Engine Bearing Replacement

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We depend on reliable service and strong performance from our r/c engines, but unfortunately sometimes things don’t go that way. Recently my new OS 50 SX engine started making an ‘extra’ noise. At first I thought (hoped) it was one of the bearings on my helicopter. But after replacing them and still having the noise I concluded it must be a bearing in the engine. I took the engine apart and discovered a rough rear bearing. There are two bearings in most engines, the front and the rear bearing. At first sight you might think it’s impossible to remove them for replacement. If you look at the rear bearing you will see there is no place you could get a bearing puller on it and you are right. It may seem difficult to remove these bearings but as you will see it’s actually very simple. You have to use a ‘think outside the box’ technique. Which in this case involves sticking the engine in the oven…. hold on there, you can’t do that just yet  :)  hahaha First you have to strip the engine. Take a look at the picture to the right, yep that’s what an engine looks like before it’s all put together. To begin first take off the carburetor. It is held by a bolt in the engine case that is just below the carb. Usually you can just loosen this bolt and pull the carb out, but in the case of my OS 50 SX I had to remove the bolt and put it in backwards to use it to pull the wedge that hold the carb in. Once it was removed I used an allen wrench from that end to push out the other wedge. Then with a good pull and a little twist I got the carb out. Normally they are not this difficult but this carb design has a great sealing effect. On my engine, it has a flat washer that sits under the carb, If you have something like this remove it and don’t lose it. Next remove the back plate from the engine. It’s held on with four bolts, note that there is a thin gasket that you need to take care of when you remove the back plate. So don’t just jerk the back plate off, slowly remove it and look to see if any part of the gasket is trying to stick to the engine case. Next remove the heat sink. There are usually 6 bolts that hold it in place. When you lift it off watch for a thin brass washer. Some engines use a different material so yours may not be brass. It will more then likely stay in the head, don’t lose it. Next is the sleeve. The sleeve should be keyed, meaning that there is only one way it will go back in. On my engine there is a notch on one side of the sleeve that fits around a stud that is on the top of the engine case. The sleeve should pull up easily but if you can’t get it started you can push it up from inside the back of the engine (where the rear plate was). Now that the sleeve is out, you will have enough slack with the piston to angle the piston rod so that it can be slide off the wrist pin. Now you can remove the piston, but make sure you remember which way it came out. You need it to go back in the same direction so the wear patterns will match up in the sleeve. On my OS 50 SX, the piston has a notch cut out on one side so that it clears the counter weight so there is only one way to put it back in. However on my Thunder Tiger 36 the piston does not have any distinguishing marks so it could be accidentally installed backwards. Next you can push the crankshaft out the back. This should leave you with a bare bones engine case with the front and rear bearings. Make sure there is nothing else on the engine, especially no rubber seals. Clean up the engine so there is no fuel/oil residue then place the engine on a cookie sheet and place it in the oven that has been preheated to 350F. Bake it for 15 minutes then take it out and give the engine a small hit against a block of wood. The rear bearing will fall out. Use a long screwdriver, wooden dowel rod, or whatever through the back of the engine to push the front bearing out. Next to install the new bearings, re-heat the engine, then place the new front bearing on the crankshaft and use it to push the bearing into place. Make sure you get it in the right direction. The side that does not have a seal will go toward the inside of the engine. Next place the rear bearing on the crankshaft and insert it through the back of the engine. Note: I did have a case where the inside diameter of the rear bearing was just a fraction too small and would not slide all the way on the crankshaft. I used a high speed Dremel cutting bit to shave off a few molecules. It didn’t take much, I just went around about 3 times and then the bearing would slide just like it should on the crankshaft. Note that you don’t want a loose fit!!! The remainder of the engine assembly is just the reverse of how you took it apart. Remember to install the connecting rod/piston in the same direction you took it out. Usually pistons have a dot on the top that will align with a dot on the top of the engine case (the dot usually is toward the front of the engine). If you have a ring type engine like this one then be sure to position the ends of the ring so that the pin on the piston is in the middle. When you go to tighten the 6 bolts that hold the head on or the 4 that hold the back plate on, just snug all the bolts first then tighten then non-sequentially (the star pattern). Also remember to not leave out the head gasket And if you had a seal under the carb, make sure and put it back.