Piston Seizure On Pro Saws
Pro saws with high power-to-weight ratios are what every professional saw user wants. Today's saw manufacturers work long and hard to produce new pro saws that offer better performance than previous designs.
Power-to-weight improvements are accomplished by designing saws that utilize modern lightweight materials and by designing engines that operate at higher and higher RPMs. These high speeds combined with today's fuel make it more important than ever for pro saw users to understand what causes an engine to seize and how it can be avoided.
This is where the piston started to stick to the cylinder wall. Notice the seizure marks are located near the exhaust port. This is typical damage caused by a lean seizure. Operating conditions that can cause a lean seizure include: incorrect carburetor settings, a clogged fuel filter, stale gas, alcohol in fuel, an air leak, part throttle operation, or sometimes a combination of factors.
As engines run at higher speeds and produce more power, the factors that cause piston seizure also increase. At 13,000 RPM, a piston makes 430 trips up or down the cylinder each second. Heat generated by combustion and the friction between the piston, rings, and cylinder wall, build as engine speed increases. If this is not offset by better lubrication, and improved engine cooling, the piston will simply get so hot it will try to weld itself to the cylinder wall.
Today, high-performance pro saw engines are built to survive at this speed. A well maintained saw engine, being operated properly, using proper gas/oil mix, with a carburetor properly adjusted, will not seize. But remove or change some of these conditions and you run the risk of an expensive meltdown.
Since most piston seizures occur because of the way a saw is operated, saw manufacturers rarely cover seizure damage under warranty.
The photo illustrates how the surface of the piston can become so hot metal actually transfers from the piston to the cylinder wall. The "light" spot is from metal transfer. Some of it has melted metal across the rings, too.
Our Advice
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Adjust the carburetor properly. This helps keep the saw from building excess RPM. It also reduces or eliminates lean detonation which causes heat buildup in the cylinder. Contrary to what many cutters believe, the saw actually produces more horsepower when it is adjusted properly. It may not scream like a "leaned out" one, but it will make more stump power. Resist the temptation to make it sound good. (Please look at the section on carburetor adjustment.)
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Refill the saw before it runs out of gas. Some pro saw users make a practice of shaking their saw when it starts to run out of gas. Shaking helps pick up every drop of fuel in the tank, but it also causes an extremely lean running condition. This is a bad practice and should be avoided. Today's pro saws have semi-transparent gas tanks, so the fuel level can be easily seen without loosening the cap. After you have run a tank for a while, glance down and check the fuel level before making a new undercut. There is no excuse for starting a cut without plenty of fuel.
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Clean the cylinder fins, flywheel fins, and fan housing. Certain working conditions cause these to become blocked by sawdust and debris. When air does not flow around the cylinder, seizure becomes a threat. The cooling system can be cleaned with compressed air or scraped out with a bar tool.
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Use supreme grade unleaded fuel. Today's pro saws require 89 octane or better. We recommend using supreme grade fuel which is 91 octane at most gas stations. (See the section on Mixed Fuel for more information.)
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Use the manufacturer's brand of mix oil. Use it in the proportion the manufacturer recommends. (See the section on Mixed Fuel for more information.)
Reduce the risk of damaging your pro saw engine by following these simple tips. Our goal is to help you get top performance and many hours of trouble-free service from your pro saw engine... That's why we're the Pro's Choice.