The Engine Shop
135 Waugh Avenue, New Wilmington PA 16142
Half a year after my first "Engine Shop" column appeared in MA, I've received nearly 100 letters from readers. Most had questions, and I wrote a personal reply to every one. (Please enclose an SASE if you send me a query that needs an answer.) More than a dozen readers asked about model fuels, lubrication, rust protection, and after-run oils. Because misunderstandings seem common in this area of model engine operation, I'll explain some of the "wet" aspects of model airplane engines as thoroughly as I can.
Lubrication: barrier films and cooling
The oil in model fuel does much more inside our engines than just keep moving parts slippery. It forms a barrier film over surfaces to prevent corrosion and wear. With proper lubrication, metal parts never touch—the oil film keeps them separated by its own thickness. If that thickness is greater than the height of the maximum surface imperfections (no machined surface is perfectly smooth), metal-to-metal wear cannot occur.
Oil also provides cooling. In full-scale machining you may remember the flood of cutting oil that keeps parts and tools at uniform temperature; the oily discharge from a model engine's muffler does a similar job. The cooling fins remove some heat, but the oil carried by the fuel (glow, diesel, or spark) carries away much of the waste heat from combustion.
Key functions of oil in model engines:
- Forms a protective barrier film to prevent metal-to-metal wear and corrosion.
- Cools internal parts by carrying heat out of the combustion area.
- In some oils, helps prevent detonation and provides heat-resistant lubrication.
Castor oil: detonation prevention and film properties
Castor oil does a special job in many high-revving competition engines: it helps prevent detonation. In normal operation the fuel/oil/air mix ignites and burns smoothly, delivering relatively uniform pressure to the descending piston. Detonation is an almost instantaneous energy release that generates shock waves, uneven pressures, and hot spots in the combustion chamber; it is inefficient and can cause catastrophic damage (for example, blown holes in piston crowns).
An analogy: nitroglycerin is an ultrasensitive explosive; dynamite is nitroglycerin absorbed into an inert filler (diatomaceous earth) that moderates the energy release. Castor oil plays a similar role in model engines: microscopic droplets of castor suspended in the combustible mixture act as shock absorbers during the millisecond explosions that produce power impulses. They do not burn and thus moderate peak pressures.
Castor oil is often misunderstood but has two important, almost-unique properties:
- It forms a tenacious film that stays on metal surfaces in the hottest areas.
- Unlike most oils, it increases viscosity (film strength and thickness) as operating temperature rises, so it gets thicker and tougher with heat.
This temperature-dependent thickening is particularly valuable at the little end of the connecting rod and the rod/piston interface—the second-hottest region inside the engine and one of the hardest to lubricate because the fuel/oil/air mix cannot flow freely into the enclosed piston underside. Oscillating-type bearings (such as some small-model connecting rods) are especially difficult to keep lubricated because they tend to squeeze lubricant out under pressure, whereas rotary and reciprocating bearings more readily maintain their oil films.
Castor oil: downsides and blends
Castor's tenacious film-forming ability has downsides. Internally it can form varnishlike coatings that increase friction between moving parts. Externally it often forms a dark brown patina from the combined effects of heat, oxygen, and time; that baked-on residue is more tenacious than most vegetable oils. Castor is chemically similar to linseed oil, and within a few days both can form a virtually insoluble surface film.
To balance pros and cons, many glow fuels blend castor with synthetic oils: castor provides the tough, heat-resistant film while synthetics help maintain slipperiness. Common blends are two parts synthetic to one part castor, or primarily synthetic with a small percentage of castor.
If you want to retain a clean-looking engine while using castor:
- Immediately after a day's flying, clean the engine exterior with isopropyl alcohol and a stiff toothbrush.
- Some "airplane cleaners" (mixtures of water, ammonia, detergent, and alcohol) work well but can cause internal rust if they enter the engine. Seal the inlet and exhaust tightly before using such cleaners. Alcohol alone will not cause rust; water/ammonia/detergent can.
- If a brown coating has formed, two solvents that remove it without harming engine parts (when used according to directions) are Z-Best Engine Cleaner and Sears-Roebuck's SOKOFF.
Cox engines and castor requirement
Cox engine instructions historically stated that the guarantee was void if the fuel used lacked castor oil. If a damaged Black Widow were returned to the factory, the technician could inspect the upper end of the rod: a blue discoloration from excessive heat proved lack of castor in the fuel. Only castor could provide the tough, heat-resistant lubrication that Cox engines' ball-jointed conrods required to survive.
Teflon-containing oils
Never add any lubricant containing Teflon particles to your fuel. At temperatures over about 600°F, Teflon decomposes and can release fluorine-containing gases—one of the most corrosive elements. While oils with microscopic Teflon particles are sometimes promoted, Teflon decomposition at high temperatures makes them unsafe in hot-running internal combustion engines.
I have used Teflon-containing oils successfully in CO2 and compressed-air motors (which never get hot), and I have used a high-Teflon-content oil called Terra Gun Oil to spot-lubricate crankshafts and crankpins in disassembled engines. Terra Gun Oil (milky and requiring thorough shaking before use) reportedly leaves micron-size Teflon particles that lodge in microscopic surface pits and lower friction. In my tests, after cleaning and thoroughly working the oil into plain bearings, engines gained several hundred rpm. Terra Gun Oil has been available by mail order from M. E. Williams, Ltd., 7040A Bembe Beach Rd., Annapolis MD 21403; Tel.: (410) 268-4197. However, do not add Teflon-containing oils to running-engine fuel.
Fuel oil percentage
For long life and maximum power during that life, I feel a minimum of 25% oil by volume in fuel is needed—except perhaps in ABC-type engines. A little excess oil in old model engine fuel does not detract from its power content. The quantity of combustible vapor available in the cylinder is controlled by the needle valve and throttle settings, not by the oil percentage.
Closing tip: buy engines when you see them
Whenever you see a model engine that looks like what you need, buy it while it's still available. I've missed out on engines I wanted by waiting ("I'll get one next month"), only to find the model discontinued or the company gone (as happened with G-Mark and HB, and with Cox discontinuing some lines). If you particularly want to own an engine, buy it quickly—while you can.
Transcribed from original scans by AI. Minor OCR errors may remain.
