The GF40 is the first ever OS four-stroke petrol engine and is designed to offer the benefits of four-stroke operation to a new size of model. That softer, scale sound can now be used in models designed for 33cc 2-stoke engines. In fact, the GF40 has the same mounting bolt configuration as the GT33 as well as the same distance from the rear of the mount to the propeller driver.

Unlike full size 4-stroke engines (car and aircraft) the GF40 does not use an oil system, it relies on oil mixed in the fuel like a two stroke. To do this requires very careful design and some special features to ensure that lubrication meets the riight surfaces at all times. Two strokes get a constant stream of fuel-oil mix around the big end and bottom end of the engine. A four stroke has to rely on on ‘blow by’ or other methods to achieve this. OS has thought of this and incorporated internal features to make sure oil reaches the right places. This is partly achieved through their PCV system (Positive Crankcase Ventilation) plus other oil galleries.The PCV system also reduces oil ejection from the exhaust and limits pollutents.

Carburation is controlled by a special carburettor developed between OS and Walbro especially for the GF40. Being a ‘pumped’ style of carburettor, there is no need to keep the carburettor close to the centre line of the fuel tank.

Like all four-strokes, the GF40 is all about torque and the ability to accelerate larger propellers more effectively. Sizes from 18 x 6 up to 20 x 8 bear testament to this.

With amazingly low fuel consumption and an effective silencer, was there ever a better time to go four-stroke in bigger models?

Specifications
Weight	1408g w/silencer and ignition
Displacement	39.96cc
Bore	40mm
Stroke	31.8mm
Practical RPM	1,800 – 9,000 RPM

Spec

• Brand: OS Engines
• Part No: L-OS39400
• Barcode: 4531028022805
• Format: Petrol