Degand22
Active Member
Hello guys I'm new to the waverunner world. Just got a 96 GTX 787 an was wondering why my pistons look like this. It still ran great an both cylinders had 150psi compression. Lack of oil or heat? Thanks for any input.
Piston scoring
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Degand22
Active Member
Was cleaning the rave valve an replace manifold gasket an seen the scoring.
Degand22
Active Member
Mean water getting into the cylinders.
The oil pumps don't go out and if it did it would have most likely seized solid.
What color is the oil you are using?
By the scuffing and discoloration is sure looks like they are just worn out and the large piston to cylinder wall clearance is causing the oil film to be lost. I would have the pistons and cylinders professionally measured for clearance and see if you are out of spec.
What color is the oil you are using?
By the scuffing and discoloration is sure looks like they are just worn out and the large piston to cylinder wall clearance is causing the oil film to be lost. I would have the pistons and cylinders professionally measured for clearance and see if you are out of spec.
Degand22
Active Member
The color of the oil is red. An I just got it last Sunday. Rode it for a hour on the lake ran great. Was just cleaning the rave valves an seen the scoring so I tore it down to find this.
I found the same problem with my first ski. Compression was above 140 and to my surprise look at the damage. Lots of scouring and the top ring was broken. If I pull the top end, it gets new pistons and depending on the cylinder size and condition... bore or hone. This was my first jet ski and being an old two stroke motorcycle guy, my first move was nix the oil injection. 
hon
hon
I found the same problem with my first ski. Compression was above 140 and to my surprise look at the damage. Lots of scouring and the top ring was broken. If I pull the top end, it gets new pistons and depending on the cylinder size and condition... bore or hone. This was my first jet ski and being an old two stroke motorcycle guy, my first move was nix the oil injection.
honView attachment 44427View attachment 44428View attachment 44429
As you ( and many others here) have proved is that good compression means very little as far as piston condition on these engines.
Chester
Amen to that brother. In the last picture you can see the wrist pin clip failed and the wrist pin was digging a hole in the cylinder bore. The owner just putted around on the ski (35mph) and didn't notice any issues. I think compression was around 110 or 115. Unbelievable.
In the last picture you can see the wrist pin clip failed and the wrist pin was digging a hole in the cylinder bore. The owner just putted around on the ski (35mph) and didn't notice any issues. I think compression was around 110 or 115. Unbelievable.
R-u-n2it
Active Member
Deegand22 pistons were cold seized, engine wasn't warmed up and heavy throttle and likely fairly cold water. When you see majority of damage/scuffing on sides of piston adjacent to wrist pin boss, there is more aluminum there and aluminum transfers heat faster than the cast iron sleeve can expand and the piston distorts. A jetting seizure will have the majority of the scuffing on the piston face at the exhaust port, that extrudes the aluminum over the rings the 1st time then 2nd ride the rings don't seal and you get flame/combustion and heat which quickly can transfer aluminum to the cast iron sleeve. Ever been riding a 2 stroke that locks up (seize) but after it cools it starts up and seems ok only to seize again and it's toast? On a 2 stroke that has an exhaust port bridge you see the deepest scarring on the piston where the bridge contacts if lean and/or new bore and bridge not relieved enough. Listen to Miki, especially good synthetic 2 stroke oil.
HEY, ANYONE GOING TO THE WORLD FINALS?
HEY, ANYONE GOING TO THE WORLD FINALS?
Actually cold seizures are actually old wives tales. On a stock engine it’s almost impossible to cold seize an engine unless the clearances are setup wrong or the water is freezing and you run WOT from the moment it hits the water.
Back in the day Wiseco pistons were notorious and everyone said cold seizure but that’s only because the machine shops were setting them up with the wrong clearance for cast pistons when they were forged.
Back in the day Wiseco pistons were notorious and everyone said cold seizure but that’s only because the machine shops were setting them up with the wrong clearance for cast pistons when they were forged.
R-u-n2it
Active Member
Lack of good oil can cause seizure, but Deegands pics show more scuffing near the pins and indicate insufficient warm up due to unequal rates of thermal expansion, and if a 2 stroke with clearance setup wrong (tight) just riding it hard after it warms up can cause it to seize but would likely differ in ballistics since pistons are not machined round to start with. Lean can cause seizure but etemplets first pics you referenced doesn't look lean by the damp looking black carbon. I'm just looking at the pics and describing what I see. So with a piston thermal expansion (COE) roughly twice that of a cylinder liner you don't advise to warm up the engine before full throttle operation? Hey, maybe I've been wasting time and fuel and suffering old school syndrome.
