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O.S. 15 GT Engine Review by Brian Winch

Two stroke, single cylinder, spark ignition




904g (ready to run)

2.4 ps – 2.37 hp

2,000 – 11,000

13 x 8 – 15 x 8

95 RON petrol down to 50:1 synthetic oil

UNF 5/16 – 24

Spark plug, ignition module, silencer, instructions, decals

With the range of two stroke petrol engines currently topping off at 60cc, the O.S. company have gone back down the scale to produce an engine with a very popular capacity and physical size (mount pattern the same as AX65) to fit nicely in the well established .90 class of engines. While a 15cc glow engines is still very popular for larger size sport and scale models, a petrol version has distinct advantages for several reasons. First off, contrary to common belief (where these ‘facts’ come from is beyond me), a good quality petrol engine has less vibration than a glow engine of the same capacity. For starters, a lower compression ratio than a glow engine is required for general petrol operation, they have controlled ignition timing, pre-ignition due to lean tuning is generally not an issue, and as well, the carburettor requires almost no re-adjusting once set, so the chances of incorrect tuning are greatly lessened. Due to the controlled ignition timing and that a flooded petrol engine will not generally kick over if the plug is wet, hand starting, if you use this method, is so much easier and safer and no bitten fingers. This is also a factor of the lower compression ratio as the engine is not so likely to baulk (commonly called a ‘kickback’) when the propeller is flicked as can happen with a glow engine.

Screen Shot 2014-07-07 at 11.10.47 amBecoming more of a problem now in Australia, particularly when you use larger capacity engines, is the cost of fuel for a methanol engine. Using around 2.5 times more fuel than a petrol engine, a glow engine can hit the back pocket at times. I hear of modellers paying $60 for 4 litres of mixed fuel and I know of several large engines that use one litre of fuel for around a 12 minute flight. As such, you are looking at $15 per flight – not an inconsiderable amount. For a model petrol engine you use common pump petrol – middle of the RON range, a modicum of synthetic oil and… that’s it… nothing else to add… nothing else needed and one litre would keep a very large engine running for much longer than you would care to fly. As an example, with this engine running at full throttle for a full 10 minute flight you could expect to use around 200cc of fuel – 5 such flights per litre. While we are in the fuel department, tank position is not so critical as it generally is for a methanol engine. Actually a high tank position is an advantage as the gravity feed will keep the petrol up to the pumped carburettor much the same as a motorcycle, or as an example, a Tiger Moth with the wing top tank.

Another example of your control over the engine is being able to stop the engine from your transmitter should a problem occur when flying a model. I know you can cut the throttle on any engine, but not if there is a servo, linkage or connection problem. A switch on your TX is flicked, the ignition power is immediately switched off and, naturally, the engine stops immediately. Next consideration is a double header and it is the exhaust sound and efflux of the engine. A petrol engine has a quieter exhaust note than a glow engine, almost no efflux (exhaust residue) and O.S. have produced a matching muffler for this engine that cuts the sound quite a lot more without reducing the power.


Established modellers will well know the almost legendary quality of O.S. engines that goes right back to the very first engines produced. New-comers to the flying field might have yet to learn this but can be assured by long term modellers and then be further assured when they purchase their first OS. I recommend this engine as an excellent first petrol engine when the beginner modeller has gotten over the first tentative steps, so to speak, of flying an i.c. powered model and is looking for something nice – away from trainer type models – and an engine that will be user friendly, have a certain class about it and will be as reliable as the engine in the car he drives to the field. I hear, at times, from modellers asking my advice about a suitable engine for a project, and when I mention O.S., they remark that the engines are ‘nice’ but the cost is high. No, the cost is not high – it’s a reasonable cost for a high quality product, one that will provide reliable service for many years, an engine that doesn’t need replacing due to rapidly worn parts or the economy of disposing of the engine rather than having it repaired. If you do have a mishap, say a hard crash, if needed, your O.S. engine is well worth repairing, the original parts will have it back to new condition and performance, and to me that is the investment in quality you should consider. As with all other O.S. engines, there are no short cuts taken in the production of this 15GT – it has the same high quality manufacturing of so many engines before it – the same expectation of service use and the same expectation that it will still be running reliably years from now when many lower priced engines have been long gone and forgotten.

