Introduction

I will say this. If you are afraid of engines and refuse to learn about them, you will be missing a large part of modeling, will make a lousy instructor (imo), and waste a LOT of money and flying time waiting for someone else to clean out your motor after a small crash. Well after you get the plane back together, you will still be looking in the mail everyday, waiting for them to finish doing to your motor what you could have accomplished yourself in ten minutes. 

To take a typical model engine apart:

2. Remove the muffler. 

3. Remove the engine backplate. Save the gasket if there is one. 

4. Remove the cylinder head. Loose the screws in a criss-cross pattern. Save the gasket if there is one. 

5. Turn over the motor until the cylinder is down as far as it goes. Place a hardwood dowel or servo rail or such in the exhaust port, so that it protrudes into the cylinder above the piston. Turn over the motor, and the piston should push the sleeve up so that you can get a grip on it. Don't touch the sleeve with pliers, please. Remove the stick, but leave the sleeve sticking up out of the bore for now. 

5a. Note the orientation of the piston. There is often an arrow or some other mark on the piston face. Notice this and remember it, or if you have to, use a grease pencil on the piston face to mark the position. You'll want to wipe this off just before final reassembly, though. 

6a. If the engine has a separate front and rear crankcase, separate them. Save the gasket. The crank will stay with the front half, and the connecting rod should now be free. 

6b. If the engine does not have a separate front and rear crankcase, and if it has rings, you should be able to slip the connecting rod off of the crankpin with the piston all the way down. If it doesn't have rings, you can pull the sleeve out, leaving the piston in the motor, then slip the rod off the pin and remove the piston. When pulling out the sleeve, notice which way the ports are pointed. (This will be important for reassembly.) Don't use tools on the sleeve. 

7. Remove the carburator from the engine. 

8. If you remove the prop nut and washer, the prop thrust washer (the knurled thing behind the prop) sometimes just slips off. On other engines you have to get a three-pronged gear-puller to get it off. When it does come off, be careful not to lose the woodruff key that sometimes comes out with it, and the brass bearing washer if it's there.

9. At this point, the crankshaft will often just slip out. If not, it is a press fit, and is more difficult, and I don't suggest you attempt it unless you have a hydraulic press. If you have a motor like this and need to replace the bearings, just send this assembly in for servicing. 

10. If it does come out, the next thing is the bearings, if you motor has ball bearings. The rear bearing comes out by heating the case in a 350degree oven for fifteen minutes, removing it with a pot holder, and rapping it on a peice of wood, rear of the motor down, and the bearing should just pop out. Careful not to burn yourself. Let it cool. You should then be able to use a steel rod or drift punch and tap out the front bearing from the rear of the motor. 

11. The piston comes off of the rod by slipping the wrist pin out. That has probably slipped out by itself already, though. :-) 

Inspection

Look for cracks anywhere, and replace the part if you find them. This is rarely necessary. 

Look carefully at the connecting rod. Look at the bearing surfaces at the end, and fit them on their respective pins to look for play. This is common with older engines. 

 The piston should have no scratches, only polished areas. Same for the inside of the sleeve. This would indicate dirt ingestion, and the piston and sleeve should be replaced. 

 If the piston has a ring, it should be polished, with no nicks, and obviously be unbroken. If you replace the ring, it's a good idea to replace the sleeve, too, and vice-versa. 

 The backplate will have marks from the crankpin. That's okay, as long as they are not deep scratches. This would indicate bad bearings or a torqued crank. Rare. 

 Clean any crap out of the inside of the motor. Don't worry too much about the castor varnish--just use a castor/synth blend for a while. The motor should be spotlessly clean inside before reassembly. 
 
 

Assembly

 2. Drop the crankshaft in. Oil. Hold it on there with the thrust washer, a prop hub, nut washer and nut. 

 3. If you have a ringed engine, oil the piston, ring, and inside of sleeve. Find the pin in the ring slot, and place the ring gap there. (That is the only way you can compress the ring all the way around. Press the ring in with your fingers and slip the piston into the bottom of the sleeve. There should be a shoulder on the inside of the bottom edge of the sleeve to help you with this. Be gentle, it doesn't take force, and you can easily break the ring. Get the orientation right, same as you noted earlier. 

 4a. If you had to remove the sleeve (6b, above) before the piston, just reverse the process. The sleeve will go in easier if you oil the outside. 

 4b. Otherwise, slip the sleeve in there (pay attention to orientation) and put the connecting rod on the crank pin. Make sure the ports all line up just perfectly. Push the sleeve up using the wood in the port and turn it if you have to. 

 (Oh, you needed to reattach the front and back crankcase halves in here somewhere. :-) 

 5. Install backplate. 

 6. Install cylinder head, criss-cross pattern on the screws. If there was a piston fence (a raised ridge on the piston face), get the orientation of the head so the slot matches up to it. 

 7. Install the carb. Push pretty hard when seating it; you don't want air leaks there. 

 8. Glow plug. 

 And there you have it. Maybe an hour, assuming you need to replace the bearings. Don't kill the screws, and use non-permanent locktite if you are worried about them backing out. 

 Dave Svoboda, Palatine, IL