Bearing condition

by Had Robinson

Note: check the bearing replacement page for more info and a simple test to check main bearing condition (see the "Installation" section step #7).

For an explanation of what the bearing codes and suffixes mean, go to the MicroBlue Bearings reference page.  For example, the flywheel side main bearing in a Minari 180 has a code of 6304 C3.  This means that the bearing is a typical deep grove ball bearing (6), for medium loads (3), bore size (04), and has internal clearances greater than normal (C3).  The bearing does not have its dimensions stamped on it so it must be measured.

The condition of the bearings in a 2 stroke paramotor is something pilots must keep track of when their engines have more than 100 hours.  Much depends on the care with which the engine was operated e.g. did the pilot use AVGAS or premium ethanol-free gasoline with synthetic oil or did he use lawnmower 2 stroke oil with cheap gasoline loaded with ethanol?  A well-taken care of engine can easily go 500+ hours without any major overhaul i.e. when the engine case must be split and the crankshaft and main bearings replaced.

There are (4) important bearings in 2 stroke paramotor engines: (2) large main crankshaft bearings, (1) lower connecting rod bearing, and (1) upper connecting rod bearing.

The bearing that wears the fastest is the upper connecting rod bearing (a.k.a. the piston roller-cage bearing), then the lower connecting rod bearing, and then the main crankshaft bearings.  Unfortunately, the entire crankshaft assembly must be replaced if the lower connecting rod 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

On automotive engines, the general condition of the bearings can be checked 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) indicates that one or both connecting rod bearings are excessively worn.  But 2 stroke paramotor engines are not so easy to check because they are inherently very noisy.  For this reason, an imminent failure of the upper connecting rod bearing is difficult to predict – and why this bearing should be changed out on a regular basis.  Don't ask me how I know this....

The lower connecting rod bearing and the general condition of the main crankshaft bearings can be checked by grasping the engine flywheel and rotate it back and forth slightly.  Move the flywheel from side to side.  Lastly, pull the flywheel in and out.  There should not be any play in any of these tests.  If there is, the engine requires a complete overhaul.  Pilots may notice excessive wear in these parts by the presence of engine knocking when it is running but this is very difficult to hear.

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

Replacing the upper connecting rod bearing (along with the circlips/springs) should be done every 100 hours on most engines (200 hours for the Minari).  I have found that this bearing will last much longer than a 100 hours.  However, it is an easy and inexpensive job to replace it compared to overhauling the entire engine if this bearing should fail.  If the piston pin (rod) has any wear (grooves), replace it.  A micrometer can be used to check if the pin is worn in the middle.  Unfortunately, the pin may not be available separately so the entire piston assembly must be replaced.

I attribute the long life of the lower connecting rod and main bearings in my engines due to the fact that the best fuels and oils are used and engines are not flown at full throttle but for brief periods (<2 or 3 minutes).

When you replace the upper connecting rod bearing in the Top 80, also check the clutch connectors and springs for wear.  For other engines, check the clutch for drag and/or noise.

Turkey Vulture

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