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Bearing condition

by Had Robinson

The condition of the bearings is something pilots must keep track of when their engines have more than 250 hours or so.  Much depends on the care with which the engine was operated e.g. did the pilot use AVGAS with fully-synthetic oil or did he use Wal-Mart grade lawn mower oil with cheap gasoline loaded with ethanol?  A well-taken care of engine can easily go 500+ hours without any major overhaul.

The bearing that wears the fastest is the upper connecting rod bearing (a.k.a. the piston roller-cage bearing).  The lower connecting rod bearing is not replaceable.  Ordinarily, the entire crankshaft assembly must be replaced if the lower bearing has failed.

Below is a photo of a Minari lower connecting rod bearing that was destroyed when the pilot overheated the engine.  The entire engine had to be overhauled and all moving parts replaced, including the main bearings.

Minari lower connecting rod bearing failure

Pilots can check the general condition of the bearings by listening closely to the engine while it is running at medium or slow speeds.  Any knocking sound (like hitting a piece of metal with a hammer) must be checked out promptly.  An easy way to get some idea of the shape of all the bearings is to remove the clutch housing, grasp the crankshaft (the clutch), and turn it back and fourth a bit.  There should not be any play or very little (< 1mm) in the crankshaft in any direction before the piston begins to move up or down.

WARNING: When bearings start to fail, they will do so quickly and can destroy the engine if not repaired.

Replacing the upper connecting rod bearing is supposed to be done every hundred hours on many engines.  I have found that this bearing will last much longer than a hundred hours.  However, it is an inexpensive job to replace it compared to replacing the engine if the connecting rod should fail at the upper bearing.  I attribute the long life of this bearing in my engines due to the fact that I use the best fuels and oils I can buy and do not fly at full throttle but for only brief periods (<2 or 3 minutes).

When you replace the upper connecting rod bearing, also carefully check the clutch connectors and springs for wear.