Rod Bearing Failure On Pro Saws
The crankshaft rod bearing, or "big end" bearing, as we like to call them, is an expensive part. Not only is it expensive to replace, but if allowed to fail and break, a good cylinder and piston can be ruined, too. Parts of the broken bearing get loose in the bottom end and try to transfer to the top end. They usually don't make it and end up wedged between the piston and cylinder wall. When this happens, the crankshaft, cylinder, and piston all have to be replaced. The only thing more expensive than this repair is the price of a new saw.
The red arrow points to a place where part of the bearing "cage" is missing. While this saw still ran, the missing parts got loose in the engine and damaged the piston shown below.
On some saws, a worn rod bearing could be replaced without changing the whole crankshaft assembly. On today's pro saws, this bearing is not serviceable. The reason is a separable rod end is weaker and heavier than today's one-piece design.
These fine scratches and "peppering" is caused by the lower rod bearing failing. Small, but hard pieces of the bearing and race are breaking loose and causing this piston damage.
Why Rod Bearings Fail:
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Wear. All bearings will eventually wear out. With time and use, big end bearings fail. This usually occurs after hundreds of hours of use. When failure occurs, most saw owners do not repair the saw motor because all the other components are worn out, too. Chances are, it's also an obsolete model at this point.
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Excessive RPM. Big end bearings can fail in minutes if subjected to excessive RPM. The reason is: During regular operation the rollers (in the big end bearing) roll on the outside surface of the crankshaft journal and on the inside surface of the connecting rod. A light film of mix oil coats the bearings with every rotation the crankshaft makes. This bath of oil (and fuel) lubricates the bearing, which removes friction and keeps the bearing running cool. When the engine is subjected to excessive RPM (like limbing with the throttle wide open), the rollers roll as fast as they can and then start "skating." This skating or sliding activity scrapes away the cushion of oil on the bearing surfaces. The bearing gets hot instantly. If the engine speed does not come down to where the rollers can roll again, it fails quickly. Even if the speed returns to normal, the bearing sustains some damage.
Our Advice
Have you ever worked on a crew where some guys seem to make a saw last forever, and the next guy goes through cranks every few months? The guy with the short crank life probably allows the saw to spin too fast at some point in his routine.
Excess RPM does not occur when the saw is buried in the cut. We do not recommend that you operate your saw at part throttle when the engine is loaded. But if you have a tendency to over-speed your saw engine because you like chain speed when you limb, back off and your saw engine will last a lot longer.