Piston Destruction! Trying to figure out the cause...

[mtomas]

Hi - My 2000 25 hp Merc 2 stroke developed severe problems after a 15 minute run at ~4700 rpm. Wouldn't idle, wouldn't accelerate, etc.

A compression test showed 0 psi on cylinder #1 . I ended up tearing into the PH for a look - and the attached images are what I found.

About half of the top ring on that piston just fell out when I pulled the crank assembly from the block ? so I?m not entirely sure where it was ?resting? in there. Also, the bottom ring is definitely much tighter to the piston than normal, almost like it?s been smashed into the ring groove (but still intact).

I'm trying to get a bead on what caused this failure so it doesn't happen again after I rebuild. Gas was mixed 50:1, so lubrication should have been fine. Tell tale flows strong. Doesn't really look like it overheated imo..

This just random/unfortunate ring failure? Or are there other 'clues' in the pics I'm not seeing?

Thanks for any input you might have!

 

[CharlieB]

Re: Piston Destruction! Trying to figure out the cause...

The rings failed because the overheated piston was seizing in the bore, wiping aluminum into the ring lands and pinching the ring.

Notice the one side of the piston dome is much cleaner than the other, this should be on the exhaust side, the flame 'wash' across the dome as the exhaust port opens burnt these areas clean.

This does not appear to be an overheat caused by a lean fuel mixture as the dome of the piston is not melted, but more likely a cooling problem.

I suspect this is the upper cyl, as a restricted cooling flow the upper cyl suffers from the reduced cooling sooner.

Service your water pump.

I have seen a number of motors lose cooling flow due to a piece of plastic drifting, covers the water inlets until the engine is stopped, then drifts away, leaving nary a clue as to what went wrong, other than showing a small amount of melt on the very ends/edges of the impeller.

 

[mtomas]

Re: Piston Destruction! Trying to figure out the cause...

Wow - what a great response! You were spot on - that is the upper cylinder.

I'll definitely service the water pump/cooling system during my rebuild. Thanks so much for the review and advice! You guys rock!

 

[mtomas]

Re: Piston Destruction! Trying to figure out the cause...

Figured someone might find this interesting... I checked out the water pump as CharlieB recommended. The bottom face plate had severe grooves worn into it, a couple of nylon washers were either missing or worn away, and surprisingly enough, 2 of the 5 rubber blades on the impeller were actually compressed in the reverse direction. How that even happens - I don't know. Maybe the previous owner tried to replace the impeller by just forcing the blades under the pump assembly cover with a screwdriver as opposed to spinning it in? Who knows...

Either way - the rebuild is done, a whole new water pump assembly is in place, and all is well . I never could have done this job myself without this board. Thanks again.

Mike

 

[jeffersondodson561]

Re: Piston Destruction! Trying to figure out the cause...

Looks like it snagged a ring as well, all of those pits on top of the pistion caused major pre ignition and at that point timing is no longer a factor, on your first pic, see that long streak vertically that may have been where the ring locator pin cam loose. How old was the gas last time you ran?

 

[RadarRick]

Re: Piston Destruction! Trying to figure out the cause...

Out of curiosity, what were the major steps in the rebuild?

Thanks for reporting the outcome - it is very informative to hear the final analysis.

 

[Faztbullet]

Re: Piston Destruction! Trying to figure out the cause...

Something I have noticed on these blocks(mercosil) is if they get real hot they seem to shed a gritty substance in the cylinders.