Not wasting time at all, and I am also "Old School" and warm up all my skis because.... well... I'm Old School and it makes me feel better even though I know better,LOL.
Take a look at this form one of the masters, Harry Klemm...
Some fundamentals - Many people believe that piston seizures occur when engine heat causes the piston to expand larger than the size of the cylinder bore...this is not true. If you could freeze your engine "in motion" in the middle of a long full throttle pass, and disassemble it for micrometer measurement, you would find the piston to measure at a .0005" to .0015" press fit into the bore. That's right, a slight press fit! The reason that it doesn't seize is because the premix oil has such a terrific film strength that it acts as an irremovable buffer between the piston and the cylinder. That is, the bare metal surface of the piston never actually touches the bare metal surface of the cylinder because the oil stays between them. Many mechanics have experienced this phenomenon while cleaning a freshly bored cylinder. Completely dry without cleaning solvent, the piston moves through the bore with difficulty. After rinsing the piston glides all the way through with no resistance at all. This is because the solvent acts as a film between the piston and cylinder.
A piston seizure can only occur when something burns or scrapes away the oil film that exists between the piston and the cylinder wall. Understanding this, it's not hard to see why oils with exceptionally high film strengths are very desirable. Good quality oils can provide a film that stands up to the most intense heat and the pressure loads of a modern high output engine.
The difference between seizure and scoring Seizure and scoring are two different stages of the same problem. When the oil film on a cylinder is momentarily burned or brushed away, the metal surfaces of the piston and the cylinder wall will actually touch. When this happens, there is a sort of scraping that takes place between them. When the oil film is resumed, the marks from this scraping will often remain on the piston and (or) the cylinder wall. This momentary scraping or "scoring" seldom causes any permanent or performance robbing damage. No significant damage takes place because the oil film is resumed before the piston and cylinder have a chance to start exchanging material onto one another.
Scoring is commonly seen on the piston face directly below the piston ring end gaps. The blast of combustion can get between the large end gap of a worn out ring and burn the oil off the piston and cylinder in that area...Hence the surface scoring. In most cases, score marks can simply be sanded off of the piston and cylinder. However when ever you see scoring, it's a good idea to find the source so that it doesn't develop into a full blown seizure.
Seizure is a case of scoring where the oil film does not immediately return. After a few moments of constant scoring, the piston and cylinder will scratch each other hard enough to remove material from each other. This floating material grinds itself into the piston and the cylinder as it continues to grow in size. As this snowballing material grows, it will drive the opposite side of the piston against the cylinder wall with a pressure so terrific that scoring begins to take place. While all this is going on, your engine is still running wonderfully at full throttle.
The death blow comes when the mass of material between the piston and the cylinder wall finds it's way to the piston ring. This nearly molten mixture of aluminum and iron will instantly lock the ring in it's groove. This ring locking, not the piston surface seizure, is what actually causes your engine to quit. When the piston ring becomes locked back in it's groove, it's incapable of providing compression sealing against the cylinder wall. This instant loss of compression, while the engine is at speed, causes a dramatic loss of power. That power loss, along with the added drag of the badly seized piston, makes the engine quit or lock up in a nanosecond. In fact this entire seizure process, from the first scoring scratch to the piston locked solid, takes less than a second at full rpm. That's not even enough time for you to utter the first syllable of your favorite profanity.
There are many different causes or sources for piston seizure. Each cause has it's own symptoms and it's own visual results. The following is a description of several very common types of seizures, and the most common problem source for each one. It should be understood that diagnosing piston seizures is not done with any precision by even the best engine builders. However this information may allow many of you to make a more educated and accurate guess.
Four corner seizure - This is by far the most common type of seizure found in personal watercraft engines. Both sides of the piston will show heavy scoring and seizure marks on each side of the wrist pin hole. The pattern of these four seizure points often appears to be a perfect square, hence the slang term "four corner". The scoring takes place in this pattern because those areas of the piston casting are the thickest. When the piston is seriously overheated, the thick areas will expand and distort the most. High output motorcycle engines usually experience this type of seizure pattern when a piston has been fitted with too little clearance. Most experienced , and well meaning, motorcycle mechanics would take one look and immediately say that insufficient piston clearance is the cause. However that diagnosis, on watercraft engines, would be wrong about 99% of the time.
