The Engine Shop
Eric Henderson | [email protected]
Exploring O.S. engines
In my October column, I explored the impressive but complex YS 140. That engine has metamorphosed into many variants, including a pump instead of a regulator and even a computerized-ignition version. The latter was a YS 175 that won the 2011 FAI F3A World Championship, held at AMA Headquarters in Muncie, Indiana.
The O.S. 160FX is a less-complicated engine with similar power. Although there is also a fuel-injection version of the 160 that uses a computer module, the basic O.S. 160FX has changed little since its inception.
The O.S. 160FX is less expensive to own and run than the equivalent four-cycle engine. It will operate on the cheaper 10–15% nitro/methane-based two-cycle glow fuel. With a standard muffler, it will turn propellers in the range of 15 to 18 inches in diameter with pitches of 6 to 14 inches.
The cubic-capacity designation of two-cycle glow-plug engines is often stated in cubic inches. The O.S. 160FX name tells you that it is a 1.6-cu. in. displacement engine. The cubic capacity of an engine is the volume in the cylinder between the piston at bottom dead center (BDC) and top dead center (TDC).
Many people refer to engines by cc (cubic centimeters). The O.S. 160FX is actually a 26.23 cc engine and may be the optimum size for a two-cycle, glow-fuel-powered engine. O.S. has the larger BGX, and MARK has the 2.1, but you run into the law of diminishing returns on power vs. weight when you factor in the large two-cycle glow-fuel tanks required for a 10-minute flight. It is not unusual to see a 32-ounce tank on a BGX. The O.S. 160FX will give you 10 to 15 minutes on a 20-ounce tank at full power—subject to the propeller and nitro content being used.
This is a two-cycle engine and has fewer moving and static parts than the YS four-cycle, yet it will turn similar-sized propellers depending on the fuel and muffler systems. The standard O.S. muffler and scale “can-type” mufflers give good sport power.
When the O.S. 160FX is used with a tuned pipe, you can get a pattern airplane to go vertical, do a snap roll, then open up to full throttle and resume vertical climb with the same speed used prior to the snap. A lightweight carbon tuned pipe will give an 11-pound pattern model unlimited vertical with a 17 x 13 APC propeller.
The O.S. 160FX is an easy and straightforward engine to use. After it has been set up, it needs little attention. In its standard configuration, it uses muffler pressure to encourage fuel to reach the carburetor.
I have owned and operated more than 20 of these engines and still run them in my Hangar 9 106-inch Cub and Airborne Models 80-inch Mustang. I still have a few in boxes as well.
You can run this ringed engine as it comes out of the box. After breaking in and filling a tank of fuel, you are ready to fly. When you use the standard muffler it is best to keep the tank as close to the engine as possible. The big 60F carburetor provides a good mixture throughout the range, but like many large two-cycle, glow-fuel-powered engines, it does not draw fuel well on its own.
The standard muffler contains a baffle that many users remove. This may seem to give more power, but it disrupts muffler pressure that feeds the tank. What you see is a good top-end run that is plagued by gurgling and hesitation in the midrange during flight.
Many O.S. 160FX operators accidentally run their 160s too hot. This leads to problems such as mufflers coming apart, head bolts loosening, and failed bearings. In extreme cases, the engine can seize in the cylinder liner or jam the ring in the piston groove.
One obvious cure is to put the baffle back in the muffler. Another solution is to fit an O.S. "F"-type plug. The "F" plug is designated as a four-cycle glow plug which, by design, runs hotter. This means it keeps glowing brighter between ignition moments. The supplied plug is an A3, which does not like to get too rich. If you set your transition and mid-range needle just right, you may end up overheating the cylinder at sustained full throttle. With an "F" plug, the engine can be set for full throttle and will recover from a slightly rich mid-range run.
Using aftermarket mufflers can be tricky. The canister-type muffler with two down-pipes exacerbates the carburetor-tuning situation. I have seen Perry carburetors and Kline pump systems used to remedy the problem.
