Imminent motor rebuild, advice/tech info needed.

[BruceSki]

You want to check rod play in the up and down direction with the crank in a fixed position. I usually check this still in the cases by holding the drive coupler and trying to wiggle the rod up and down. The left and right movement of a half mm between the thrust washers is normal. Once your piston goes into the sleeve the rod should never move side to side anyway.

It's possible that those marks could be from detonation, but it looks a lot more like a piece of bearing cage let go and bounced around in the combustion area to me.

You should have pulled the drive coupler before removing the crank, it's much more difficult to do without damaging the crank when it's outside of the cases.

Hard to tell the shape of your crank without doing a visual inspection myself but everything looks okay at a quick glance at the pictures.

 

[Nuklear6]

If crank does check out ok. I'm thinking throttle or choke screws came loose. It Def ingested something metal. Deto is not to blame for this one

 

[TheBuzzard]

its the back piston thats chewed so its one of the back 2 bearings. probably the one that spins the best and is the loudest.

 

[Harbor]

. Just wishful thinking has me hoping the crank was replaced when those rings were done and for some insane reason the piston wasn't swapped too. I don't see how it could have scarred that piston and not damaged the ring.

You may be right, that it's from a prior failure that was repaired the way you've stated above. When I look at the pistons, It looks like some horizontal and non horizontal sanding/scotch bright marks in the area of the piston scarring. Strange how the damage is in the area of the ring gap and the distance from the gash over to the ring locking pin is about the same as the distance from the ring land down to the bottom of the vertical gash. That nice clean dent in the dome above the ring lock pin reminds of a ring, like a hung ring was the previous failure

 

[jaxrider]

that looks like old damage, someone put it back together with old piston. hows the upper rod bearing surface look?

 

[Nuklear6]

You guys are totally correct. I see pistons come out like that after bearing or ring failure so often i skipped right past the fact that the rings look perfect. Crazy to think someone would do that. Reassemble an engine with those Pistons and the bore looking like that. If they had the mechanical ability to clean the ring glads up for the rings to fit back in, they more then likely knew better but didn't care.

 

[Sparkplug]

I put pistons back in if they have dings in the top. I smooth the sharp edges. I put worn cylinders back together also. As long as they hold compression. I run old cranks. I put 6m6 bottoms on 62t cases. I run cranks without all the pins. I've never once bought a carb rebuild kit.

I spend more time on the water riding, and less money on parts and downtime, then any other rider I know. It's all about spec.

All my skis are yamaha engines tho. They aren't 10k engines. If they were I'd be a tad more anal about what goes into them. But I will always always still use parts that are in spec and pass visual inspections.


Download the fsm and go buy some tools to check things, and you will be alot further ahead than asking all us what's wrong with your engine with just a few pics. Your going to get 500 different answers. Othing teaches better than experience.

 

[Nuklear6]

Running used, carefully inspected miss match parts, and Pistons with dings in the top is a bit different then what was done with this engine. There are severe gouges in the piston and cylinder wall. Some one took time to re open the ring gland for the new rings. This is something i could see myself doing in a hotel room to my or a friends engine to get through a freeride. But to sell it knowing it was this way and not disclose to the potential buyer is wrong.

 

[Benwaahh]

Interestingly enough, my car finally gave out today :/ so there's a chance I'll be selling this ski once I get it where it needs to be. There's no way I will put this ski back together any way but with quality, new parts with machine work done by the best so that whether I decide to ride it, or sell it I can do so with confidence. I deal with too many customers that try to diagnose/fix their own hvac systems, and end up worse off than they started, to try and do this myself. We are fortunate enough to have professionals on here that do this for a living help us diagnose issues without expectation of payment, and when the time comes they'll be the ones who get my business.

 

[Sparkplug]

I wouldn't put that engine back together either. Never recommended that in any way. I gave advice to get a manual so you can learn the proper specs and way to disassemble your engine. Properly. There is a proper sequence for taking all the parts off. Parts you already took off.

Any machinist is gonna machine it to yamaha specs, unless you get one of those guys that knows more than yamaha team of engineers. There seems to be a few...

If you buy new for reliability, do not buy anything other than the exact manufacturers that yamaha uses. Otherwise your getting parts that don't see near the qc.

I work with machinists and engineers all day long. Don't believe all the hype you read on here. Very few on here are actual pwc "professionals"

Good luck and hope all goes well for you with your rebuild! Cheers!