Four corner seizures in watercraft engines are almost always a result of the engine creating more heat than the cooling system can exchange away. That is not to say that most cooling systems are under built, but rather that it's easy to make a modification that creates too much internal heat for even the most beefed up cooling systems. Even though a constant feed of cool water is being moved through the cooling system, the cooling system must be capable of exchanging the engine heat away at a rate quicker than the engine is creating it.
The engine factors that have the greatest seizure related effect on operating temperature or excessive compression ratio or ignition advance, high rpm, insufficient fuel octane level, insufficient cooling, or any combination of these. Properly adjusting these same factors will have the greatest effect on total power output. The job of the professional engine builder is to find the right combination, or "blend", of these factors that will result in strong overall power output at a pace that your cooling system can keep up with. There are many mechanics and engine builders who have mastered their own combination "blend" that can get you all the power your after without risking a seizure.
A group of mismatched modifications is a first class ticket to "seizure-land". Any inexperienced individual who sets up your engine with over 200 psi of compression and advanced timing, is also guaranteeing your arrival. If your big mouthed motorcycle buddy down the street tells you that he can make any engine "roost"...you should think twice. You could be in for a very expensive lesson.
Lean seizures - Despite what most people think, lean fuel mixture seizures on personal watercraft engines is actually a very rare occurrence. The high speed circuit on almost all personal watercraft carbs is responsible for delivering fuel in the 30%-100% throttle range. If the high speed circuit is lean enough to cause piston seizure, it will also cause an almost un-ridable hesitation or laziness in mid-range throttle response. Dangerously lean high rpm racing motors can sometimes offer acceptable mid-range, however they will accelerate to peak speed very slowly.
The classic lean seizure exhibits heavy scoring and seizure along the entire width of the exhaust port with only light scoring on the opposite piston faces. In lean mixture conditions, the exhaust gas temperatures escalate quickly into the meltdown stage. Those high temperature gases can compromise or completely burn off the oil film on the exhaust piston face as the exhaust port is being covered up. With the oil film weakened or gone, scoring quickly turns into seizure and ring locking.
Very interesting comments. I too spend about 5 minutes warming up my skis on the water, then gently apply throttle. Half-speed for a bit before I go wide open. I also seldom hold any engine of mine wide open for long periods. I back off to about 3/4 throttle and find the sweet spot. Good Stuff.
R-u-n2it
Active Member
Glad to hear you do a warm up, I could not believe after all your posts I've read that you wouldn't. Funny you bring up Group Ks info, probably been 20 years but I found something on his site back when he was into the old 2 stroke road race bikes, been so long don't remember what it was exactly but ended up on the phone with Harry and he thanked me for pointing out something. 2 signs of aging: 1) short term memory loss and 2) I forget what the other one is at the moment.
"Most experienced , and well meaning, motorcycle mechanics would take one look and immediately say that insufficient piston clearance is the cause. However that diagnosis, on watercraft engines, would be wrong about 99% of the time." Actually there IS too little clearance for that particular engine when one is determined to extract as much power as possible. Imagine the result of no warm up on an engine like that. I remember looking at Chris Fishetti's World Finals winning 787 XP and it had 2 waterlines going into the middle of the head near the carbs and it exited at the oem in and out positions. I even saw water hose routing to the counter balancer front bearing. These were in carbon fiber hulls back then. I built my ski following a lot of Pops Fischetti advice. Klemms remarks about what causes overheating beyond stock cooling systems capability is absolutely right. Ignoring any of those and one will get a lesson of what is detonation is and how it differs from pre-ignition (pinging). I went as far as having a computer program to plot timing curves for the MSD to be able to run higher compression and advance timing in the lower through mid range and then retarded at high rpm to get the 7880 rpm back. I finally went from the X to XO Solas (about 1/2 deg more pitch on the trailing end) as the RacePak recorded 2 hits of 8000 rpm, enough to explode the starter flywheel and go through housing and hull. That Doo was all I did every day in the summer back then. Anyways, enough of my rambling.
