Piston Seizure On Pro Saws
Power to weight ratios are what sell professional saws. Both STIHL & HUSQVARNA know this. Their engineers work long and hard to produce new models of pro saws that offer better performance than previous designs. They achieve power to weight improvement by designing saws that utilize modern lightweight materials and by designing engines that operate at higher RPMs.
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, slightly to the left on the piston, is not light reflecting off of it. It's now shiny from metal transfer and has melted metal across the rings. Some of the metal is now impregnated in the cylinder wall as well.
Practice the Following to Reduce Seizures:
- 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.)
- Some cutters 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 causes an extremely lean running condition. This is a bad
practice and should be avoided. It
is best to refill the tank
before it runs out of gas. Today's pro saws
have semi-transparent gas tanks, so the fuel level can be 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.
- 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.
- 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.)
- 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.
Got questions or comments? Call or stop in.