Piston seizures - by the technicians of Group K. this company offers the public a valuable service they can found online. they sure seem to know there stuff
It's a common sight. You see a boat on the beach with the hood upside down and the top end off. A group of technical "guess men" are assembled in a circle passing around a seized piston as if touching it can give them greater insight as to the reason for it's failure. Unfortunately, when this whole mess of parts gets dragged down to the shop on Monday morning, the local mechanic may not be able to provide much more insight unless he is very familiar with that particular engine's "sources of seizure". It's a lot to ask.
Even among mechanics, there's plenty of confusion about what causes a particular piston to seize. This confusion is compounded when the mechanic only gets to see the remnants of the meltdown instead of the entire machine. The following information will help to dispel some myths, and shed some light on the understanding of piston seizures. The objective of this article is to make piston seizures a part of your past.
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.
Air leak seizures - These are very common seizures because air leaks themselves are so common in watercraft motors. If you could pressure check every engine that showed up at a local racing event, you would find over 50% of them to have an air leak. Because of the varying degree of these leaks, some will result in seizure, others will only cause poor carburetion or slight overheating. The varying effects of these air leaks makes this a difficult diagnosis.