Honda GX25

Honda GX25The 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:

  • 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: 0.06-0.10 mm (0.0024-0.0039 in)
Exhaust valve clearance: 0.09-0.13 mm (0.0035-0.0051 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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