99 GP800 Top End Rebuild questions

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99 gp800
#1
Hey guys!

So our GP800 decided to be glutenous and devoured a power valve (typical of this ski I know). We bought it used from someone 2 months ago and they had assured us that the Wave Eater clips had been installed (that turned out to be a complete lie)...

Anyways I dropped in an endoscope and saw a ridiculous amount of shavings and just assumed Id have to replace the entire top end. However I opened it up today, vacuumed up the debris and for the hell of it pushed down on the seized piston (nothing happened). Pushed down on the good piston and voila it moved! Vacuumed again then pushed the bad piston down and it slid down with the same amount of effort as the good piston..... When I pushed it down I could see large pieces that had wedged the piston and even damaged the edge of the piston...

So here is where I need a little advice please.

See the pics below. Obviously I still need to open up the crank case to inspect and look for metal, take out the piston, get the tolerances measured, pressure test etc .... but I am amazed that there is no scratch on the piston cylinder wall. All marks that you see I can actually rub away with my hand... Yes even that double black mark you see in the photo wiped off with a paper towel. Feels smooth (could still be damaged)...

Where could I take the piston cylinder/sleeve it to have it inspected/measured? What do you think the odds are that the sleeve doesn't need any work and I could just slap in a new piston or even piston ring and be good to go? I even slid my nail around the entire inside of the piston cylinder and it didn't catch on anything... I'm kind of baffled really that there isn't more damage.
 

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mikidymac

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#2
Ok, all that metal came from somewhere and I bet it is from the piston skirt.

Pull the engine then the cylinders and you are going to have to split the cases to inspect and flush the crank bearings.

Any machine shop can measure and inspect the cylinder bores. The service manual will have the bore tolerances. Keep in mind these are plated aluminum cylinders so you can't bore them and any damage or wear means the cylinder is basically junk. There are a few companies that replate them like Millennium Technologies but you really can't sleeve them or they will run too hot.
 
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99 gp800
#3
If you look at pic "bad piston clean 1" in the exhaust port whole there is a circular object, that's the support shaft of the power valve. All that metal was the power valve, and there is a good chance the base of the piston is toast.

Thanks for the help I started calling around and an couple shops offered free inspection until i told them it was a waverunner then they said $50 haha... obviously they were hopeful for an engine to work on. When I open it all up this winter and finally see all damage I'll be back to post more.

Thanks guys
 

mikidymac

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#4
My local shops will measure the bopres for free as long as I bring them in clean and have the specs printed out for them.
 
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99 gp800
#5
Oh man... wife let me wrench over the weekend so I opened her up today and took the engine out.... not a good day haha.

On the plus side I found the power valve!... well one of them at least... it appears when they rebuilt the motor they just didnt put the 2nd power valve back in... I don't know how I didnt notice this when riding, you'd think it would be bogging down on the low end when I hit the gas.

Now to replace the broken block, new piston, power valves and WE clips, or just part it out and find someone to buy the hull as it's in great shape... decisions decisions...

66E00 turns out was the same for all the Yamaha 800's internationally... new block from America costs about $800, but in Japan I can find the exact same block with piston for $350 shipped... crazy.
 

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seadoosnipe

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#6
Wow, surprised it even ran.. There are a few ways you can go about this. The post was made in September, so I'm not sure what you did so far. You can actually buy a top end kit, that comes with everything complete to build, pistons, jugs, wrist pin bearings, rings, connecting rods, all gaskets, for right at $475, which if you think about it, all the time you'll save, it's actually worth it. When you tear these engines down, being careful what you purchase, because you can't take the bottom end casing, and change it out with another. The casings are actually formed as one. If you open the bottom up, and it looks ok, then to get this entire upper end, for the price they are asking for, you can't beat it. Even if you go to a machine shop, have them bore it over .030, you'd still have to buy 30 over pistons. I think I can attach it here. I saved it as a PDF; mechanic I am, paperwork artist I'm not. But, good luck! Post your results! If you want to look at these parts, go to Short Block Technologies. I've used them for years!
 

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