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:

  • Overhead valve (OHV) 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 modelGX31
Engine type4-stroke, OHV, single-cylinder
Displacement31 cm3, (1.9 cu·in)
Max. horsepower1.5 PS (1.12 kW; 1.5 HP) at 7,000 rpm
Max. torque1.64 N·m (0.2 kg·m, 1.2 ft·lb) at 6,500 rpm
Cooling systemForced air
Air cleanerDry type
Lubricating systemPumping spray
Ignition systemTransistorized magneto ignition
Fuel systemDiaphragm type carburetor
Fuel consumption250 g/HPh
Fuel tank capacity0.65 L (0.17 US. gal. 0.14 Imp. gal.)
Fuel usedUnleaded gasoline (octane number 86 or higher)
Starting systemRecoil starter
Stoping systemIgnition primary circuit ground
PTO shaft rotationCounterclockwise (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 weight3.4 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:OHV
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:0.12 ± 0.02 mm (0.005 ± 0.001 in)
Exhaust valve clearance:0.15 ± 0.02 mm (0.006 ± 0.001 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 gap0.35-0.5 mm (0.0138-0.0197 in)
Author: Sergei Wers
About: Co-founder and CEO of a multi-brand auto repair shop (engine repair, chassis repair, body repair and painting) for over 15 years. I have been developing internet sites with technical specifications for more than seven years.

ATTENTION! Dear visitors, this is an informational database of technical specifications, we are not a parts supplier or a marketplace.

We try to use official documentation and verify the published data, but errors may occur. In addition, specifications of machinery and equipment may vary depending on markets and modifications. We do not recommend the use of this data for spare parts or repairs. Use official service manuals or parts catalogs for your product.

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