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787 Piston damage

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kicker

kicker

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Looking for some help diagnosing what happened to this piston

My PTO piston from a 787 is toast, most of the damage is along the squish band, basically took out the top ring and out edge of the piston. The cylinder wall dont look too bad, but I did see damage on the head as well. I have not taken it apart cause it has to go back for warranty, so I cant show any detailed pictures.

If you guys can have a look and let me know what you think went wrong, that would be great. That way I can make sure it doesn't happen again if it was my fault.

I'm thinking that it is from debris of some kind, possibly a needle bearing?



 
Too blurry and a bad angle. My best bet is the edge of the RAVE contacted the piston.

Pop the head off, and post a pic of the crown.
 
Ya sorry, I will try to get some better pictures. I did check the Rave and could not feel anything along the edge that would have caught the ring.
 
My guess is something was bouncing around in there and beat the piston crown up. Unless that's melted piston, but I'd expect more discoloration. Sure is dry looking in there (not oily), are those score marks in the cylinder wall?
 
Sportster-2001-951C-Stock said:
My guess is something was bouncing around in there and beat the piston crown up. Unless that's melted piston, but I'd expect more discoloration. Sure is dry looking in there (not oily), are those score marks in the cylinder wall?
Click to expand...

I am not very familiar with piston damage but I would have to agree. There is not too much damage on the crown or in the middle of the head, most of the damage is around the outside edge of the head and cylinder. The outer edge looks dry but the cylinder and cylinder wall is coated with oil. What looks like score marks on the wall is just streaks of oil, no real damage from what I can tell at this point.

At this point I cant pop the head off because of the warranty, so I can only look through the exhaust port.
 
Was this an SES engine? How many hours do you think you had on it?


Long live the 2 stroke Seadoo!
 
Here are some better pictures, could not get one of the crown. The crown itself does not have much damage, and from what I can tell has normal carbon build up........it doesnt look like it was running lean.



You can see the fingerprint in the on the side of the cylinder, anything below the bottom ring seems to have good oil and no real discoloration.



 
AdamA96XP said:
Was this an SES engine? How many hours do you think you had on it?


Long live the 2 stroke Seadoo!
Click to expand...

Yup, I think we put about 40-50 hrs on this one. It ran like a top all summer and then died in Sept. What I hate is that I am not sure why this happened, especially if the problem came from the seadoo itself, carbs, oil injection, fuel, etc. If the problem came from the rebuild itself, that is easy, then a least I know I dont have to troubleshoot the rest of the seadoo.

To me they dont look like it was melted from heat, it looks more like it wash mashed with debris.
 
141.jpg


Yea, that was mine at the end of summer. It took a beating


Long live the 2 stroke Seadoo!
 
griz400 said:
is the inside of your exhaust pipe rusty inside, and flaking into cylinder ???
Click to expand...

I just looked and there is no rust in there, just some carbon build up. The PTO side of the manifold has what looks like waves or streaks of a heavy, hard dry carbon stuck to it, where the mag side does not have the same look. It almost looks like water or something was mixed with the oil/gas fumes going into the manifold.

I will try and get a picture.
 
if you got water in cylinder, the best tell tail sign is a real clean spark plug, versus the other one would have brown/black deposits from 2 stroke oil -- when a head leaks water, the cylinder is real clean ...
 
kicker said:
I just looked and there is no rust in there, just some carbon build up. The PTO side of the manifold has what looks like waves or streaks of a heavy, hard dry carbon stuck to it, where the mag side does not have the same look. It almost looks like water or something was mixed with the oil/gas fumes going into the manifold.

I will try and get a picture.
Click to expand...

What does the other piston look like? Did you ever run a compression check on this engine before this happened?
 
kicker said:
Yup, I think we put about 40-50 hrs on this one. It ran like a top all summer and then died in Sept. What I hate is that I am not sure why this happened, especially if the problem came from the seadoo itself, carbs, oil injection, fuel, etc. If the problem came from the rebuild itself, that is easy, then a least I know I dont have to troubleshoot the rest of the seadoo.

To me they dont look like it was melted from heat, it looks more like it wash mashed with debris.
Click to expand...

Then must be something was in there and crushed the top ring land, pinched the top ring, and compression would've dropped off once ring seal was lost. Whatever the debris was, then got spit out of the exhaust port into the exhaust pipe.
 
griz400 said:
if you got water in cylinder, the best tell tail sign is a real clean spark plug, versus the other one would have brown/black deposits from 2 stroke oil -- when a head leaks water, the cylinder is real clean ...
Click to expand...

Ok Thanks, that will not be the case then, cause both plugs had the same brown color. Minus the flecks of aluminum stuck to the plug that was in the bad cylinder. Both cylinder walls look the same as well.
 
dan87951 said:
What does the other piston look like? Did you ever run a compression check on this engine before this happened?
Click to expand...

The other piston looks great, I did a compression test in mid Aug and had 152 in both only used it a couple of times after that then it died. My daughter was driving it and said it just quit, she then tried starting it and killed the starter in the process.
 
Is it just me, or does it look like rust on that piston?

It might just be the way the photo looks, but it looks like rust on the pictures


Long live the 2 stroke Seadoo!
 
The ground electrode was bent from the debris flying around. They both had what I would call a medium brown color to them. Based on the little knowledge I have they look like it was running with proper tuning, but I could be wrong. The plug from the bad cylinder has flecks of piston stuck to it so the color looks slightly different, the picture doesnt show it that well but my thought is that prior to the cylinder going down I would say they looked identical in color and buildup of oil/carbon.

Here is some pictures, let me know what you guys think

Blue arrows show the one from the bad cylinder







 
Last edited by a moderator:
Maybe running a little lean? Possibly small amount of water ingestion? nothing missing from Carbies that could of caused damage?
 
The carbs were rebuilt in the spring and it ran like a top all the way until it blew out the cylinder. No bog or hesitation what so ever. I have not taken them apart yet but will be doing so before I re install the motor again.

As far as water injestion the ski has never been tipped, its a GTX, they don't roll over easy. I checked the exhaust/manifold gaskets which were new in the spring and there is no evidence of a leak by looking at the gaskets. Do both plugs look lean or just the bad one?

If it is a bit lean would the low speed adjuster be set too low? I believe they were set at 1 3/4 turns out and it idles smooth.
 
Something came apart somewhere and it chewed it up. I wouldnt sweat the plug color I bet its aluminum people are thinking is white electrode.

If you are rebuilding it it needs to be tore all the way down to be cleaned...you will find something missing. Broken ring, Piston pin clip, rod thrust washer, maybe a screw off a carb butterfly...something.
 
I think he is still under SES warranty. So hopefully Tom will make it right and it will be back running in no time.


Long live the 2 stroke Seadoo!
 
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