Bad Day at the Field

Note: Click on the photos for an enlarged image.

 

Sorry to say, but it has happened to all of us. Worse yet, it will most likely happen again to every model pilot. Something goes wrong. The elevator servo dies or the elevator linkage breaks. Maybe the receiver battery quit or the plane snap-stalled (more on that in another article). Perhaps the pilot simply lost orientation and ran out of altitude and inspiration all at the same time?

Whatever the reason, your plane took that long, final plunge into the not-so-Good Earth. Every model is born with an expiration date and your plane’s date has arrived. In the eerie silence that follows, you slowly hand your transmitter or control line handle to your now sad-looking friend and begin the long, lonely walk, plastic trash bag in hand. You know you will need it this time.

Sure enough, when you arrive at the impact site, you immediately notice two things. First, how can a 5-foot long, 7-pound plane with a 6-foot wingspan suddenly look so tiny? It barely takes up 2 feet of space! And second, you can’t see your engine because it’s buried 6 inches deep into the soft dirt.

Actually that last might be a blessing in disguise. The softer the dirt, the less chance the engine experienced severe mechanical damage. The deeper the engine is buried, the less chance something important (read expensive) was destroyed. The only way to know is to take the engine home without causing further damage. Then disassemble and clean it,

Preventing further engine damage however, begins at the impact site. Do not allow the engine’s crankshaft (the part that the propeller bolts onto) to rotate. Not for any reason. The engine is now full of dirt. Dirt and topsoil are really crushed rock. Right now, the dirt is inside but it is not mobile, not scraping against the engine’s precisely machined parts. As long as the engine is not rotated, no further damage is occurring.

Carefully remove the engine from the ground. Avoid rotating it even if you have to dig it out by hand. If part of the propeller is intact, make sure it isn’t bumped on that lonely carry back to the pits. Even one-quarter turn will scratch the cylinder wall or crankshaft. Be careful when you pack the pieces to take home as well.

It’s time to clean and inspect the engine. Do not remove the engine from the damaged airframe or engine mount. The idea is to remove most of the exterior dirt first, without rotating the engine. The less handling at this point, the easier that is to do. Photo 2 shows the tools you will need to complete this task. Wooden toothpicks are good for tight places, such as inside cooling fins, and will not scratch aluminum. A toothbrush and disposable epoxy brushes are handy as are paper cups and old coffee cans for soaking parts. A household vacuum cleaner is also handy but was too large to fit into the picture.

Photo 2

There are two solvents that can be safely used to clean the engine. The first is kerosene. Kerosene removes dirt and rust stains without attracting water. It also leaves a slight lubricating film that prevents surface rusting during the cleaning process. But it can only be used in a very well vented area, or even better, outside. Do Not Smoke when using kerosene!

Warm soap and water may also be used, but each piece must be dried with the hair dryer as it is cleaned to prevent surface rust. Do not use alcohol as it attracts water and can form surface rust before the piece can be dried.

Believe it or not, of all these tools, the very first one you will need is the electric hair dryer. With the engine still mounted, use the hair dryer to carefully dry the wet dirt covering the engine (Photo 3). DO NOT use a “plastic film” type heat gun. These heat guns can reach temperatures in excess of 500 degrees and can easily warp an aluminum engine.

 
Photo 3    Photo 4

Once the dirt is dry, carefully remove the impacted dirt using a strong, flat toothpick. Metal picks will also work but may scratch engine surfaces (Photo 4). The epoxy brush is used to remove loose dirt from inside cooling fins and other hard to reach areas (Photo 5). It sometimes helps to cut the bristles to half-length, increasing their stiffness. As the final exterior cleaning step, use the toothbrush to remove ALL the remaining exterior loose dirt. (Photo 6)

 

Photo 5   Photo 6

Note from photo 6 that the dirt in the carburetor opening is left undisturbed. Trying to remove that dirt at this point just results in pushing more abrasive materials deeper inside the engine. The carburetor will be cleaned after it is removed from the engine. Give the engine’s exterior a quick vacuum to insure all dirt is removed prior to disassembling the engine (Photo 7). Clean the dirt from your work surface before disassembling the engine or carburetor.

