how did i get a chunk out of my piston???

[craig4wd]

I have 93-95 sportjet 90. I put it in to the Merc. dealer to have the carbies tuned. I got it back, it ran great for about 2hrs and died. <br /><br />I lost compression in cylinder 2. I got the manual (thanks CrazyJames), took it apart and there's a big chunk in the piston (see link).<br /><br />The top rings missing about 2 inches, the bottom ring is jammed in.<br /><br />My question is how did this happen? Could the ring have just broken apart and scored the piston or has the mechanic dropped a screw down the inlet when doing the carbies? What should I do? Pictures of the piston

 

[steelespike]

Re: how did i get a chunk out of my piston???

Craig, In my opinion it would be pretty hard for your motor to injest something that far into the engine.There are to many other things it would tear up before it got that far.My guess is some kind of spontanious failure.If some kind of foriegn matter got in the piston it would be<br />more evident. Im not an expert,just going by the pistons Ive seen after swollowing a part.<br /> good luck.

 

[fireman 757]

Re: how did i get a chunk out of my piston???

the shop would have no reason to be any where near the pistons if they just rebuilt the carb. Pull the head and piston. have the cylinder rehoned and replace the piston, rings, and bearings. Check the crank to make sure it isn't bent.

 

[Franki]

Re: how did i get a chunk out of my piston???

Thats a strange shape for a spontanious failure..<br /><br />it certainly looks like something got stuck between the piston and the exhaust port lip..<br />if a ring got carboned up, and stuck, then caught on the exhaust port, that would result in something like what you are seeing..<br />was there alot of carbon???<br /><br />I'd also be checkin your reed valves while you are in there.. (though if your motor is that "apart" you probably already have the reed out...)<br /><br />last time I saw a chunk like that missing, it turned out that the skirt was missing from the piston and had exited the exhaust, (that was on a motorcross bike) and it took most of the exhaust header off on the way out..<br /><br />regards<br /><br />Frank.

 

[G DANE]

Re: how did i get a chunk out of my piston???

Hi Craig<br /><br />The only time i saw something like that was a left over screw, that was sucked thruogh the intake and got snapped in the exhaust port. Do you have alle your screws from intake silencer ? Does any of them look new ? The center screw from reed plate ? Have a chat with the ones that rebuild carbs.

 

[12Footer]

Re: how did i get a chunk out of my piston???

It could've also injested a reed, so check the reed block too.

 

[IndySpecialty]

Re: how did i get a chunk out of my piston???

Craig4WD: How long did you run it after this happened?, Its hard to see in the pic. but,If you look real close there is a big burn of blow-by right under that spot. and the ring-land looks hammered apart like the ring was broke for a long time & as the ring starts breaking into smaller & hooking the port as it goes by, its only a matter of time before a peice gets caught. It looks like the damage happened with the piston moving down, Almost all ingested objects will get nailed by the piston when its on its way up. So sense I can't see the picture very well you will have to deside which way the piston was going by the damage to the port & the piston. if the port as a fair amount of damage on the top & bottem of the port right at the point where the piston suffered the damage, you can bet it was just carbon build-up behind the ring that restricted its movement & the ring just broke. & sense the factory doesn't Chamfer the ports very well. its easy for them to hook when they start getting sharp.<br /><br />Good luck.

 

[ODDD1]

Re: how did i get a chunk out of my piston???

Craig, looks like a classic top ring failure...these rings have a very steep taper on the upper inside edge, making them very active in the ring land [groove]...they will usually wear away the lower side of the land starting at the ring locating pin until the ring can rotate over the pin, break, and fall partially out the exhaust port..the end result is what you see. I really doubt just a hone and new piston/rings will be all, this does not happen without bore damage...the beginnings of this failure were very likely there before you took the motor in....why would you take a well running engine in?