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A look at the well-known engine maker's second gas offering
Featured in the May 2013 issue of Model Aviation magazine
by Jim Rice

When Jay Smith asked me if I wanted to review the O.S. GT-33 gasoline engine, I jumped at the opportunity. I have flown nearly all kinds of RC aircraft during the past 55 years, and many have been powered by O.S. glow engines.

I have settled into the 20-30cc gas category and three-cell park flyer electrics. I currently have two airplanes with 20cc engines and two with 30cc engines. I am about to test-fly a new biplane I designed with both those engine sizes in mind, depending on the pilot’s performance desires.

The GT-33 retails for $419, which is slightly higher than some of its competition, but when I opened the box I was taken by its appearance, packaging, and degree of completeness. I was so excited to get to work that I forgot to take a picture of it either in the box or setting on the bench. I did take the time to weigh everything and discovered that the advertised weights were close to what my scale reported.

The engine comes with a NGK CM-6 spark plug and Walbro WT1024 carburetor installed, which I think are both pluses in the gas engine arena. It has user-friendly choke and throttle arms installed. These are often items that must be fabricated or purchased separately on similar engines.

The engine case has a built-in choke rod guide, so installing a choke rod is simple. The engine manual has a picture with choke rod installed. It would be nice if O.S. included it. The wraparound Pitts-style muffler was a nice surprise, allowing neater cowl installation and providing quiet operation.

The ignition is rated to use 4.8 to 7.4-volt (4.8 to 6.0-volt Ni-Cd or NiMH, 6.6-volt Lithium Iron, or 7.4-volt LiPo) battery pack. Current consumption is rated at 600 mAh at 6000 rpm and it recommends 1,000 mAh or larger ignition battery. The ignition module is set to not operate below 120 rpm for safety.

I used a single 2,600 mAh LiPo to power the ignition and receiver and servos. I did use a voltage regulator for the flight system, but connected the ignition without a regulator.

I was excited to try the engine in my son’s Sbach that had been sitting on my bench without a radio or engine for approximately two months, waiting for me to return it to flying condition for him.

The spark plug is canted toward the rear of the engine, which helps keep the plug wire away from propeller arc and inside the cowl.

The wraparound Pitts-style muffler is included and provides a clean installation.

When I compared the mounting lugs for the GT-33 with those of the DLE-30 that had been on the airplane before, I discovered they matched perfectly and all I needed was an extra 1/8-inch spacer to get the spinner backplate outside of the cowl. It went on with little effort and looks good.

The plug is canted toward the rear of the engine, which should help keep the plug wire away from propeller arc and inside the cowl. The cowl went back on the airplane with this engine and only required minor cutout modifications. It may not have required any, but I wanted to mount the muffler to the engine before installing the cowl and it wouldn’t quite fit. The modification is unnoticeable, but it makes muffler/engine/cowl assembly much easier.

Regular gasoline with Evolution synthetic oil in a 32:1 ratio was used for all engine runs. For the first start, the choke was closed and carburetor opened wide with the ignition on and it fired after roughly six flips. When the choke was opened and the carb closed it started on the fourth flip, but died quickly.

That repeated for six or seven iterations with the low throttle at various settings. There were only minor differences in run time before it stopped. I was convinced that the carburetor needed to be full and the diaphragm soaked and it would run perfectly.

Finally, I reluctantly opened the low-end needle approximately a quarter turn and it immediately fired. A Vess 18 x 8S propeller provided good transition and fairly reliable idle with only minutes of run time. I have provided a comparison of propellers.

After 30 minutes of running at different throttle settings, I refilled the tank and noted the following readings:

• Vess 18 x 8S 7,700: 1,800 rpm
• Evo 18 x 6 8,440: 1,830 rpm
• Xoar 20 x 6 7,140: 1,700 rpm

The tachometer reading prior to the first flight is 7,200 rpm on a 20 x 6 Xoar propeller.

Transition was good with every propeller, but idle was best with the bigger and heavier Xoar propeller. Throttle curve was used to set the sweet spot for my aircraft and flying style.

I installed the cowl after two tanks and took the 11-pound, 4.5-ounce Sbach to the field.

The carburetor adjusting screws aligned perfectly with the firewall, so adjusting with the cowl on became impossible. Despite the engine running slightly rich, it performed well for two flights and provided superior power-to-weight ratio, despite not being fully broken in or sufficiently leaned out. The airplane and pilot were keen on the engine.

Final thoughts

I definitely like the GT-33. Although its displacement is 10% greater than the engine it replaced, I believe it provides more than a 10% increase in performance. The aircraft had lead in the nose to balance with the other engine, so installing this more powerful engine allowed me to remove some of the dead weight.

Knowing the condition of my shop and the stiffness of my old fingers, I accept that someday I will lose the crankshaft woodruff key and have to get a new one, something I am sure the rest of you never do. Fly safely!


Size: 33cc gas engine
Output power: 3.85 hp; 8,000 rpm
Idle speed: 1,800 rpm
Maximum speed: 8,000 rpm
Displacement: 33cc
Bore and stroke: 35.99mm x 32.41mm
Weight: Engine 34.72 ounces; muffler 5.65 ounces; ignition 3.70 ounces
Propeller: 18 x 10 to 20 x 10
Ignition battery: 4.8 to 7.4 volts; 1,000 mAh or larger
Spark plug: NGK CM-6 or equivalent
Mount: Firewall
Street price: $419

—Jim Rice


Box 9021
Champaign IL 61826
(800) 637-7660


Vess Propeller

Evolution Propellers

Xoar Propellers

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