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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.
In any situation where an engine has seized for no apparent reason, the motor
should be pressure tested before any other teardown work is performed. If a
mechanic does not have the equipment to pressure test your seized engine, it's
very unlikely that he will have the finesse to accurately diagnose your problem
either. In fact, pressure testing should be a standard finishing procedure for
any major engine reassembly work. Race engines should be pressure tested no less
than every 20 operating hours.
The air leak piston seizure, depending on the severity of the leak, can look
like a four corner type or a lean mixture type of scoring pattern. If an engine
is operating on the ragged edge of overheating, a small air leak can easily
cause the extra overheating that will result in a four corner seizure. On the
other hand, a huge air leak will draw in so much additional air that even an
over rich engine can experience a lean type seizure at full rpm's.
Detonation seizures - If a modified engine has been prepared
with too much compression or spark advance, or if it's run on unacceptably low
octane fuel, it will begin to "ping" or detonate. Detonation is a big
subject the merits another article of it's own. For now we only need to
understand that it causes a terrific amount of internal heat in a very short
time, as well as physical damage to the combustion chamber. If you have ever
seen the outer diameter of a cylinder head dome that looks as if it's been eaten
by termites, you have seen the results of detonation. It packs a physical force
that is roughly equivalent to hitting the edge of the piston crown with a full
arm swing of a ball peen hammer. In a short amount of time, this detonation
pounding will collapse the ring land and lock the ring in place (usually on the
hotter running exhaust side of the piston). As soon as the ring is locked, the
flames of combustion burn the oil film off of the cylinder wall, and the
scoring/ seizure process begins. Because of the exhaust side scoring and the
swift overheating caused by detonation, you'll have a 50/50 chance of a four
corner seizure or a "lean mixture" appearing seizure. Only an
experienced watercraft engine builder will be able to accurately diagnose this
seizure source.
Cooling system seizures - This cause cuts into the gray area of
piston seizure. A clogged cooling system on any machine can cause swift and
serious temperature problems. However, no engine will ever experience a seizure
purely as a result of inadequate cooling from the stock system. I have seen
several 50+ mph modified boats run an entire trouble free season with a bone
stock cooling system. These engines are not a statement of the effectiveness of
the stock cooling system, but rather a statement to the benefits of having a
professionally prepared high performance combination. The larger line and dual
line cooling kits certainly have their merits on high output race engines that
will be run at full throttle for extended periods of time. Their ability to more
rapidly exchange away engine heat is a great asset on modified engines that are
run at full throttle only. However, none of them can exchange away the excess
heat created by a poorly prepared engine package. If you are experiencing
chronic piston seizures of any kind, increased cooling may temporarily stave off
the problem, however it will almost never cure it.
Piston clearances - As mentioned earlier, too little piston
clearance is one of the most common "wrong" diagnoses made on seized
watercraft engines. Most of today's watercrafts come brand new with cylinder
clearances that are .001"-.002" over the recommended factory setup
clearance. This extra clearance is an added protection against riders who don't
follow the proper break-in procedure. If the clearance of a bored cylinder has
been set at the factory recommended clearance, the close piston clearance by
itself will not cause seizure. There is usually an added factor such as
excessive compression or an air leak. If a piston if fitted with too little
clearance, it will usually experience a four corner type seizure pattern. In
most cases the ring will experience little or no damage. If this is the case,
it's entirely safe to sand the score marks off the pistons and re-use them in
the freshly honed cylinder.
Too much piston clearance can also result in piston scoring and seizure. A
piston ring, in an excessively large cylinder bore, will have a very wide end
gap not to mention very weak ring tension against the cylinder wall. The flame
of combustion can easily burn past this weak ring seal as well as down the end
gap opening itself. If this flame burns off a significant amount of the oil film
on the cylinder wall, the scoring seizure process begins. Piston seizure caused
by excessive clearance is much more common than you might think, particularly on
iron cylinder 550's.
Break in seizures - The most common break in related seizure is
usually caused by the ring not the piston. Most new piston rings come with a
gray or green Teflon coating on their outer sealing surfaces. This coating seals
to the cylinder wall in just a few operating minutes, which provides better
power during the break in period. As the engine is breaking in, the Teflon
eventually wears away and lets the hard surface of the ring come in from behind
to provide the long term seal. The down side of this Teflon coating is that it
makes for a dangerously small end gap during the first hours of operation. If
the engine is run too hard too soon, the heat will cause the ring to expand in
diameter which will drive the ring ends together and drive the ring surface hard
against the cylinder wall. A piston ring that is being overheated in this
fashion will easily have enough tension against the cylinder wall to scratch off
the oil film which will begin the scoring/seizure process. A piston seized in
this way will have heavy scoring around the entire diameter of the piston, with
the ring usually locked into the groove all the way around as well.
Lubrication related seizures - By now it's apparent how
important the oil film strength can be. Equally important is the amount of oil
that is present in the engine. Most watercraft engines carry factory
recommendations for oil premix ratios between 40:1 and 50:1. For the rpm's and
operating temperatures of a stock engine, these mix ratios work fine. However if
your rpm range is significantly raised by aftermarket modifications or bolt on's,
you may need to increase your mix ratio slightly.
