Honda GX31

Honda GX31 engineThe Honda GX31 is a 31 cc single-cylinder air-cooled four-stroke internal combustion ultra-small gasoline motor manufactured by Honda Motor Company since 1997. In 2002 the GX31 engine were replaced with the Honda GX25.

This engine is a suitable power source for handheld applications like brush-cutters and non-handheld applications like concrete screeds, tillers, and pumps.

The Honda GX35 engine has an OHV design with valve rocker arms and camshaft driven by a maintenance-free, low-noise timing belt, makes the engine more compact and light.

It has a recoil starter, diaphragm type carburetor, and transistorized magneto ignition. Bore is 39.0 mm (1.5 in) and stroke is 26.0 mm (1.0 in). The GX31 produced 1.52 PS (1.5 HP, 1.12 kW) at 7000 rpm and 1.64 Nm (0.17 kg·m, 1.21 ft·lb) at 6500 rpm.

General information

Engine Specifications
Type4-stroke, OHV, single cylinder
Displacement31 cm 3 (1.9 cu-in)
Max. horsepower1.52 PS (1.5 HP, 1.12 kW) at 7000 rpm
Max. torque1.64 Nm (0.17 kg·m, 1.21 ft·lb) at 6500 rpm
CarburetorDiaphragm type
Cooling systemForced-air
Ignition systemTransistorized magneto ignition
Lubricating systemPumping spray
Starting systemRecoil starter
Stoping systemIgnition primary circuit ground
Fuel usedUnleaded gasoline (octane number 86 or higher)
Fuel tank capacity0.65 L (0.17 US gal, 0.14 lmp gal)
Fuel consumption340 g/kWh (250 g/HPh, 0.55 lb/HPh)
PTO shaft rotationCounterclockwise (from PTO shaft side)

Dimensions and Weight

Length, mm (in)210 (8.3)
Width, mm (in)251 (9.9)
Height, mm (in)250 (9.84)
Dry weight, kg (lb)3.4 (7.5)

Cylinder Block and Head

Cylinder block
Compression ratio:8.0:1
Bore x Stroke39 X 26 mm (1.5 x 1.0 in)
Sleeve internal diameter39.00 mm (1.535 in)
Piston skirt outer diameter38.99 mm (1.535 in)
Ring end gapTop/second0.10-0.25 mm (0.004-0.010 in)
Ring widthTop/second0.98 mm (0.039 in)
Top/second0.015-0.050 mm (0.006-0.0020 in)
Cylinder Head
Warpage (max)0.2 mm (0.01 in)
Valve Arrangement:OHV
Stem outer diameter (standard):INTAKE3.985 mm (0.1569 in)
EXHAUST3.950 mm (0.1555 in)
Valve guide outer diameter4.000 mm (0.1575 in)
Cam height:INTAKE22.79 mm (0.897 in)
EXHAUST22.79 mm (0.897 in)
Camshaft outer diameter:5.020 mm (0.1976 in)

Maintenance data

Idle speed3100 ± 200 rpm
Cylinder compression2.9-5.9 kg/cm2 (0.3-0.6 MPa, 41-84 psi) at 1200 rpm
Valve clearance
Intake valve0.12 ± 0.02 (0.005 ± 0.001)
Exhaust valve0.15 ± 0.02 (0.006 ± 0.001)
Main jet#40
Oil system
Oil typeHonda 4-stroke or an equivalent (SG, SF)
Recommended oilSAE 10W-30
Oil capacity0.1 L (0.1 US gal, 0.1 lmp qt)
Ignition system
Ignition timing27 ° B.T.D.C (Fixed)
Spark plugNGK: C5HSB, CR5HSB
Spark plug gap0.6-0.7 mm (0.024-0.028 in)
Primary ignition coil resistance0.7-0.9 kΩ
Secondary ignition coil resistance3.6-5.5 kΩ
Ignition coil air gap0.35-0.5 mm (0.014-0.020 in)

Tightening torque specs

Tightening torque specs
Flywheel nut (M8)15 Nm; 1.5 kg·m; 11 ft·lb
Crankcase bolt (M5 5×55 mm)7 Nm; 0.75 kg·m; 5.1 ft·lb
Crankcase bolt (M5 5×18 mm)5 Nm; 0.55 kg·m; 3.6 ft·lb
Camshaft cover bolt (M4)3 Nm; 0.35 kg·m; 2.2 ft·lb
Recoil starter pulley (M8)8 Nm; 0.8 kg·m; 5.8 ft·lb
Clutch bolt (M8)15 Nm; 1.55 kg·m; 11 ft·lb
Cylinder head cover bolt (M5)4 Nm; 0.4 kg·m; 2.9 ft·lb
Standard torque values
Bolt, nut (5 mm)5.5 Nm; 0.55 kg·m; 4.0 ft·lb
Screw (3 mm)1 Nm; 0.1 kg·m; 0.7 ft·lb
Screw (6 mm)9 Nm; 0.9 kg·m; 6.5 ft·lb
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We try to use verified sources and official documentation, however, differences between sources or errors in entering information may occur. We do not provide advice on technical issues related to the engines operation or repair. We do not recommend using provided information for engines repairing engines or spare parts ordering, use only official service manuals and spare-parts catalogs.


  1. Looking for a small engine GX31, Could you help me with that. That is going on a Concrete creed.

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