An RC Model Airplane Engine: The O.S. FS-120 MK.3 Surpass

Maybe the most popular 4-stroke model airplane engine in the O.S. range. A full description, along with CAD drawn 3-views.

OS FS 120 SurpassThe O.S. FS 120 Surpass

The "Surpass", in the name means this model aircraft engine is the second series of 4-stroke engines offered by O.S. The Surpass engine has 30% more power than it's predecessors.

I have owned 3 of these engines and can testify to the great service and long life you can expect if you own one. 


  • Specially developed PD-07 Pump. This unit ensures a reliable fuel feed to the engine. You no longer have to be concerned with tank position.  the fuel feed is always constant.
  • new 70N carburetor.
  •  Ball bearing supported crankshaft and camshaft.
  • New F-5020 Silencer.
  • Two year warranty.

 Box Contents:

  • OS. F-type glow plug.
  • One muffler with manifold.
  •  One manual
  • One additional needle with coarse taper for less sensitive adjustment


A good ,substantial plastic mount is recommended for this type of model aircraft engine. This type of beam mount can then be bolted to a ply box (1/4" thick) or dowel stand-offs can be used.

Great Planes adjustable engine mount

The photo shows the Great Planes Adjustable Engine Mount 1.20-1.80.

I have used these on my last 2 projects and found them ideal. I prefer to tap the mounts and use # 8 or # 10 socket head capscrews.

If space is required to achieve the correct spacing for a cowl, then I have had great success with dowel stand-offs. Take a look here for more information- RC Engine.

Tank Size for this model airplane engine

A tank size of around 15oz will be good for this model airplane engine and this amount of fuel should last for 14-15 minutes.

Click here to visit the Sullivan Products site for more tank details


Nitromethane content should be between 5 and 15%. 

Oil content should be 18%. Synthetic or a synthetic/castor blend. I would not suggest an all castor oil mix, as this tends to produce varnish in the model airplane engine and makes a real sticky mess in the exhaust! 

Glow Heat

I have not found it necessary to provide glow heat at low throttle settings, even with the engine mounted  inverted. The O.S. "F" type plug is very good in these conditions.

When the engine is fully cowled, I do prefer to use a remote glow connector. This makes glow hook-up safer and easier.


  • Very smooth running engine with a minimum of vibration.
  • Nice exhaust note.
  • Easy starting with an electric starter.
  • Usual O.S. first class materials and construction.
  • Long life.
  • Two year warranty.
  • Non critical tank position because of the fuel pump.


  • Higher price ( Worth it, in my opinion).
  • Messy clean-up- typical of all glow engines.

3-View Drawings

The 3 drawings below were drawn using Autocad. If you click on the drawings you will have access to a PDF version.

I have tried to make these drawings as close as I can to full size, but there are a lot of variables at play here!

The inch and metric scales you see on the drawings should help in scaling any dimensions. and/or scaling the print scale.

OS FS 120 Surpass CAD front
OS FS120 CAD side view

The OS FS120 Surpass is in stock

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