My recommendations for this engine are sport flying in larger and easier to see models, mid range scale applications and applicable competition flying. Again certainly a very good introduction to spark ignition, petrol fuelled engines for advancing modellers and modellers changing their model power to i.c. – a ‘real’ engine. Let us now look at why I describe the quality so highly.


The main case is a common style casting comprising the crank area, outer finned cylinder and the front housing. Beam type mount plates are either side of the lower case, the exhaust manifold is on the side of the cylinder and the inlet manifold is incorporated on top of the front housing. In all, a well established model internal combustion engine shape much the same as we’ve known for 70 plus years – only the size varies. Of the usual high quality casting long established by O.S., the case is sturdy, has a pleasant external appearance and the high internal shine of fine machining and high grade aluminium alloy. The front housing is fitted with two ball bearings for the crankshaft, has two threaded holes for the ignition sensor and incorporates the original design manifold for the unique 61H petrol type carburettor.

Due to the necessity to isolate the carburettor from the main engine block, the manifold is a two screw pattern with an isolation spacer fitted. This is necessary to prevent heat from the engine causing a vapour lock (petrol partially vaporises) in the carburettor as petrol fuelled engines run at a slightly higher temperature than methanol fuelled engines and use much less fuel. The extra fuel used in a methanol engine and the lower vapour temperature (of fuel flowing through the manifold) serves to lower the temperature in the area of the intake manifold as well as lowering the general operating temperature.

On the rear of the lower case the rear cover (of common design) is fitted and sealed with a gasket (petrol fuel tends to seep out unsealed joints). In- side the case a PD-08 diaphragm type fuel pump is fitted and it is pulsed by crankcase pressure via a short length of Tygon tubing from a nipple cast in the rear case. All neat and trouble free.

The cylinder head is also of the common style except for the fins that are longer to cope with the dissipation of the higher running temperature. The plug is a CM6 with a 10mm thread and it does not need elbow straining effort to tighten it – a good nip up when the thread is seated is quite adequate.


The muffler is the well established E-4040 known as the POWERBOX and comes with good ancestry as it is used on a range of O.S. engines to good ef- fect. Briefly, it has a very effective internal baffle and a four position outlet pipe. Due to my not wanting to upset the neighbours too much, I fitted a 300mm length of 12.7mm bore silicone tube to the outlet and had it pointing downwards. Certainly tamed the final exhaust sound and did not make a scrap of differ- ence to the engine performance. With the designed effectiveness of the O.S. muffler and my extension, the final exhaust sound was extremely low and – to me – a good engine sound. Modellers who consider using this engine for scale models might find the optional E-4051 flat muffler a good choice as it would fit in many cowls, and depending on how you orient the engine, you can have the outlet in several positions – even like a Pitts style.

The carburettor is a new and interesting piece of equipment designed specifically for this engine (at this stage). At first glance it appears to be a common style carburettor – the reliable O.S. twin needle design – but a second look reveals that on the rear of the carburettor body is an integral regulator and this is the first port of call, so to speak, for the fuel supply. Fuel is pumped from the tank by the rear fuel pump up to the regulator where it stops until there is a demand from the engine. According to the RPM (demand) of the engine, the regulator pulses in the fuel charge which is then finely tuned to demand by the high and low speed needle adjustments. On test, I found the fuel supply system to be faultless with a reliable supply even though my test bench tank was considerably lower than the carburettor. Tuning was very indicative in that the monitored RPM reached the maximum for the particular propeller and did not change when the needle was turned, on average, 4 clicks leaner, then further lean tuning was indicated by a gradual drop in RPM. A twist back to the richer setting had the engine respond almost instantly. I do not advise tuning like this all the time – it was purely for my testing observation for this report. As the engine was supplied to me I had no need to touch the idle mix tuning and I’m sure you’ll find the same result regardless of your fuel mix (I used 20:1 and 32:1). Of interest, on the second day of testing, I tried out a new item from RotoFlow tanks – the QuikFire Fuel Balancer and Filter supplied by Christian Traders. Simply, it is a metal hopper tank with an inbuilt filter that you mount higher than the carburettor. When full of fuel it supplies a smooth flow of fuel (especially for extreme flying styles), and as it doesn’t naturally empty there is fuel instantly when you crank the engine. I will fully review this in a later article.


