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
Model GX31
Type 4-stroke, OHV, single cylinder
Displacement 31 cm 3 (1.9 cu-in)
Max. horsepower 1.52 PS (1.5 HP, 1.12 kW) at 7000 rpm
Max. torque 1.64 Nm (0.17 kg·m, 1.21 ft·lb) at 6500 rpm
Carburetor Diaphragm type
Cooling system Forced-air
Ignition system Transistorized magneto ignition
Lubricating system Pumping spray
Starting system Recoil starter
Stoping system Ignition primary circuit ground
Fuel used Unleaded gasoline (octane number 86 or higher)
Fuel tank capacity 0.65 L (0.17 US gal, 0.14 lmp gal)
Fuel consumption 340 g/kWh (250 g/HPh, 0.55 lb/HPh)
PTO shaft rotation Counterclockwise (from PTO shaft side)

Dimensions and Weight

SC SE
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 Stroke 39 X 26 mm (1.5 x 1.0 in)
Sleeve internal diameter 39.00 mm (1.535 in)
Piston skirt outer diameter 38.99 mm (1.535 in)
Ring end gap Top/second 0.10-0.25 mm (0.004-0.010 in)
Ring width Top/second 0.98 mm (0.039 in)
Top/second 0.015-0.050 mm (0.006-0.0020 in)
Cylinder Head
Warpage (max) 0.2 mm (0.01 in)
Valve Arrangement: OHV
Valves: 2
Stem outer diameter (standard): INTAKE 3.985 mm (0.1569 in)
EXHAUST 3.950 mm (0.1555 in)
Valve guide outer diameter 4.000 mm (0.1575 in)
Cam height: INTAKE 22.79 mm (0.897 in)
EXHAUST 22.79 mm (0.897 in)
Camshaft outer diameter: 5.020 mm (0.1976 in)

Maintenance data

Engine
Idle speed 3100 ± 200 rpm
Cylinder compression 2.9-5.9 kg/cm2 (0.3-0.6 MPa, 41-84 psi) at 1200 rpm
Valve clearance
Intake valve 0.12 ± 0.02 (0.005 ± 0.001)
Exhaust valve 0.15 ± 0.02 (0.006 ± 0.001)
Carburetor
Main jet #40
Oil system
Oil type Honda 4-stroke or an equivalent (SG, SF)
Recommended oil SAE 10W-30
Oil capacity 0.1 L (0.1 US gal, 0.1 lmp qt)
Ignition system
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 (0.024-0.028 in)
Primary ignition coil resistance 0.7-0.9 kΩ
Secondary ignition coil resistance 3.6-5.5 kΩ
Ignition coil air gap 0.35-0.5 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
ATTENTION! Dear visitors, this site is not a trading platform, an official dealer or a spare parts supplier, so we do not have any price lists or spare parts catalogs. We are an information portal and provide the technical specifications of gasoline and diesel engines.

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.

6 Comments

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

  2. I LIVE IN SLIDELL,. LA. I would like to buy a Carbreator form my GX-31 Tiller.
    Do you have a local address?

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