Photo 7

Now that the exterior dirt has been cleaned, remove the engine from the mount. Remember not to rotate the crankshaft. If there was a spinner installed, insert a screwdriver into the propeller opening to prevent the crankshaft’s rotating and remove the spinner (Photo 8). Using a pair of adjustable pliers, firmly hold the propeller hub, not the engine’s thrust washer (the serrated piece behind the prop hub) and remove the prop bolt and prop hub (Photo 9).

  

Photo 8     Photo 9

The prop hub may be difficult to remove without rotating the crankshaft. Use a small screwdriver to pry it off without turning. Once the propeller is removed, you may need to insert a foam spacer onto the crankshaft and replace the crankshaft bolt and propeller washer. The idea is to prevent the thrust washer from sliding forward on the crankshaft. Not all engines have secured or tight thrust washers (Photo 10).

Photo 10

Begin disassembly by removing the muffler (Photo 11). Point the muffler downwards during removal to prevent any exterior dirt from entering the exhaust port. Try to leave the muffler bolts in place and remove the muffler. Then carefully stuff some paper towel into the exhaust port (Photo 12). While the engine is still pointing exhaust-side down, remove the muffler bolts.

 

Photo 11    Photo 12

Turn the engine upside down, to prevent dirt inside the carburetor from falling into the engine, and remove the carburetor mounting screws (Photo 13). While still holding the engine inverted, carefully slide the carburetor off the engine and remove the rubber air-seal washer (Photo 14).

 

Photo 13   Photo 14

Then remove the idle adjusting screw. This screw usually has a spring for secure positioning (Photo 15) and slide the throttle barrel out of the carburetor (Photo 16). Make sure not to lose the throttle barrel tension spring (Photo 17). As you remove the spring, note its orientation. Here, the smaller side fits against the carburetor’s interior wall.

Photo 15
 

Photo 16   Photo 17

Unscrew and remove the high-speed needle valve assembly if it is mounted in the carburetor. Remove the high and low speed needle valves. Count the number of turns to permit re-installation to about the same point after the carburetor has been cleaned.

Remove the impacted dirt from the throttle barrel with a toothpick and “epoxy brush”. Pour some kerosene or soapy water into a cup and clean all the carburetor pieces, except the rubber washer. NOTE – If the throttle barrel has an “O” ring around it, clean the throttle barrel only in water and then air dry. Use a second cup of solvent to rinse each piece after it is cleaned. Remember to clean the high-speed needle assembly inlet with a small plastic “twist tie” (Photo 18). Cotton swabs work well inside the carburetor body and inside the needle valve housing (Photo 19).

 

Photo 18   Photo 19

As the final carburetor-cleaning step, use an eyedropper to push solvent through all the fuel inlets, especially the low-speed spray bar (Photo 20). Immerse the high-speed needle assembly in clean solvent and use the eyedropper to flush it with solvent (Photo 21). Even a small speck of dirt in the fuel delivery system can prevent reliable running.

 

Photo 20    Photo 21

Wipe the carburetor to remove any remaining solvent and reassemble. Put a few drops of rust preventing air tool oil on the throttle barrel before reinserting it. Make sure the slot in the throttle barrel lines up with the idle throw adjusting screw (Photo 22). If the high-speed needle valve assembly is inside the carburetor, make sure the assembly is in the exact center of the throttle opening (Photo 23). The low-speed needle valve’s spray bar is part of the throttle barrel.

 

Photo 22    Photo 23

Put the clean, assembled carburetor aside. Unbolt the cylinder head before removing the engine’s backplate. Most of the remaining dirt is usually found inside the head’s cooling fins. By removing it first, there is less chance dirt will fall inside an open back plate. Carefully note the amount of force needed to unbolt the head. You will try to match this amount of force when reinstalling the head. Make a small scratch on both the head and crankcase (Photo 24) to insure the head is re-assembled exactly as it was removed.

