Honda GX25

Honda GX25The Honda GX25 is a 25 cc (1.5 cu-in) single-cylinder air-cooled four-stroke internal combustion ultra-small 360° inclinable gasoline engine manufactured by Honda Motor Company since 2002.

The Honda GX25 engine is a suitable power source for different applications like commercial lawn and garden equipment, tillers and cultivators, construction and industrial equipment, agricultural equipment, and water pumps. This engine has an OHC 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 with the accelerator pump, transistorized magneto ignition.

The cylinder bore is 35.0 mm (1.4 in) and the piston stroke is 26.0 mm (1.0 in). The Honda GX25 mini-motor produced 1.1 PS (0.81 kW) at 7000 rpm and 1.25 Nm (0.13 kg·m, 0.94 ft·lb) at 5000 rpm.

General information

Engine Specifications
Model GX25
Type 4-stroke, OHC, single cylinder
Displacement 25 cm 3 (1.5 cu-in)
Max. horsepower 1.1 PS (0.81 kW) at 7,000 rpm
Max. torque 1.25 Nm (0.13 kg·m, 0.94 ft·lb) at 5,000 rpm
Carburetor Diaphragm type
Cooling system Forced-air
Ignition system Transistorized magneto ignition
Lubricating system Oil mist
Starting system Recoil starter
Stoping system Ignition primary circuit ground
Fuel used Unleaded gasoline (octane number 86 or higher)
Fuel tank capacity 0.55 L (0.14 US gal, 0.12 lmp gal)
Fuel consumption 340 g/kWh (250 g/HPh, 0.55 lb/HPh)
PTO shaft rotation Counterclockwise (from PTO shaft side)

Cylinder Block and Head

Cylinder block
Compression ratio: 8.0:1
Bore x Stroke 35 X 26 mm (1.4 x 1.0 in)
Sleeve internal diameter 35.00 mm (1.378 in)
Piston skirt outer diameter 34.970-34.990 mm (1.377-1.378 in)
Connecting rod small end ID 7.978-7.989 mm (0.3141-0.3145 in)
Ring end gap Top/second 0.10-0.25 mm (0.004-0.010 in)
Ring width Top/second 0.970-0.990 mm (0.038-0.039 in)
Side clearance Top/second 0.015-0.056 mm (0.0006-0.0022 in)
Cylinder Head
Warpage (max) 0.2 mm (0.01 in)
Valve Arrangement: OHV
Valves: 2
Stem outer diameter (standard): INTAKE 3.470-3.485 mm (0.1366-0.1372 in)
EXHAUST 3.435-3.450 mm (0.1352-0.1358 in)
Valve guide internal diameter 3.50-3.52 mm (0.1378-0.1385 in)
Valve spring free length 20.66 mm (0.8134 in)
Cam height: INTAKE 22.097 mm (0.8700 in)
EXHAUST 22.097 mm (0.8700 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

Maintenance data

Idle speed 3100 ± 200 rpm
Cylinder compression 9.2-11.2 kg/cm2 (131-159 psi) at 2,000 rpm
Valve clearance
Intake valve 0.08 ± 0.02
Exhaust valve 0.11 ± 0.02
Main jet #35 (SAU and SAUT types)
#34 (For others)
Oil system
Oil type Honda 4-stroke or an equivalent (SG, SF)
Recommended oil SAE 10W-30
Oil capacity 0.08 L (2.7 US oz, 2.8 lmp oz)
Ignition system
Ignition timing 30 ° B.T.D.C (Fixed)
Spark plug NGK: CMR8H
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 6.1-9.3 kΩ
Ignition coil air gap 0.35-0.5 mm (0.014-0.020 in)
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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.


    You are requested to provide same as much as possible.

  2. I have a tiller with the GX25. I cannot seem to get it started. I can’t figure if the choke should be in the up or down position. In the middle it will run for about 2 seconds.
    by the time I get to the switch, it stops running. Please help me!

    • I had the same problem, motor is on a mantis tiller. Bought the service kit for it. Kit comprises of air filter, fuel filter, spark plug, oil, gaskets .
      Try the fuel filter first

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