Hirth F-23

The Hirth F-23 is a twin cylinder, horizontally-opposed, two stroke, carburetted or optionally fuel injected aircraft engine designed for use on ultralight aircraft.[1][2][3]

Hirth F-23
Type Twin cylinder two-stroke aircraft engine
National origin Germany
Manufacturer Hirth
Unit cost US$4423 (base price 2009)


The F-23 is intended to compete with the 50 hp (37 kW) Rotax 503 and is differentiated from the Rotax powerplant by offering a horizontally-opposed cylinder layout. The F-23 uses free air cooling and piston-ported induction, with dual Bing 34mm slide or optional diaphragm type carburetors. 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-23 produces 50 hp (37 kW) at 6150 rpm and 40 hp (30 kW) at 5500 rpm.[1][2]


Specifications (F-23)

Data from Recreational Power Engineering[2]

General characteristics

  • Type: Twin cylinder, two-stroke, aircraft engine
  • Bore: 72 mm (2.8 in)
  • Stroke: 64 mm (2.5 in)
  • Displacement: 521 cc (31.8 cu in)
  • Length: 247 mm (9.7 in)
  • Width: 570 mm (22.4 in)
  • Height: 338 mm (13.3 in)
  • Dry weight: 84 lb (38.1 kg) including reduction unit, full exhaust system and electric start


  • Valvetrain: piston-ported
  • Fuel system: 2 X Bing 34mm slide or optional diaphragm type carburetor or fuel injection
  • Fuel type: unleaded 93 octane auto fuel
  • Oil system: 50:1 fuel/oil premix
  • Cooling system: free air
  • Reduction gear: cog belt with 1.8:1, 2.2:1 and 2.5:1 ratios optional



  1. Cliche, Andre: Ultralight Aircraft Shopper's Guide 8th Edition, page G-3 Cybair Limited Publishing, 2001. ISBN 0-9680628-1-4
  2. Recreational Power Engineering (n.d.). "F-23 2 cycle 50hp". Retrieved 17 January 2013.
  3. Purdy, Don: AeroCrafter - Homebuilt Aircraft Sourcebook, page 72. BAI Communications. ISBN 0-9636409-4-1
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