Glow Engine Tear-Down

A reliable running engine makes for a popular one but often this is the result of proper care and operation. Many modelers never miss a flying session due to a failed engine even when flying the same old unit year after year. What is sometimes not fully understood is that these people care for and maintain their engines. We shall look at proper operation, general handling and engine rebuilding techniques in the following text.

The glow engine relies on the oil contained within the fuel for lubrication and cooling. Although these small power plants are primarily air-cooled the oil also carries heat away from the metal which it makes contact with inside. Running a slightly richer mixture by way of correct oil content and a reasonable fuel/air charge will help to cool things off.

Trying to squeeze that extra smidgen of power from an engine through leaning of the main needle and/or lowering the oil content will cause problems. The glow engine has limited cooling fins around the cylinder and unlike a gasoline engine it needs much more oil for cooling. A glow engine exploits bushings instead of needle bearings on the connecting rod dictating higher oil content for long-term survival.

Methanol by nature attracts moisture and as we know it is the main ingredient in glow fuel. Fuel should not be left in the tank as it may siphon off into the engine through the carburetor. Since the fuel tank is vented to atmosphere through the muffler, moisture will enter the system. You may think the hole is small but ambient temperature changes will accelerate an air exchange as fuel vapours heat and expand out the fuel nipple.

When things cool off air is pushed back into the tank by atmospheric pressure as cooling, condensing or the contracting of vapour occurs in the fuel tank. It is best to seal the fuel system if it is not fully emptied by way of a shut-off valve at the muffler and carburetor inlet fuel lines. Even though many people empty the fuel from the helicopter the shut-off valve is still a good practice.

Engines are usually shut down by sealing off the fuel/air mixture flowing into the engine cylinder by way of closing the throttle. The problem with this method is that while it kills the engine, it also leaves residual fuel in the engine crankcase. The remainder will attract moisture as it lays around bearing areas eventually leading to internal corrosion. Corrosion pitting of bearing races causes the bearing to wear out before it should. We may extend bearing life by pinching the fuel line off at the end of the day with a shut off valve during a medium idle configuration, thus allowing all the fuel in the crankcase to burn off.

As an extra measure the glow plug may be energized at the end of a flying session to assist in burning off the last bit of the fuel/air charge, which normally would be too lean to burn off unassisted. This will leave the engine in a better condition for short-term storage since adequate oil still coats the precision internal parts.

Fuel filters protect the engine carburetor from dirt but they do need servicing or routine replacement just as any other IC engine might. Because filters are so tiny often they will need attention every case or so of fuel, depending on if the fuel is filtered coming out of the field box and into the helicopter fuel tank. The aircraft should never be fuelled from the filter side near the carb. Any dirt forced into the front side of the filter will be redirected into the carb during normal fuel flow when the engine is running. I prefer fuel filters, which thread together and tend to pick the larger ones because the time between servicing is greater.

Some modelers utilize air filters but this reduces engine performance or maximum power output. For the budding flyer this is a good idea as he or she will spend many hours close to the ground disturbing ground laden abrasive dirt through rotor down wash. It is best to learn to fly off a moist grassy surface rather than a dusty sandy one for this reason. Sand and crud also reduce the life of the helicopter in general. A competent modeler spends little time inches from the ground and so the air filter has little use, unless of course one flies at a dry desert area.

Engine bearings eventually wear out regardless of how they are operated and cared for. The more abuse the engine receives the sooner they will be replaced. The more thrashing the engine sees by way of high rpm and temperatures with the more often than not full carburetor barrel openings, the more care it will need. Think of the candle that burns with the brighter flame. It may be most excellent fun, but you will pay for it! This needs to be recognized through a proper maintenance schedule. Alcohol carries none of the heavy metals and sulfuric acid that gasoline and diesel exhausts do. Because combustion by products are not a neutral PH they will promote corrosion but this will be a smaller factor in bearing life than say moisture accelerated corrosion.

Be cautioned as the rear bearing may be directional

Bearings come in different grades and some, which are made from superior metal, may utilize non-corroding plastic bearing cages. OEM bearings are usually fine but can be upgraded should the bearing quality be in question. A plastic cage is most important in the rear bearing since this vicinity is subject to corrosive combustion chemicals. Popular engines come with both bearing cage versions as factory new. My main machine receives a bearing change annually after many cases of fuel are burned, while my son’s night machine might go 2-3 years or 3 cases of fuel.

