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How to Disassemble a Model Airplane Engine

May 05, 2015



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Airfield Models (http://www.airfieldmodels.com/)How to Disassemble a Model Aircraft Engine

Before you even consider disassembling an engine you should have a very good reason.  Every time an engine is taken apart there is the risk of doing more harm than good.

Likewise, if you don't have a very good reason to disassemble the carburetor then leave it alone even if you disassemble the rest of the engine.  Don't remove the needles because you do not want to change the settings if you do not have to.

The only reasons for taking an engine apart are:

  • An internal component is damaged such as a broken connecting rod.
  • A part is worn and needs replacement such as bearings or the piston and liner.
  • The engine was crashed and dirt got inside (very unlikely).
  • You are curious and it is worth the risk of damaging it to see what's inside.
  • You are writing a how-to article for your website.
 
 

Taking the Engine Apart

Note:  Information contained in the instructions supersedes information presented here.  Be sure to review the parts diagram and instructions that came with your engine before taking it apart.

Remove the engine from the aircraft and use a stiff bristled brush and compressed air, if you have it, to remove as much dirt from the outside of the engine.  Gather up necessary tools, a couple clean rags, a can of oil, a container to put small parts in and another container filled with whatever cleaning solution you plan to use.  I normally use denatured alcohol.

This article will not cover cleaning the engine, but if you want to do that, then a good method is shown here.

Important! Never use any type of metal tool or object inside the engine.

Before you begin taking the engine apart, be sure to make a small scratch on the head and the cylinder so you know how it goes back on.  Do the same thing with backplate.

 
 
Tools used to disassemble a model aircraft engine. A few items needed to take the engine apart.  Lay out a clean rag and use a second rag to wipe oil from the parts.

A toothbrush is helpful for removing varnish.  A hardwood dowel is useful for a couple of purposes that will be discussed later.

Remove the propeller nut, carburetor and muffler first. Begin by removing major items such as the air filter, muffler, carburetor, propeller, prop nut and washer, thrust washer and the glow plug.  Put small items in a container so they do not get lost.

Do not put items from inside the engine in the container.  You do not want to scratch the piston, liner, crankshaft or anything else that needs to have a smooth surface.

Remove the engine head and backplate.

Remove the head and backplate.  Normally there are more bolts for the head than the backplate.  Put the bolts in the container.

If the head and backplate have the same number of bolts and they are different sizes, then be sure to note which are which so you can put them back in the proper location.

There is a good possibility that the backplate gasket will tear when taking the engine apart particularly if the engine has a lot of hours on it.

How to make a backplate gasket

When the head has a gasket it is usually made from thin metal.  Put the gasket somewhere safe so that it does not get bent or damaged.

Remove the piston liner. Often there is an alignment pin in the top of the cylinder that is used to key the liner.  If not, then mark the liner and cylinder so that it can be reassembled facing the exact same way.

Most engines are designed to have the liner removed before the piston can come out.  If the liner will not slide out, then push on it from underneath with the dowel.

If it still will not come out, then put the engine in a 300 oven for about 10 minutes.  The aluminum will expand from the heat and the liner will slide right out.

 
 

Important! Before removing the piston, look at it from the top and through the backplate opening.  Often the connecting rod has an oil hole in it and goes on one way.

After the engine has been run, the piston is mated to the cylinder and should go back in facing the same direction.  If it is turned around, the engine will lose compression and wear out faster.

 
 
Remove the piston. Typically the piston needs to be as high as possible to be removed.  Turn the crankshaft to move the piston up.

To remove the piston, tilt then engine backward and pull the connecting rod from the crank pin.  Turn the engine upside down and let the piston fall into your hand.

Do not remove the piston ring unless you plan to replace it.  Some pistons have small circlips holding the pin in place.

If you want to remove the pin, then use a pair of needle-nose pliers to carefully remove the circlip. Put it someplace safe because if it gets lost, the engine is out of service until it is replaced.

Remove the crankshaft. If the crankshaft does not slide out, then set the back of the engine on a piece of wood.  Put another piece of wood over the crankshaft and tap it lightly with a rubber mallet.  It should come out easily.  If it does not then 10 minutes in the oven should loosen it up.

Some engines have a key holding the thrust washer.  These engines must have the thrust washer pulled off first with a bearing puller.  The key is then removed and then the crankshaft.

My O.S. 1.08 is like that.  I pounded on the crankshaft for several minutes before looking at the parts diagram and realizing that no amount of hammering was going to knock the crankshaft out until the key was removed.  Live and learn. Engine manufacturers don't tell you that the secret to engine disassembly is a good hammer.

Heat the engine to remove the bearings. Usually the ball bearings do not come out easily.  Find a wooden dowel that will can be inserted through the back of the engine and push the front bearing out.  Again, heating the engine makes removal easier.

The rear bearing is removed by rapping the engine against a piece of wood.  If the engine is heated it should fall right out.

The engine is now ready to be cleaned, have parts replaced or whatever work done that is necessary.

 
 

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Maintaining and Cleaning a Model Aircraft Engine
How to Disassemble a Carburetor

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Copyright 2003 Paul K. Johnson