Honda GX31

Honda GX31The Honda GX31 is a 31 cc (1.9 cu·in) natural aspirated single-cylinder four-stroke air-cooled internal combustion small gasoline engine with a horizontal shaft, manufactured by Honda Motor Company for general-purpose applications.

Cylinder bore and piston stroke are 39.0 mm (1.54 in) and 26.0 mm (1.02 in), respectively. The compression ratio rating is 8.0:1.

The Honda GX31 engine produced 1.5 PS (1.12 kW; 1.5 HP) at 7,000 rpm of maximum output power and 1.64 N·m (0.2 kg·m, 1.2 ft·lb) at 6,500 rpm of peak torque.

The Honda GX31 features:

  • OHC design
  • Transistorized magneto ignition system
  • Diaphragm type carburetor
  • Centrifugal governor
  • Pumping spray lubrication system
  • Forced air cooling system
  • Recoil starter

General information

Engine Specifications
Engine model GX31
Engine type 4-stroke, OHC, single-cylinder
Displacement 31 cm3, (1.9 cu·in)
Max. horsepower 1.5 PS (1.12 kW; 1.5 HP) at 7,000 rpm
Max. torque 1.64 N·m (0.2 kg·m, 1.2 ft·lb) at 6,500 rpm
Cooling system Forced air
Air cleaner Dry type
Lubricating system Pumping spray
Ignition system Transistorized magneto ignition
Fuel system Diaphragm type carburetor
Fuel consumption 250 g/HPh
Fuel tank capacity 0.65 L (0.17 US. gal. 0.14 Imp. gal.)
Fuel used Unleaded gasoline (octane number 86 or higher)
Starting system Recoil starter
Stoping system Ignition primary circuit ground
PTO shaft rotation Counterclockwise (from PTO shaft side)
Dimensions (L x W X H) 210 x 251 x 250 mm (8.3 x 9.9 x 9.8 in)
Dry weight 3.4 kg (7.5 lbs)

Cylinder Block and Head

Cylinder block
Compression ratio: 8.0:1
Cylinder bore: 39.0 mm (1.54 in)
Piston stroke: 26.0 mm (1.02 in)
Cylinder internal diameter (standard): 39.000 mm (1.5354 in)
Piston skirt outer diameter (standard): 38.990 mm (1.535 in)
Number of piston rings (compression / oil): 2/1
Ring end gap (Top): 0.100-0.250 mm (0.0039-0.0098 in)
Ring end gap (Second): 0.100-0.250 mm (0.0039-0.0098 in)
Ring width (Top): 0.980 mm (0.0386 in)
Ring width (Second): 0.015-0.050 mm (0.0006-0.002 in)
Crankshaft center distance: 13.0 mm (0.51 in)
Cylinder head
Valve arrangement: OHC
Valves: 2
Valve steam diameter (INTAKE): 3.985 mm (0.1569 in)
Valve steam diameter (EXHAUST): 3.950 mm (0.1555 in)
Camshaft lobe height (INTAKE): 22.790 mm (0.8972 in)
Camshaft lobe height (EXHAUST): 22.790 mm (0.8972 in)
Camshaft journal diameter : 5.020 mm (0.1976 in)

Service Data

Idle speed: 3,100 ± 200 rpm
Cylinder compression: 2.9-5.9 kg/cm2 (0.3-0.6 MPa, 41-84 psi) at 1,200 rpm
Valve clearance
Intake valve clearance: 0.10-0.14 mm (0.0039-0.0055 in)
Exhaust valve clearance: 0.13-0.17 mm (0.0051-0.0067 in)
Oil system
Lubrication system: Pumping spray
Oil type: Honda 4-stroke or an equivalent (SG, SF)
Recommended oil: SAE 10W-30
Oil capacity: 0.1 L (0.11 US. qt, 0.09 Imp. qt.)
Fuel system
Type: Diaphragm type carburetor
Main jet: #40
Ignition system
Type: Transistorized magneto ignition
Ignition timing: 27° B.T.D.C. (Fixed)
Spark plug: NGK C5HSB, CR5HSB
DENSO U16FS-UB, U16FSR-UB
Spark plug gap: 0.6-0.7 mm (0.0236-0.0276 in)
Ignition coil air gap 0.35-0.5 mm (0.0138-0.0197 in)

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