OS 40 FSR

OS 40 FSR schnuerle motor is one of the most popular over 35 years ago. Schnuerle scavenging combustion system, good case designing and precise finishing made it's reputation unmovable, also it's carburetor had fuel squeezing mechanism and it was operated so stable and so smooth well not only on ordinary flying models but also on stunt maneuver contest scenery. 
In spite of the holding of crank shaft main bearing is not fully covered, the thickness of front case mainly supporting crank shaft is stout with no vulnerability leading to some certain operation.
Crank shaft bearings F/R are R6Z and #6902 proving enough gas port area grooved in the shaft and sophisticated designing carried out to establish a perception as the first class operating product.
So, ultimate performance could be obtained not only by an experienced flyer but also by a layman pilot so far as the rust is avoided unexpected appears in the bearings, piston ring.
ABN P/C model could be chosen in later model, the difference between a ring piston one is not be distinguished, on the other hand operation noise of  ABN model is higher than the former model.
The technology for producing this kind of Schnuerle motor must have established at that period, though one piece case technology and machinery precision have remarkably advanced these days.
Noisy performance operation is not accepted by ordinary inhabitant these days because of Noise Pollution, the quiet and calm operation is desired except some specific field such as Pylon Race and Rat Race. Probably, this is why 4 cycle motor or Brushless motor source are accepted including breaking effect while downing phase.