Note: Click on the photos for an enlarged image.

The ball bearings that support the spinning crankshaft in a model glow plug engine will, at some time, need to be replaced. Crankshaft bearings usually wear out after a few hundred flights or perhaps may succumb to early rust damage from post combustion moisture. Bearing rust is common, especially if the engine is not “run dry” after every flying session.
The signs of a bad bearing are symptoms such as metallic noise, (a metal humming sound loud enough to hear from the ground), when the engine is running or a roughness that can be felt when the engine is turned over by hand. Another indication is that the propeller can be rocked at the tips. In severe cases, you may see black residue coming from your muffler. If this happens, the bearins are not only worn-out, but may be coming apart inside the engine. Such extremely worn out bearings can damage the crankcase, piston and cylinder lining. At this point the engine may be beyond economical repair.
You could send the engine to the manufacturer and wait until they get the job done. But it is a lot quicker, and less expensive, to change bearings yourself. It is much easier than you might think. An added bonus is that you learn more about your engine and how it works. This article assumes that you can find a bearing in your local hobby shop or local bearing supplier but that is usually not a problem. There are several model engine bearing suppliers as well the parts department of your engine’s manufacturer who can supply these parts.
This article will only cover changing the bearings in a two-cycle model engine. Changing bearings in a four-cycle is similar, but requires a much more involved disassembly and assembly.
A two-cycle engine has few parts to remove and put back in the right place. Most trainer-type planes come with a two-cycle engine that can be serviced by a newcomer to RC flying. The engine chosen for this report was actually an OS 1.40 RX because it is larger, which makes it easier to photograph on the inside. (Plus, this OS 1.40 is my 2004 Pattern Competition engine and this was a good opportunity to get stainless steel bearings installed in it by an engine expert. Ed.)
However, the principles of bearing replacement apply to all two-cycle engines. The basic tools (photo 1) needed to take an engine apart are the correct size Allen wrenches (usually metric and usually were supplied with your engine), a block of wood, a heat gun or oven, hand protection and a torque-wrench. (Most modern electric screwdriver/drills have torque clutch settings that can be dialed to different degrees of relative torque.) A torque wrench is not an absolute necessity however.

Photo 1

To change the crankshaft bearings you need to take the engine apart. To put it back together you will need all of the parts. It is a therefore a very good idea to use a series of plastic containers to “capture” the parts as you disassemble the engine. You can label these containers with names such as carburetor parts, head parts, backplate, piston liner, etc. A flat oven tray, lined with a paper towel, provides a good work surface and catch-all when working on an engine. The paper towel prevents parts from bouncing and makes them easy to see.

Begin by removing the carburetor. You will be heating the engine to loosen the bearing so the “soft-bits” found in the carburetor need to be protected. Carburetors are usually held on by one pinch bolt (photo 2) or by two screws on either side of the crankcase (photo 2A). Loosen the nut on the pinch bolt to remove the carburetor or remove the two screws. Lift the carburetor up and put it aside (photo 2B)

Photo 2   Photo 2A

Photo 2B   Photo 3

Next, mark the top of the backplate, to insure it is reinstalled with the same side up, and then remove it (photo 3). Keep the screws and the backplate together. The screws are often a specific length and should not be mixed with the rest of the screws. In most cases, you will find a gasket on the backplate cover. Take care to keep this gasket in one piece.