out the side.


Yamaha Lower Unit Crack Cracked Lower Unit

Pictured at left is a closer view of the same crack when dry. Now I'm wondering whether to fix it, and how to fix it. A neighbor knows how to weld. We may just wind up replacing it, not sure...


To be continued, no doubt.


  1. I learned on Yamaha outboard discussion forums that this is a common type of crack. It is caused by salt water penetrating behind the bearing carrier, resulting in a reaction between the two kinds of metals and corrosion between bearing carrier and lower unit housing.

    Repair options quickly approach the price of a rebuilt lower unit, since it has to be disassembled, welded, then machined to accept the bearing carrier back in place.

    A local mechanic told me that sometimes these cracks will continue until a U shaped piece of metal falls off. He said that as long as the crack does not get past the oil seal and cause leakage, one option is to ignore it.

    This is an old engine. Something else could fail while I’m waiting for this problem to finish killing the lower unit. I’d rather save pennies for a new engine, so ignored it shall be!

    • I have a 1994 Yamaha 75hp 2 stroke with the same lower unit crack. I just purchased the boat and probably wouldn’t have if I knew more about motors when I bought it. The guy I bought it from told me that the crack had been there for years and it hadn’t changed. Anyway….just curious to hear if you are still running this motor and if there has been any change in the crack in the lower unit.

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