The total amount of time that it takes for a drop of oil to get from the
carburetor, to the lower end bearings, to the top end, and out the exhaust port
is called "oil migration time". As peak rpms increase, the amount of
time that a drop of oil remains inside the engine is drastically shorter. In
other words, a 9000 rpm race engine would need a mix ratio of about 32:1 to
maintain the same internal lubrication presence that a stocker would have with a
50:1 mix. There are several oil brands that claim that their oil can provide
equal lubrication at a leaner mix ratio (60:1 or 80:1) because of a claimed
better lubrication quality. In my 20 years of working on racing motors, I have
never experienced this to be true nor has any oil manufacturer, to my knowledge,
proven it to be true. It's like running your truck on two quarts of a special
oil instead of four quarts of a standard type oil. The quality cannot make up
for the quantity...ever.
Fables and untruths: Cold seizure - this is by far the most
over used "seizure scapegoat". It some how implies that the rider ran
his engine in a way that caused the failure. At least 95% if the "so
called" cold seized engines that I've seen have had a very apparent problem
elsewhere in the engine that the original technician failed to see. Telling a
customer that he cold seized the engine is an easy way for a mechanic to
immediately reverse the guilt and the responsibility.
If a freshly bored engine or a high performance engine were started from
stone cold, and then run hard at high rpm within 30 seconds of the start up, it
could very likely experience a true cold seizure. This happens because the
aluminum piston would experience a radically faster rate of expansion in that 30
seconds than the cylinder does. The reason for this difference in expansion rate
is two fold.
First and foremost, the internal temperatures that the piston crown is
exposed to at full load are on the order of 800°F - 1100°F. The gases passing
through the exhaust manifold ports is also in this 800° -1100°F temperature
range. The expansion rate caused by these temperatures is usually not a problem
when the water entering the exhaust manifold is being preheated by the very hot
exhaust port passages. However in the first 30 - 50 seconds of operation, these
internal exhaust ports have not yet absorbed the heat that's needed to properly
pre heat the cold incoming water. During the first 30 operating seconds, this
cold incoming water will briefly maintain the water jacket around the cylinder
at "stone cold diameter" while the piston is becoming "full
temperature diameter". On engines with properly sized pistons, the
difference in these diameters becomes much more than even the best oils can
withstand. Any engine that has been warmed up for 60 seconds or longer, would be
virtually incapable of a "cold seizure"
Cold water seizure - Like many other people, I have operated
stock as well as heavily modified machines in water as cold as 51°F with no
seizure or scoring problems of any kind. These are all typical machines that
were all originally prepared to operate in 60° - 80° water. It is my opinion
that as long as proper warm up is performed, blaming water temperatures down to
51°F for any piston seizure is just another case of scapegoating. I can
honestly say that I have absolutely no expectations of ever testing any high
output engines in water under 50°F. I would be keenly interested to read an
article from any thawed out individual who has.
Hot water seizure - the hottest water I have ever operated high
output race machines in is 92°F. These particular machines were equipped with
the same single cooling line system that they had used in the 62°F ocean only
two days earlier. Both test sessions put these machines (a 550 modified and a
650 modified) through numerous closed course and full throttle radar tests.
Subsequent internal engine inspections showed no signs of any piston scoring or
seizure what so ever. There was no doubt that the warmer water caused both
machines to lose peak speed ability after repeated radar passes. However these
losses leveled out after 4 or 5 passes. When the machine was permitted to cool
down briefly, the higher peak speeds would return. While this one experience may
not necessarily qualify as the gospel for every machine in the nation...It
qualifies for me.
Leaning out - this is a term for a phenomenon that doesn't
exist. It implies that a carburetor, whose needle/seat and high speed metering
screw is properly set, will suddenly begin to meter slightly less fuel to the
engine for no apparent reason. This does not happen...ever. In most cases what a
rider is actually referring to is the way his machine begins to slow down
noticeably during a long full throttle pass. In most cases this slowing is the
result of a serious overheating problem caused by excessive compression,
ignition advance or poor quality gasoline.
Seizure by running out of gas - as many people already know, a
larger size needle and seat must often be installed into a carburetor to contend
with the increased fuel demands of a moderately modified engine. If a modified
engine is operated at full throttle with a stock size needle and seat, it will
usually carry full rpm for about 2 or 3 seconds and then shut off as if someone
hit the kill button. When the machine comes to a stop, the rider touches the
start button to see what the problem is. The engine, no longer in fuel deficit
caused by the undersized needle and seat, unexpectedly starts right up.
This instant high speed shut off is caused by the carburetor literally
running out of gas. It is sometimes implied that during this shut down moment of
fuel starvation, the engine is also starved of the oil that is pre mixed in.
This perceived oil starvation is then blamed for any subsequent piston scoring
or seizure. Watercraft pistons do not seize in this way...ever.
At the moment that the fuel starved engine shuts down, combustion and all the
heat associated with it "ceases". Unlike a motorcycle whose rear wheel
traction continues to move the pistons in the bores at a very high
"friction causing" speed, the propeller and pistons in a fuel starved
watercraft stop turning almost instantly. The heat and movement friction that
can cause seizure are virtually non existent in a fuel starved watercraft shut
down. This same concept applies to any machine that simply runs it's main tank
dry before having to switch to reserve.
It is possible for a rider, whose carb has an under sized needle and seat, to
induce a piston seizure. However this would require a great deal of combined
skill and stupidity. Once the rider has established that extended full throttle
operation causes his engine to quit, he might make the very poor choice of only
applying enough high speed throttle to avoid starving the engine. When he does
this, he will be capable of maintaining about 90% throttle which will hold the
engine endlessly on the lean thresh hold of fuel starvation. As this rider
eventually masters this throttle position, he will be able to maintain a very
high rpm with the carb feeding a horrifically lean mixture. Ultimately his
finesse will be rewarded by one of the most abrupt and destructive lean mixture
seizures that his mechanic has ever seen.