How to Assemble a Model Aircraft Engine — Part I

If you took my advice in the engine disassembly article and noted how the parts in the engine are oriented to one another before you disassembled it, then reassembly is very simple.  If not, then chances are something will be backward when you put the engine back together.

Take a close look at the connecting rod.  You will probably notice that it is not symmetrical front to back.  If it is not replaced on the piston properly, then it will either jam in the crankcase or rub against something that it shouldn't.

However, if you are now in that situation, do not despair.  Look over the parts diagram carefully and see if you can find out what the problem is.  If the diagram does not clear it up, then do not run the engine or force it to turn over.

Take the engine back apart and turn parts around to see if that fixes it.  If that does not work, then get some help from someone with more experience, call the manufacturer or send the parts in and let someone else put it back together.

Putting the Engine Back Together

Note:  Information contained in the instructions supersedes information presented here.  Before assembling the engine, be sure to review the instructions that came with it. 

Use sewing machine oil or another light oil to thoroughly coat every part while the engine is disassembled.  The parts don't need to be dripping oil, but they do need to be coated.  Sewing machine oil is safe for all parts of all engines that I am aware of.

	The crankshaft and bearings must be in place before anything else.  Shown here are the rear ball bearing (not all engines have ball bearings) and an aluminum spacer having a purpose I have no idea about.  But I know where it goes, so I do not have to know what it does. The rear bearing is not sealed.  It can go in facing either direction.
	The rear bearing and spacer in place on the crankshaft.
	The front ball bearing is sealed on one side.  The sealed side faces out to keep dirt from entering the bearing and the engine.  Shown here is the unsealed side of the bearing.
	The sealed side of the front bearing.  Again, not all engines have ball bearings.  Economy engines often have bronze bushings that should not be removed.
	The crankshaft is inserted into the engine through the backplate opening.  The bearing is a tight fit and unable to seat.  To remove the bearing, the engine was heated in the oven. Aluminum expands more than steel so heating the engine allows the bearings to fall out. The reverse is also true.  Heating the engine allows the bearings to go back in place which was what was required here.  I put the crankcase in a 250° oven for about 10 minutes. Only the crankcase was placed in the oven — not the crankshaft and bearing.
	After heating the case the front bearing falls into place easily.  The sealed side is out.
	Heating the case allows the rear bearing to slide into place more evenly.  However, you can see that it is not fully seated. When the crankshaft is seated properly the opening in the crankshaft will align with the carburetor hole or at least be much closer. To remedy the situation, I put the thrust washer on and then bolted a propeller to the engine.  Tightening the propeller nut pulled the crankshaft and rear bearing into place.
	The piston, wrist-pin and connecting-rod.  Note the cutout on the skirt of the piston.  This cut-out goes forward and prevents the piston from contacting the rear bearing. In some cases there is a relief at the back of the piston to prevent it from contacting the backplate.  This is why you need to examine your engine before disassembling it.  Putting parts in backwards is easy to do. Note that the connecting rod has two different size holes.  This ensures that the right end is in the piston, but it can still be turned around backwards.  This engine, for example, requires the connecting rod to face a certain direction. If it is backwards, the engine will not turn over.  Each engine is unique, so this may not be the case with yours.
	The assembled piston.  Note the oil hole in the bottom of the connecting rod.
	The piston goes into the engine before the liner.  If you put the liner in first, you will not be able to put the connecting rod over the pin on the crankshaft.
	The engine is shown here upside down.  Most frequently, the pin on the crankshaft has to be at its top position in order to put the connecting rod on the crank-pin.
	The connecting rod seated properly on the crankshaft pin.

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Copyright © 2004 Paul K. Johnson