The Honda GX25 is a 25 cc, (1.53 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.
The cylinder bore is 35.0 mm (1.38 in) and the piston stroke is 26.0 mm (1.02 in). The compression ratio rating is 8.0:1.
This engine produced 1.1 PS (0.81 kW; 1.1 HP) at 7,000 rpm of horsepower and 1.25 N·m (0.1 kg·m, 0.9 ft·lb) at 5,000 rpm of torque.
The Honda GX25 features:
- Overhead camshaft (OHC) design
- Transistorized magneto ignition system
- Diaphragm type carburetor with the accelerator pump
- Centrifugal governor
- Oil mist lubrication system
- Forced air cooling system
- Recoil starter
General information
| Engine Specifications | |
| Engine model | GX25 |
| Engine type | 4-stroke, OHC, single-cylinder |
| Displacement | 25 cm3, (1.53 cu·in) |
| Max. horsepower | 1.1 PS (0.81 kW; 1.1 HP) at 7,000 rpm |
| Max. torque | 1.25 N·m (0.1 kg·m, 0.9 ft·lb) at 5,000 rpm |
| Cooling system | Forced air |
| Air cleaner | Semi-dry type |
| Lubricating system | Oil mist |
| Ignition system | Transistorized magneto ignition |
| Fuel system | Diaphragm type carburetor with the accelerator pump |
| Fuel consumption | 250 g/HPh |
| Fuel tank capacity | 0.55 L (0.15 US. gal. 0.12 Imp. gal.) |
| Fuel used | Unleaded gasoline (octane number 86 or higher) |
| Starting system | Recoil starter |
| Stoping system | Ignition primary circuit ground |
| Governor | Centrifugal governor |
| PTO shaft rotation | Counterclockwise (from PTO shaft side) |
| Dimensions (L x W X H) | 192 x 221 x 230 mm (7.6 x 8.7 x 9.1 in) |
| Dry weight | 2.90 kg (6.4 lbs) |
Cylinder Block and Head
| Cylinder block | |
| Compression ratio: | 8.0:1 |
| Cylinder bore: | 35.0 mm (1.38 in) |
| Piston stroke: | 26.0 mm (1.02 in) |
| Cylinder internal diameter (standard): | 35.000 mm (1.378 in) |
| Piston skirt outer diameter (standard): | 34.970-34.990 mm (1.3768-1.3776 in) |
| Number of piston rings (compression / oil): | 2/1 |
| Connecting rod small end diameter (standard): | 7.978-7.989 mm (0.3141-0.3145 in) |
| Side clearance (Top): | 0.015-0.056 mm (0.0006-0.0022 in) |
| Side clearance (Second): | 0.015-0.056 mm (0.0006-0.0022 in) |
| 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): | 0.970-0.990 mm (0.0382-0.039 in) |
| Ring width (Second): | 0.970-0.990 mm (0.0382-0.039 in) |
| Crankshaft center distance: | 13.0 mm (0.51 in) |
| Cylinder head | |
| Valve arrangement: | OHC |
| Valves: | 2 |
| Valve steam diameter (INTAKE): | 3.470-3.485 mm (0.1366-0.1372 in) |
| Valve steam diameter (EXHAUST): | 3.435-3.450 mm (0.1352-0.1358 in) |
| Valve spring free length (INTAKE): | 20.660 mm (0.8134 in) |
| Valve spring free length (EXHAUST): | 20.660 mm (0.8134 in) |
| Camshaft lobe height (INTAKE): | 22.097 mm (0.87 in) |
| Camshaft lobe height (EXHAUST): | 22.097 mm (0.87 in) |
Tightening torque specs
| Tightening torque specs | |
| Lower crankcase | 6.4 Nm; 0.7 kg·m; 5.1 ft·lb |
| Fan cover | 6.4 Nm; 0.7 kg·m; 5.1 ft·lb |
| Reed valve plate | 3.9 Nm; 0.4 kg·m; 2.9 ft·lb |
| Ignition coil | 3.9 Nm; 0.4 kg·m; 2.9 ft·lb |
| Recoil starter pulley | 6.4 Nm; 0.7 kg·m; 4.6 ft·lb |
| Flywheel | 14.7 Nm; 1.5 kg·m; 10.8 ft·lb |
| Valve adjusting lock nut | 4.9 Nm; 0.5 kg·m; 3.6 ft·lb |
| Spark plug | 11.8 Nm; 1.2 kg·m; 8.7 ft·lb |
Service Data
| Idle speed: | 3,100 ± 200 rpm |
| Cylinder compression: | 9.2-11.2 kg/cm2 (131-159 psi) at 2,000 rpm |
| Valve clearance | |
| Intake valve clearance: | 0.08 ± 0.02 mm (0.003 ± 0.001 in) |
| Exhaust valve clearance: | 0.10 ± 0.02 mm (0.004 ± 0.001 in) |
| Oil system | |
| Lubrication system: | Oil mist |
| Oil type: | Honda 4-stroke or an equivalent (SG, SF) |
| Recommended oil: | SAE 10W-30 |
| Oil capacity: | 0.08 L (0.08 US. qt, 0.07 Imp. qt.) |
| Fuel system | |
| Type: | Diaphragm type carburetor with the accelerator pump |
| Main jet: | #35 (SAU and SAUT types) #34 (For others) |
| Ignition system | |
| Type: | Transistorized magneto ignition |
| Ignition timing: | 30° B.T.D.C. (Fixed) |
| Spark plug: | NGK CMR8H |
| Spark plug gap: | 0.6-0.7 mm (0.0236-0.0276 in) |
| Spark plug torque | 11.8 Nm; 1.2 kg·m; 8.7 ft·lb |
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