Hirth F-33

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The Hirth F-33 is a single cylinder, two stroke, carburetted aircraft engine designed for use on ultralight aircraft, including powered paragliders and ultralight trikes. the engine is noted for its extremely light base weight of 35 lb (16 kg). It is manufactured by Hirth of Germany.[1][2][3]

Hirth F-33
Type Single cylinder two-stroke aircraft engine
National origin Germany
Manufacturer Hirth



The F-33 was intended to fill the niche previously occupied by the now out-of-production 28 hp (21 kW) Rotax 277. The F-33 uses free air cooling, dual capacitor discharge ignition and reed valve induction, with a single Bing 34mm slide or optional diaphragm type carburetor. The cylinder walls are electrochemically coated with Nikasil. Standard starting is recoil start. A belt reduction drive system, fuel injection, tuned exhaust and electric start are optional.[1][2]

The engine runs on a 50:1 pre-mix of unleaded 93 octane auto fuel and oil. Recommended time between overhauls is 1000 hours.[1][2]

The F-33 produces 22 hp (16 kW) at 5200 rpm and 28 hp (21 kW) at 6500 rpm.[1][2]



Specifications (F-33)


Data from Recreational Power Engineering[2]

General characteristics

  • Type: Single cylinder, two-stroke, aircraft engine
  • Bore: 76 mm (3.0 in)
  • Stroke: 69 mm (2.7 in)
  • Displacement: 313 cc (19.1 cu in)
  • Length: 300 mm (11.8 in)
  • Width: 299.5 mm (11.8 in)
  • Height: 380 mm (15.0 in)
  • Dry weight: 31 lb (14.1 kg) (base weight) 45 lb (20.4 kg) (with electric start and reduction drive)


  • Valvetrain: Reed valve, piston-ported
  • Fuel system: 1 X Bing 34mm slide or optional diaphragm type carburetor or fuel injection
  • Fuel type: Minimum of unleaded 93 octane auto fuel
  • Oil system: 50:1 fuel-oil premix
  • Cooling system: free air
  • Reduction gear: four element cog belt



  1. ^ a b c d Cliche, Andre: Ultralight Aircraft Shopper's Guide 8th Edition, page G-3 Cybair Limited Publishing, 2001. ISBN 0-9680628-1-4
  2. ^ a b c d e Recreational Power Engineering (n.d.). "F-33 2 cycle 28hp". Retrieved 17 January 2013.
  3. ^ Purdy, Don: AeroCrafter - Homebuilt Aircraft Sourcebook, page 72. BAI Communications. ISBN 0-9636409-4-1