Let's start with pictures of the carnage, since that's what everyone wants to see...



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Background:
Boat: 1998 Yamaha Exciter 220
Engine: (2) 1100cc triples
Failure: Both engines overheated @ 3/4 throttle. Left one came back, right one did not!

So, the big end of rod #3 failed. What would have caused this? I found that the coolant passages are pretty corroded, but nothing seems to be clogging them up. I did, however, find that the long coolant line leading from the back of the head was pinched underneath the exhaust pipe, causing coolant flow to be (presumably) choked.

I'm assuming that the piston got too hot, expanded, and siezed, blowing the rod up. I find it interesting that the big end went instead of the little end! The piston is badly scarred, as is that cylinder. The bottom of the sleeve is broken off, and there is a 2" x 4" window in the upper half of the crank case.

More interesting info: the piston pulled right out when I disassembled the engine (it wasn't permanently locked), and the piston was not burned. Plugs looked normal, and there was a LOT of oil in that cylinder, from the pump still pumping, without burning it off. Also, the engine still ran with a busted rod! impressive, really! It seems I had good fuel and plenty of oil.

So, what caused this failure? I'm assuming it over-heated the rear cylinder, due to the fact that the water (if I'm interpreting the manual correctly) flows from the front of the engine to the rear). Because the coolant line on the exit of the head was blocked, coolant flow was slow, allowing the water to get very hot after passing over the first two cylinders. By the time is gets to the third cylinder, it's too hot to do much cooling, causing expansion of that aluminum piston, and eventual siezure.

I have a new (to me) crank case, as well as new crankshaft, piston kit, sleeve, and gasket set on order. We'll rebuild and go from there! I'm also going to tear down the other engine while I've got the first one out, and clean all the water jackets, checking all coolant lines.

Thoughts? Is this common? Should I be looking for anything else I haven't thought of?
-Matt