The long standing and well proven combination of a fully machined (bar stock material) aluminium alloy piston, cast iron ring and hardened steel liner assures long and reliable engine life. The conrod is like so many O.S. rods as used for many years – machined from super tough aluminium alloy (like the piston) and for this one, the big end is fitted with a caged needle roller bearing. The conrod is retained on the crankpin by a crank pin stop screw which, from past experience, would have a left hand thread.

The crankshaft is of the usual high quality produced by O.S. and of the same style as is common in front induction two stroke engines. The propeller drive hub is located (and driven) by a Woodruff key (maintains timing if the hub is removed), the propeller washer is tapered face steel into which the main nut fits and it is further secured by a collet lock nut. All super high quality that we have come to expect from O.S.


Ignition is supplied by an O.S. IG-06 module that requires battery voltage from 4.8 V to 8.4 V (rated). Current consumption is 400 mAh at 6,000RPM so you should consider no less than a 1,000 mAh battery capacity for a day’s flying. The supplied spark plug is a CM-6 with a 10mm thread and, as it was supplied, the gap was perfect for the test engine. Do not change the sensor position on the front housing as any change will alter the running and starting characteristics of the engine. Of special note, press the plug cap right down to ensure it covers and locks on the hexagon section of the plug body.

The module will not issue a spark below 120 RPM for safety reasons, so unless you are an accomplished ‘prop flicker’, use an electric starter.

APC Propellers (RPM)
13 x 7 – 11,111
13 x 8 – 10,972
13 x 9 – 9,541
14 x 8 – 8,431
14 x 10 – 7,932 or 1,100 idle
15 x 7 – 8,561
15 x 8 – 7,727
16 x 6 – 7,371 or 1,000 idle


I have been experimenting with my own design isolation mount (more on this in a later article) and I was confident that I could use it for this engine on my solid steel test bench. Yes, it worked very well, there was exhibited slight movement of the engine at certain RPM operation which smoothed out at higher and lower RPM and the transmitted vibration back to the test bench was almost zero.

Being an old hand at engine ‘stuff’, I still like to hand start most engines and I had no trouble whatsoever with this one. Hot or cold, a little suck prime, two flicks, ignition on and I had it starting in no more than two flicks every time. No surprises, no kicks, just nice friendly starts; just how I like it. The tuning was very easy and indicative (as mentioned), the vibration was quite low for a single cylinder engine of this capacity and the exhaust sound (testing without the extension) was subdued and would not be a problem except at the very noise sensitive fields. I would not say I was surprised but, I was at least, extremely pleased with the transition from low to high RPM – never a quaver – instant response. Idle was dead steady and extremely reliable once the engine was well warmed and the maximum RPM was quite steady with a mildly rich tune. The fuel pump performed brilliantly and there were no seeps or leaks on the engine even after almost two days of test running and using a considerable amount of fuel. The economy was good with my testing indicating 20ml of fuel per minute at 10,972 RPM.

At 8,000 RPM I recorded 115 degrees celsius with a laser gauge and 160 degrees celsius with a thermo-couple under the spark plug – very good readings.

Testing was carried out on the 12 &13/02/2014 with an average temperature of 25 degrees celsius and 70% humidity. I used (first mix) 20:1 – 95 RON petrol to PetroMax oil then 32:1 and later 32:1 Cool-power blue. No difference in the performance with the different oils but a very mild increase in RPM with the 32:1 averaging 100 RPM higher.

My final note reads that it is a ‘nice engine in all respects – my type of engine.’ See it running on YouTube by dialling in BRIAN WINCH.

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