However, the better solution is to fix the exhaust-pressure problem. If you look closely at a two-cycle muffler, you will see that the size of the exhaust opening does not match the opening at the engine exhaust. By size, I mean the total area of the opening.
The opening cannot be too small because it will choke the engine and reduce power output. If the opening is too large, you lose some of the pressure that bounces the exhaust gases coming out of the cylinder. This bounce keeps the fresh charge from spilling out of the cylinder before the piston closes the exhaust. The muffler provides manageable exhaust pressure that we can route into the fuel-tank air vent. This pressure is low and varies with the engine's rpm.
The exhaust pressure supplies glow fuel to the carburetor at the right time. More rpm uses more fuel and produces more exhaust pressure at the right time. If this pressure is too weak, the mixture from the 60F will not be consistent enough throughout the throttle range.
To get the same pressure as a standard muffler, you need the total muffler gas-exit area to be at least equal to that of the stock muffler. Most can-type mufflers that I have seen used on O.S. 160FXs are loud and have a large exhaust-opening area. You could block off an entire pipe on some of the canister mufflers, and the engine would run better. (A tachometer can confirm these results—quiet engines can be misleading.)
I have seen field repairs where pilots crimped the ends of these pipes to reduce the total size of the exit aperture until the carburetor could be set. A more professional modification is to make two aluminum hollow-core plugs on a lathe and pin them in the ends of the downpipes. Make the total exit area slightly less than the stock muffler to allow for the fact that there are usually no baffles in the cans.
One tip to pass on is how to better assemble a stock muffler. The unit is held together with one long bolt. During reassembly, preheat the bolt with a small butane soldering torch. The heat expands the length of the bolt. If you quickly assemble and tighten the bolt, it will be less prone to come loose during operation. I also use the red, high-temperature Loctite thread-locking compound on the threads and on the rims of the mating exhaust parts. This prevents them from rotating when hot.
Starting Tip
The starting technique for a big two-cycle engine is different from the smaller versions. A starter motor is a common accessory at the field these days; however, you will need a strong one to turn over a fuel-loaded and ready-to-fire O.S. 160FX. There is an easy way to start these motors without much 12-volt DC battery power or physical effort.
With the glow igniter removed, open the carburetor roughly 25%. Cover the venturi with a thumb or finger and slowly turn the propeller over four or five times. Don’t flick the propeller—just keep a grip on it as you turn over the engine. Move the throttle back to the idle position, then attach the glow igniter. One backward flip and the engine will nearly always burst into life.
The O.S. 160FX is not immune to ball-bearing rust. Two-cycle glow-fuel-powered engine bearings can and will suffer early rust damage from post-combustion impurities and the byproduct of moisture from hydrogen molecules combining with oxygen. An option is to replace the affected bearings with stainless-steel versions. There is an article on the Sport Aviator e-zine about changing bearings in a big glow engine. The website address is listed in "Sources."
Final Tip
Now that we have more available in-flight telemetry, the operational world of internal combustion has changed. You can add a head-temperature sensor and set your radio monitoring alarm level to 200°F, and you will know when to land. —M.A.
O.S. 160FX Specifications
- Displacement: 26.23 cc (1.60 cu. in.)
- Rpm range: 1,800–10,000
- Bore: 33.6 mm (1.32 inches)
- Stroke: 29.6 mm (1.17 inches)
- Weight: engine: 930 grams (32.8 ounces); muffler: 275 grams (9.7 ounces)
- Horsepower: 3.7 hp at 9,000 rpm
- Crankshaft thread size: 3/8-24
- Suggested propellers:
- Aerobatic: 15 x 12–14, 16 x 10–14, 16.5 x 10–13
- Sport: 17 x 10–13, 18 x 10–12
Sources
- Sport Aviator e-zine
(800) 435-9262, ext. 225 www.masportaviator.com
- O.S. Engines
(217) 398-8970 www.osengines.com
Transcribed from original scans by AI. Minor OCR errors may remain.
