MS290 Rod Bearing

[rd35]

A buddy of mine brought me his MS290. Engine was seized. He said it started slowing down like it had a load on it while it was out of the cut and then just stopped. Pulling the starter rope would not rotate the engine. I pulled the muffler only to see a single scratch on the piston and no apparent scoring...rings were still free and looked good. Went ahead and pulled the engine apart. Rod bearing was galled as the bearing rollers were flat spotted and blue, and one of the little flat side spacers (I guess that's what they are called) had broken and fallen into the pan....hence locking up the engine. On further inspection, the crank was broken where the yoke surrounds the rod pin. Catastrophic crank failure. What puzzles me is that the main bearings and the piston were in pristine condition except for the damage done to all parts by that flat steel spacer, part of which ended up in the muffler.
So, have any of you guys ever seen this happen before?

 

[Poleman]

I’ve seen it happen twice on the 290’s, never on a 310 or 390. I’ve seen the big end bearing heat up and crack the crank.
This one was tuned by a Dtihl Tech to 13,9k and they were using universal one oil does all and ethanol fuel.....toasted it pretty good.
First one I seen was all slugy in the bottom... piston scored but crank still turned.... just cracked like the one pictured.

 

[rd35]

Yep! Just like this one!
The bearing on the left has gook all over it making it look damaged but it’s not. Owner swears he uses nothing but Stihl Ultra at 50:1.

 

[rd35]

 

[Poleman]

That's interesting.....all that I've seen and yours are all cracked on the same side.......hummmmm.....