Photo 24

Actually, the cylinder head on non-baffled engines is usually symmetrical and may be reassembled in any direction as long as the cooling fins are aligned with the air stream. But engines tend to warp slightly as they are used and it is always best to make sure all parts are replaced exactly as they were originally installed. Carefully remove any head spacing washers, bronze colored in photo 24, without bending. A hobby knife works well here.

Insert another paper towel plug into the cylinder and then gently lift the steel cylinder about ¼ inch above the crankcase (Photo 25). Clean any dirt using an epoxy brush. Leave the cylinder in place for now. Next, mark the back plate with a felt tip pen (Photo 26). The back plates on many engines are symmetrical, but some are not. The felt tip pen’s ink is not soluble in Kerosene and usually survives the cleaning process to insure correct reassembly.

 

Photo 25   Photo 26

Carefully remove the back plate, rotate it slightly to help break the seal without damaging the large washer (Photo 27). Once the back plate is removed, mark the rear side of the connecting rod (Photo 28). It is extremely important that this rod be re-installed exactly this way. If it is not, the engine may bind or lubricating oil will not flow into the lower connecting rod’s bronze bushing (the small bronze inset in the photo).

 

Photo 27    Photo 28

Carefully remove the steel cylinder insert (Photo 29). Once the cylinder is removed, the piston/connecting rod assembly sort of rattles around inside the big opening. This allows the connecting rod to be removed from the crankshaft pin (the black steel pin inside the rod’s bronze bushing in photo 28). Just drop the entire piston assembly out the top. The inside of the engine should now look like Photo30, (hopefully without the rust).

 

Photo 29    Photo 30

Most 2-stroke model engines today have either a loose fitting thrust washer that is easily removed or one that is pressed onto an expanding brass or aluminum cone as the prop is tightened. In either case, hold the engine firmly in one hand and gently tap the very front of the crankshaft as shown in photo 31. This tapping pushes the crankshaft out the rear of the engine. Remove the thrust washer and crankshaft. Don’t lose the small, rectangular key that locks the thrust washer in place on the crankshaft as it turns. Some engines have such a key; it is easily removed once the thrust washer is slid forward (Photo 32).

 

Photo 31   Photo 32

If a few gentle taps do not move the crankshaft – STOP. Don’t pound on the threaded front end. The thrust washer has become firmly attached to the expanding thrust cone. The best tool to use here is a thrust washer puller (Photo 33). It is available from Central Hobbies for less than $15.

Photo 33

On just a few engines, a metal clip over the thrust washer retains the crankshaft. If this is the case, this clip is very difficult to remove without proper tools and a new clip will be required for re-assembly. With these type engines, do not remove the crankshaft. Immerse the entire assembly as if the crankshaft had been removed. This is not the best method, but it usually works. For a detailed set-by-step guide on changing your bearings, click here.

 

Photo 34    Photo 35

Once the crankshaft is removed, the rear bearing is exposed. Immerse the entire assembly in solvent and brush clean without rotating the bearings (Photo 34). Clean the front bearing with a cotton swab as well (Photo 35). Continue cleaning and rinsing until each bearing can be rotated by hand without noting any roughness or “stickiness” (Photo 36). If no amount of cleaning restores a bearing’s smooth turning, the bearing was most likely damaged and needs replacement. Immerse the piston / connecting rod assembly and clean it as well (Photo 37).

http://www.masportaviator.com/articlepics/badday/large/baddayatfield_41.jpg 

Photo 36    Photo 37

Clean the inside and outside of the crankshaft also (Photo 38). Then place the front side of the crankshaft, the mid section in photo 38 showing the inlet slot, on a flat surface such as a piece of glass or plastic cutting board. Place the counterweight, the largest piece containing the crank pin, just off an edge (Photo 39). Now roll the crankshaft slowly and note if there is any up or down movement at the front end. If so, the crankshaft is bent and must be replaced. This method may appear crude, however bends as small as .003 inches can usually be seen using this method. If the crankshaft is bent less than that, it is usually OK to use.