Another guy that's the very sharp pointy tip of the spear is Jerry Hall, if you ever have the chance to talk to him take notes. He's worked with all the big for OEMS on development projects like the "LT 500 Quadzilla". That's my winter ride that keeps me outta trouble, I need to back off both to get to the Rotax power flying machine that's sat for years. Oh yeah, the plane with 39 hours on a rebuild had 3 point seizure scoring I discovered inverting the engine before I ever flew it. He probably had an inflight engine out and he freaked then sold it. Nice guy not saying a word, after all it's an airplane...
Hey, anyone going to World Finals? Saturday at Crazy Horse and night freestyle by Kokomos, other days (7 days for all classes) go to Body Beach and check out the fast boats around the buoys. This is 1 reason to go in the past ----> " The new Sea-Doo RXT, GTX, and WAKE PRO watercraft models made their public debut at IJSBA World Finals in Lake Havasu City, AZ where guests were able to test ride the new platform featuring the ST 3 hull." ....only if you want to ride skis that you can't afford.....
"Most experienced , and well meaning, motorcycle mechanics would take one look and immediately say that insufficient piston clearance is the cause. However that diagnosis, on watercraft engines, would be wrong about 99% of the time." Actually there IS too little clearance for that particular engine when one is determined to extract as much power as possible. Imagine the result of no warm up on an engine like that. I remember looking at Chris Fishetti's World Finals winning 787 XP and it had 2 waterlines going into the middle of the head near the carbs and it exited at the oem in and out positions. I even saw water hose routing to the counter balancer front bearing. These were in carbon fiber hulls back then. I built my ski following a lot of Pops Fischetti advice. Klemms remarks about what causes overheating beyond stock cooling systems capability is absolutely right. Ignoring any of those and one will get a lesson of what is detonation is and how it differs from pre-ignition (pinging). I went as far as having a computer program to plot timing curves for the MSD to be able to run higher compression and advance timing in the lower through mid range and then retarded at high rpm to get the 7880 rpm back. I finally went from the X to XO Solas (about 1/2 deg more pitch on the trailing end) as the RacePak recorded 2 hits of 8000 rpm, enough to explode the starter flywheel and go through housing and hull. That Doo was all I did every day in the summer back then. Anyways, enough of my rambling.
Another guy that's the very sharp pointy tip of the spear is Jerry Hall, if you ever have the chance to talk to him take notes. He's worked with all the big for OEMS on development projects like the "LT 500 Quadzilla". That's my winter ride that keeps me outta trouble, I need to back off both to get to the Rotax power flying machine that's sat for years. Oh yeah, the plane with 39 hours on a rebuild had 3 point seizure scoring I discovered inverting the engine before I ever flew it. He probably had an inflight engine out and he freaked then sold it. Nice guy not saying a word, after all it's an airplane...
Hey, anyone going to World Finals? Saturday at Crazy Horse and night freestyle by Kokomos, other days (7 days for all classes) go to Body Beach and check out the fast boats around the buoys. This is 1 reason to go in the past ----> " The new Sea-Doo RXT, GTX, and WAKE PRO watercraft models made their public debut at IJSBA World Finals in Lake Havasu City, AZ where guests were able to test ride the new platform featuring the ST 3 hull." ....only if you want to ride skis that you can't afford.....
R-u-n2it
Active Member
Sorry, just remembered something on the 785/787 for guys that find got looking used parts for their ski, do a search for the rave valve itself, there's a part number that has been identified by some that is prone to breaking at the stem and allowing it to drop into the cylinder while running. I had one that broke but was before I read the warning. Miki probably knows the number even.
Madpaddler
Member
That would be part number 290854350. It was the RAVE guillotine for the 1995 XP800. The new part number that fixed the issue on the 1996 models is 290854352.
Madpaddler
Member
So basically it should only be an issue if you're in possession of the extremely rare 787X that came in the earliest of 800-class XPs.
R-u-n2it
Active Member
My XP is a 95 800, in avatar or profile, on 3rd set of holders and lost count of valves after 20 years. After figuring out detonation in 97 I've only had to rebuild once, top & bottom end, seems rotary valve 2 strokes blow up curve turns sharply upward when you exceed mid 7800 rpm every time you pull the trigger....
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