OK, it threw a rod…  A few possible reasons for this.  The first reason might be that a light oil mix was used.  These engines are designed to use a 16:1 fuel/oil mix, and it is a must for those plain rod bearings.  Ignore the hype about “today’s oils are better so less is required”.  Go light on the oil mix, and this is what happens.  Another cause for this is a slow silent killer.  The lower crankcase seal is a very complicated assembly that is often ignored/left out/mis assembled allowing water to enter the crankcase slowly wearing the rod bearing.

In this case, it seems hard to believe that the crankshaft is in good shape, I would reinspect it for sure.  The other issue is hidden damage to the inside of the crankcase.  When parts go flying around inside, usually the crankcase gets cracked/damaged creating an external leak.

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Aidan Smith

I would say you are very lucky indeed.  I would clean up the crank with some fine crocus cloth and measure the journals before re using.  I would also have a very close look inside the crankcase to be sure nothing is cracked or split.

Were the lower seal components correctly installed when you pulled the powerhead off?

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Aidan Smith

Probably ran light on oil.  How does the other rod/crank journal look?

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Aidan Smith

Aidan Smith wrote:

Surprisingly, looks amazing. Clean, no carbon or signs of heat anywhere

Well, if the engine truly has low hours, I suppose it could have been a manufacturing/assembly error, or perhaps someone revved the crap out of it in neutral