It is the combination of calendar time and running hours that necessitates evaluation. It also depends on the engine size since the fuel volumes quoted above are for the fuel frugal 39-50 TT and OS engines. Just because the bearing fits does not mean the application is best. Some bearings having the same external dimensions have a different ball complement or number of balls. I have found the larger ball complement in several applications to be better suited to engines regardless of cage material. We’ve had exceptional luck with SKF products when compared to OEM units such as the often quick wearing NTN bearings.

When you decide to change out engine bearings, often only the rear needs replacement although most people will do both. Care is needed during disassembly and reassembly but the task is relatively easy. Basic tools are required as a minimum.

The engine requires that the fan hub and any drive flange be removed from the front of the crankshaft. The best way to remove the fan nut is by holding the crankshaft with a non-marring jamming tool. This can be done by using a tooth brush handle through the carb mounting hole or by jamming the crankshaft with the back engine cover removed. Use a hard but soft implement for this procedure. Holding tools are available for this purpose but avoid piston-locking tools as they impose excessive loads on the connecting rod and piston crown.

Once the fan, back cover and carb are removed we next proceed to detach the cylinder head. Loosen off the bolts progressively in a criss-cross like manner to evenly and gradually remove all bolt tension. This ensures the parts will not warp. Bag the head and attachment screws so they will be segregated making assembly easy.

The cylinder needs to be separated from the crankcase before the connecting rod and piston may be removed as one assembly. This allows the extra clearance needed to pull the rod from the crankshaft pin. The piston and connecting rod are directional so mark the back of the rod with a felt tipped marker. Some pistons have an arrow on the crown, which always points to the exhaust port in order to facilitate a similar reference. The cylinder is usually critically aligned to the crankcase with a pin and notch so pay close attention.

Sometimes the cylinder fit to the crankcase is snug so a toothbrush handle may be used to hand push on the bottom of the cylinder accessed through the back cover area. Should this prove difficult than a hair dyer may be employed on high heat. Doing so at the crankcase causes it to expand and thus allowing easy cylinder removal. Some people utilize the oven and some carefully use a propane torch as an alternate heat source. Wear leather work gloves so you don’t burn your hands. Do not use pliers or any other tool that may mark or distort any metal engine parts.

Bag the piston and cylinder in clean soft paper towel. Put these aside with the cylinder head. The crankshaft will now slide out through the back cover unless a collet is holding it up front. In this situation a gentle tapping using a plastic hammer will free the parts, provided the drive washer is pulled first. Again, use a plastic hammer and do not use excessive force. If it does not come apart easily you are doing something wrong.

There are proper pulling tools available for removing stubborn collets and drive flanges. The smaller 30-50 machines do not normally have a collet and so respective disassembly problems are non-existent. Protect the crankshaft in a clean soft rag and bag it.

The bearings are the only items remaining in the crankcase at this point. It’s funny how parts that normally wear are the last ones to access on many of life’s mechanisms! The bearings are a tight fit in the crankcase and must be carefully removed. The casing is again carefully heated with a torch, oven or hair dryer. The hair dyer may not have enough heat, even though I have used it successfully; this certainly is best done while your wife is not at home. You might want to use the same caution when using a cooking oven, as people can be difficult about tasking domestic items for engine tooling.

After heating the crankcase rap/smack the crankcasing back surface on to a flat hard surface protected with a soft rag. When the casing contacts the surface it will stop, however the inertia developed or stored in the bearing mass will tend to accelerate the rear bearing from the casing. Once the rear bearing is removed we now have access to the front bearing. Apply heat to the casing if it has cooled and tap the front bearing out forward from the rear end using a long bolt or drift slightly bigger than the inner race. You may not reuse these bearings due to the removal process possibly damaging the races and balls.

Before assembling the engine all parts should be cleaned as necessary, however do not clean the sides of the piston as it is a precision-broken-in fit and any discoloration either by fuel dye or varnish buildup is now part of the dimensional formula. The piston crown (top) and cylinder head combustion chamber may be cleaned with scotch-brite pads. Do not scratch or polish the head gasket or mating surfaces.

