Engine Bearings

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Check your engine bearings occasionally!

It only takes a moment to check your bearings, and could save your engine! All of the better two stroke and four stroke engines have at least two ball bearings supporting the crankchaft. One bearing is located towards the outside which supports the propeller and another inside at the bellcrank which supports the crankshaft, piston & rod. Both of these bearings support the crankshaft and keep it off the crankcase. As these bearings wear out, or get damaged by corrosion they loose the ability to support the crankshaft thus allowing the crankshaft to rub the crankcase in various locations. This action produces micro particles of aluminum and steel which passes through the engine. These micro particles are abrasive and wear out the piston & liner as well as wear out the bronze bushings in the connecting rod. In a four stroke, it also wears out the camshaft, lifters and rockers. The microparticles also go back through the bearings and speed up the grinding process on the bearings themselves. In other words, they accellerate tearing themselves up at an exponential rate.

I have successfully destroyed several engines over the years simply because I ignored the "tell tail" signs of a bearing going out. Replacement bearings are a whole lot cheaper than a new engine! So, let's check some bearings...

The very first sign of bearing failure is indicated by roughness in the rolling action of a bearing then followed by a "greyish" color of the oil exiting your muffler. That "greyish" color is metal... specifically aluminum from your crankcase and piston. If these tiny specs are slightly reflective in sunlight, then your engine is on the cutting edge of ruined (if it isn't already). Prior to this, your bearing are only slightly rough, and here's how you check it...

Find a spot on your crankshaft rotation where the crankshaft moves freely. Rotate it back and forth and feel the smoothness of the bearings.

If you feel anything less than perfectly smooth motion, your bearings are likely going out. If you "feel" as well as "hear" a bearing rotating, you're in big trouble! The bearing(s) are already in self-destruction mode and have already begun to contaminate all of your moving parts as well as producing the "greyish" color of the oil at your exhaust to some extent. If it's not too far along (and you still have good compression), you may be able to salavage the engine, otherwise your engine is destined for doom if you continue to run it. You can check the "up & down" motion of your crankshaft to determine how far along the damage is. If there's any  "up & down" motion of the crankshaft, the ball bearing cage(s) are about to explode. When this happens, you will experience INSTANT and total destruction of your engine when the ball cage fragments pass through the crankcase and cylinder. These very large fragments are super destructive and will take out your piston, rod, head, and cylinder, as well as chopping up your crankcase at the lower connecting rod area. There will also be large metal fragments throughout your engine including the muffler and probably on the airplane as well.

Several things cause bearing failure, the single most common problem is an engine running too lean, too long. A simple fact of physics is: The faster a bearing turns, the more lubrication it needs. And, a "too" lean running engine is not providing adequate lubrication... plain & simple. The other problem is corrosion. Methanol & nitromethane (glow fuel) absorb moisture from the air. When this fuel saturates with moisture is becomes very corrosive to ferrous metals and aluminum to a lesser extent. You can either use an after-run oil in your engine or install stainless steel or ceramic bearings, both of which are resistant to acids.

This problem exists in ANY ENGINE that uses "glow fuel" including four strokes and two strokes alike. Even converted gasoline engines to glow fuel.

I've had several occasions where pilots have had problems with their engines and had me look them over. One engine had a strange "clicking" sound at top dead center. It turned out to be a piece of bearing cage metal stuck in the top of the piston and hitting the head. Upon close inspection, the inner bearing cage had self-destructed thereby contaminating the engine with all sorts of fragments. The engine was dameged beyond practical repair.

In another case, a guy said his piston seems to stick a little before the engine gets compression. Before I even checked it, I knew a bearing had broken up and the bearing fragments sliced and displaced the piston sidewall metal. And sure enough.. that's precisely what happened. Again, another engine ruined beyond repair.

Both of these instances were avoidable buy checking their bearings occasionally and using after-run oil. They may have also been running their engines too lean... too long.