4 Stroke Engines

The smallest in the O.S. four stroke range and, at only 9.56 oz, it's also the world's lightest production model four stroke.

It has most of the features of the larger single cylinder O.S. four strokes, but it has its camshaft carried in bronze bushes - backed up by a pair of spherical thrust bearings.

Designed to be used with 9 x 6~7 to 10 x 6 props.

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The original 6.5cc FS-40 was O.S.'s second production single cylinder four stroke and the first to have the front mounted helical driven camshaft now common to all their single cylinder O.S. four stroke engines.

The current FS-40S is an all-new design which is much more powerful and closely related to the FS-48, though smaller both internally and externally.

Supplied complete with silencer, the FS-40S is ideal for sport and scale models and is designed for use with 10 x 7.5 to 12 x 6 propellers.
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The new FSa-56 from O.S. is the first engine in a completely new series of advanced four strokes. The FS ‘Alpha’ series draws on the experience gained from the Surpass range to develop and refine standards to a new level.
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The FS-62V has the same 'classic' overall design that appeals to scale modellers that like their engines to remain in character with the model. The performance, on the other hand, is very much 21st Century and builds on the O.S. tradition of constant technological advancement with each new range. In the case of the 'V' series, there is also a strong emphasis on value for money.

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Sharing the same mounting dimensions as the old FL-70 and FS-70U, the new FSa-72 is a simple bolt-in replacement offering compact external dimensions and power/torque to most types of model designed for .46 2 strokes or .70 4 strokes.
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The new FSa-81 is a direct replacement for the FS-70 Surpass II, offering greater power and throttle precision in a compact and lightweight design. The crankcase mounting dimensions allow the FSa-81 to slot into the same mount that an FS -70S II would for an instant power upgrade.

Like the FSa-56, the crankcase breather system has now been incorporated inside the engine to negate the need for an external vent and the excess oil that eminates from it. A propeller locknut assembly is of course standard.
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Sports and scale modellers have enthusiastically embraced FS-a engines because they virtually eliminate oil mess while delivering excellent power. Now you can get the 81FS-a equipped with a fuel pump for even more consistent performance.
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This latest version of the popular FS91 Surpass features a true pressurised fuel system. Similar to the new FS 120S-III the new FS 91-II P has a pump system that is capable of delivering constant fuel via the regulatory carburettor. It is therefore ideally suited to the emerging '90' size aerobatic models. Designed to be used with 11 x 10-11 to 12 x 8 propellers. If you're looking for constant fuel feed at any attitude this is the engine to choose. Supplied with an effective large volume muffler.
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Developed from the popular FS 91 Surpass and honed to perfection in this latest model.

Like its 52, 70 and 120 stablemates, the new FS 91 Surpass II features a new carburettor, longer crankshaft, plated camshaft/crankshaft/ ring and assure it's owner of the best performance in it's class coupled to the O.S. legendary quality and long life.

This is a great engine for scale models that specify a 61 size two stroke as that four-stroke sound is particularly suited to this environment.
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Stying is deliberately 'retro' and in keeping with the style of the very first OS four strokes. The original FS-120 had twin rocker covers and so do many full size engines in older aircraft. But 'retro' stying does not mean 'retro' technology! The FS-95V is a sophisticated piece of kit, delivering strong power and torque and offers a cost effective alternative to the FSa-81.

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OS FSa 110 is designed specifically to offer more power and torque compared to a .91 size four stroke, yet still fit in the same mounting dimensions, the FSa-110 is a real compact powerhouse.
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The FSa-110-P is a great engine to increase power when you want to replace a .91 as it has the same mounting dimensions. Now you can enjoy the added benefit of a fuel pump with this new version of the 110FS-a.Plenty of torque to swing large props. Ideal for aerobatics and other situations that demand high power in all attitudes.
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This is the economy, suction-fed version of the FS120S engine - without a fuel pump or pressure regulated carb but with all the other quality features found in the FS120S series of engines - the largest single cylinder 4 strokes in the O.S. range. Excellent for models where the fuel tank can be mounted normally. Designed to be used with 13 x 11~12 to 18 x 5~6 props. Supplied with an effective muffler and double lock nut system for the propeller.
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An updated version of the FS120 MkII now fitted with an integral O.S. Type PG-03 fuel pump and carburettor with built-in fuel regulation to offer the maximum engine reliability - irrespective of the fuel tank's position and model attitude.

The FS-120 MkIII features four ball bearings and a special propeller locknut assembly as standard.

Ideal for aerobatic, sport and scale models it is is designed to be used with 14 x 9~11 to 18 x 6 props.
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The OS 155 alpha is the biggest engine in the 'Alpha' series and has been designed with both aerobatic and scale modellers in mind.

The pumped fuel system deals with the demands of aerobatic fliers who demand consistent power in any attitude and is tolerant to variations in fuel tank location. With consistant fueling, the FSa-155-P then goes about its business of delivering that stunning 2.6PS of power when you demand it. But that is only part of the story. The strong spread of torque down the RPM range allows a wide variation of propeller loads to suit both aerobatic and scale modellers alike by 'picking up' cleanly and strongly as the throttle is opened and being tolerant to those different propeller loads.

With mounting dimensions identical to 1.20 class engines, fitting the FSa-155-P as a power upgrade is pretty simple.
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The largest, single-cylinder four stroke engine that OS has ever made draws on unrivalled experience in engine design. This powerful, high-torque engine has been created with scale and sports-scale modellers in mind. The ability to turn a big propeller, yet offer compact external dimensions, makes the fitment inside the tight cowling of scale models a real joy. Fuelling is courtesy of a specially designed twin needle carburettor and like all the latest Surpass series, the internals have been lavished with the engineering care that only OS can achieve, whether that be precision machining, material selection or surface finish.
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The logical step up for the aerobatic flier that needs a bit more power for his model and a 1.20 is just not quite enough. In fact, it could be the ideal power choice for a budding F3A pilot looking to power his new 2m model.

The new PD-08, diaphragm type fuel pump delivers a constant supply of fuel to the regulator, which in turn ensures that the carburettor itself has the perfect fuel supply, no matter what the model's attitude. This precise fuel supply also ensures optimum fuel consumption, preventing the need to carry excess fuel. The pump reduces the need to keep the tank close to the engine as well so that you can maintain a constant model centre of gravity throughout the flight.

The main engine is mechanically identical to the standard FS-200 so fitment to a model designed for a 1.20 is no problem.
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