Bearing installation and general assembly is very easy compared to removal process. Heat the crankcase as per the previous method, place the front bearing over the end of the crankshaft and insert it from the crankcase front making sure it bottoms. The crankshaft is simply being used as an installation handle or tool. Next slide the larger rear bearing on to the crankshaft and install it into the heated crankcase. Be cautioned as the rear bearing may be directional, if it has a face it should point to the back of the negine. The face is the side where you can see more of the balls.

Plastic caged bearings need the balls exposed to the fuel oil mixture directly on the side of the crankcase interior. To make the job easier, the bearing and crank can be cooled in the freezer using a sealed plastic bag for 30 minutes to an hour before installing it into a heated engine casing. Push the crankshaft and rear bearing in from the engine aft cover side and fully seat the bearing, then rotate the shaft as you wait for the temperatures to equalise. This helps to insure a best bearing alignment between front and rear. Install the crankshaft nut loosely so it will not fall out through careless handling or positioning before the connecting rod is installed.

Two situations exist; one is for a ringed piston and the other for the ABC/ABN etc ring-less configuration. The ringed piston needs extra attention before the cylinder is installed. The ring is positioned in a radial manner in the piston groove by a piston location pin. This positioning is subject to the piston and connecting rod assembly also being installed correctly on the crankshaft pin, with the arrow if so marked pointing to the exhaust flow direction. The reason for positioning the ring is so that the ring gap will not run over an incorrect port opening, causing hang up and breakage.

The cylinder location pin in the crankcase serves the same purpose and additionally aligns the cylinder to the crankcase porting cavities. This does not excuse the ABC piston from a correct assembly alignment process either because it too has an exhaust side. Failure to directionally assemble will result in costly damage. Usually the piston skirt is precisely cut away to clear the crankshaft throw and/or the back engine cover.

The piston and cylinder require a coat of oil prior to assembly as does the connecting rod bushing. Install the connecting rod/piston assembly on the crankshaft. Slide the indexed notched cylinder over the piston held at top dead center gently wiggling to compress the ring into the cylinder chamfer if non-ABC. If the cylinder fit to the crankcase is tight heat the case. If the ring seems to hang up remove the piston/rod and place it into the cylinder for a ring pre-alignment. Pull the piston/rod carefully straight out in the operable alignment position and reinstall on the crankshaft.

Heat the case as needed and assemble carefully with little force. Do not tap the cylinder, use light hand pressure only when locating the cylinder to the crankcase alignment pin and when you start to first push the cylinder down. If need be apply more heat to the crankcase should the fit between the cylinder and casing get tight. You’ll know if the ring is in the cylinder correctly by rotating the crankshaft a few degrees back and forth from TDC before the final positioning (full push down) of the cylinder.

Install the head with the gasket in the correct position, first making a final check that the cylinder notch is surrounding the locating pin then install the bolts until they are seated. Tighten the bolts a little at a time in a cross like manner. All bolts should have about the same rotation, otherwise the head or head gasket may have been canted. There is a correct bolt torque but it can be estimated by use of common sense. Use the torque standard for the bolt size utilized in the engine is my advice.

Finally install the rear cover with an undamaged gasket, new or used but only after applying a tad bit of oil to the rear bearing. Remember to first install the fan so you may hold the crankshaft either from the carb opening or the rear end of the crankshaft. Install the carburettor with a serviceable ‘O’ring. Rotate the engine gently by hand to check for smoothness making sure the glow plug is removed. Be cautioned that some back covers have an internal flat which must face up towards the cylinder head for correct piston skirt clearance near BDC.

Add the clutch, and check that it is run-out free before installing the engine. The engine requires no break in for the new bearings although the carb settings might be off a tiny bit. This is possibly due to slop in the old bearings changing internal gas flow along with a previous higher friction.

A final note on bearings. Some come with two shields or seals, some with one, and some with none. This is coded in the bearing manufacturers part number. The rear bearing should have no shields or seals while the front bearing needs but one at the forward end. Seals must be pried out before bearing installation. OE bearings are usually best but some economy engines use lower quality bearings and so an upgrade in quality may be worth considering.