Honda GX31

The 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