The Honda GX31 is a 31 cc (1.9 cu-in) single-cylinder, air-cooled, four-stroke cycle, 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 7,000 rpm and 1.64 Nm (0.17 kg·m; 1.21 lbf·ft) at 6,500 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 7,000 rpm |
| Max. torque | 1.64 Nm (0.17 kg·m; 1.21 lbf·ft) at 6,500 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 liters (0.69 US.qts, 0.57 Imp. qts) |
| 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 | 39.0 X 26.0 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) | |
| Piston ring end gap (Top): | 0.10-0.25 mm (0.004-0.010 in) | |
| Piston ring end gap (Second): | 0.10-0.25 mm (0.004-0.010 in) | |
| Ring width (Top): | 0.98 mm (0.039 in) | |
| Ring width (Second): | 0.98 mm (0.039 in) | |
| Piston ring end gap (Oil): | 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 | |
| Valve stem outer diameter (INTAKE): | 3.985 mm (0.1569 in) | |
| Valve stem outer diameter (EXHAUST): | 3.950 mm (0.1555 in) | |
| Valve guide outer diameter | 4.000 mm (0.1575 in) | |
| Camshaft cam height (INTAKE): | 22.79 mm (0.897 in) | |
| Camshaft cam height (EXHAUST): | 22.79 mm (0.897 in) | |
| Camshaft outer diameter: | 5.020 mm (0.1976 in) | |
Maintenance data
| Engine | |
| 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 | 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 | 0.10 liters (0.11 US.qts, 0.09 Imp. qts) |
| 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 Ω |
| Secondary ignition coil resistance | 3.6-5.5 kΩ |
| Ignition coil air gap | 0.35-0.50 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 |
Author:
Looking for a small engine GX31, Could you help me with that. That is going on a Concrete creed.
I’m looking for the engine G 31 4 stroke for my bicycle how much for the engine ?
$130.00 as is……
I love this engine because it is good
I LIVE IN SLIDELL,. LA. I would like to buy a Carbreator form my GX-31 Tiller.
Do you have a local address?
I would like to buy a carburetor for my Honda GX-31 Tiller.
I have a GX31,I bought It in year 2004.I have Been using it since now and it is very good and perfect. It is the best. From Brasil…
I repair Honda products. The GX-31 is far superior to the GX-25. The GX-31 was certainly more expensive to manufacture. It is crucial to keep the oil level full and changed frequently. The Walbro Carburetor will need the Metering and fuel pump diaphram replaced for not start or poor running conditions. I have 2 Tillers with the engine myself.