 

Photo 38    Photo 39

If the engine is more than 6 months old, or has been flown more than several hundred flights, it may be a good idea to install new bearings. Bearing replacement is its own story and will be covered separately in an upcoming Sport Aviator article. Before beginning reassembly, clean the thrust washer itself (Photo 40). Note all the dirt that found its way inside this tight fitting piece in photo 40. Apply air tool oil to both bearings, both inside the bearings and coating the “race” (the part the crankshaft slides into) (Photo 41). Coat the crankshaft exterior as well. Slide the crankshaft into the bearings. Install the thrust washer and locking cone (if any). Install a propeller and tighten firmly to seat the crankshaft. The crankshaft should turn freely without binding in any position.

 

Photo 40   Photo 41

Insert the piston/connecting rod assembly and place the lower bronze bushing back over the crankshaft pin. Make sure the mark is facing towards the rear. Coat the inside of the steel cylinder and work it onto the piston. It helps to rotate the cylinder a bit. If the engine is ringed, the cylinder will have a tapered bottom edge. This edge will help compress the ring into the cylinder as it is lowered. Take your time and use the oil freely. Eventually, the piston slides into the cylinder (it is not really hard to do, just takes a little feeling around is all).

Photo 42

Note that the cylinder slips onto a locating pin in photo 42. The slot in the cylinder fits over this pin, aligning the intake and exhaust ports. Roughly aligning this slot and pin before trying to slip the piston inside the cylinder usually makes it much easier to accomplish this task. Every engine warps slightly as it is run and both piston and cylinder wear slightly “egg shaped.” In plain English, that means they both “ain’t that round no more.” Aligning the slot and pin also roughly aligns the warps and allows easier cylinder installation.

Rotate the crankshaft and piston to insure everything is working properly. The piston should move freely in the cylinder. Oil all parts again and re-install the cleaned back plate. Make sure the mark is installed in the proper direction.

Photo 43

Install the head spacing washers, if any, and reinstall the head. Head bolts are tightened in a specific manner. Hold the head in the proper position, remember the mark, and install all head bolts without fully tightening any of them. Tighten the first bolt only until it just contacts the head. Then tighten the opposite bolt to the same point (Photo 43). Continue tightening each bolt in the same manner, always opposite the last bolt tightened. Aluminum cylinder heads are prone to warping during re-installation. Alternately tightening each bolt helps prevent any warps from occurring. Once the bolts are snug, begin tightening. Use the same amount of force first used to remove them. If you have an inch-ounce torque wrench, (most of us don’t) tighten to the same torque used to remove the bolts.

Remove any oil from the outside of the carburetor body’s lower section, the part that slips inside the engine. Re-install the rubber sealing washer. Many oils may attack this seal so try to prevent oil from contacting it. Align the carburetor so that the body is perpendicular to the crankshaft housing. Press down firmly to compress the washer and tighten the carburetor mounting screws (Photo 44).

Photo 44

Put a few drops of oil into the open carburetor and down the glow plug hole. Rotate the engine to spread the oil and install a new glow plug. The old plug may still light, but the glow element may have been shock damaged during impact.

Since all the engine’s internal parts have been disturbed, a minor “break-in” procedure should be observed. Start the engine and run it slightly rich for the first few flights. ABC engines (the piston does not have a ring) should be run at 1,000rpm off-peak power settings. The high-speed mixture for ringed engines should be at the alternating 4-cycle/2-cycle point. After the first few engine runs, reset the high-speed mixture to the usual 4-500 rpm off peak for both ABC and ringed engines.

 

Q

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