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     OS 140 RX   

The most used 2-stroke engine for F3A today, is the OS140RX.  It is a light engine with plenty of power and ease of operation.  Even though it is easy to operate, as with all engines there are a learning-curve and a few small tricks.

    Setup and configuration

  • Glowplug : Use OS-F or another long reach plug, with the supplied (at least earlier) OS-A5, idle and transition is a bit rough.

  • Compression : The OS-F plug is actually a bit to hot, and the engine runs a bit hard.  This is fixed by adding 1 or 2 extra head shims.  

  • Fuel : The engine likes nitro, I'm using 15% and 15% Aerosynth 2 oil.  

  • Exhaust : This engine is very uncritical regarding exhaust system. Both the original Hatori 900, Hatori 650, Bolly EQ140 works well.  All these are pipes and must be set fairly long, to avoid too much throttle hysteresis.  With a muffler some  (not much) power are sacrificed for improved throttle response.

  • Propeller : The flexibility of this engine is great, but for F3A only some props are usable.  We can not select the prop that gives best power, it will be too noisy. The best prop is different from plane to plane, but I have good experience with 16x14, 17x12, 17x13 and even 18x12, all APC.  My personal favorite is the 17x13.


  • Setup : Originally, the 140RX has a ringed piston in a steel liner.  From Performance Spesialties one can buy a AAC setup with the same porting and perhaps some more power with a pipe.  As an added bonus, the setup is 30-40 gram lighter than the original.

  • Twin-plug head : First seen at WC-1999 in Hatta's plane, this option can either be manufactured from an original head, or from scratch. The  central plug is an OS-A5, the rear one an OS-F.  The power is improved with this head, together with even better throttle response.  Nothing is for free, and the downside is increased fuel consumption.



Some time or another things is not working as usual.  This can be frustrating, especially with a generally trouble free engine as this. Some special behavior of this engine are :

  • Lean idle : It is more or less impossible to do the last fine adjustments to the carburetor on the ground.  If the idle is too lean, the engine can detonate in the muffler when pushing to level inverted flight and advancing the throttle.
  • Worn piston ring : A worn piston ring can give symptoms of lean idle.  This can also be due to a blocked carburetor.