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
Type4-stroke, OHC, single cylinder
Displacement25 cm 3 (1.5 cu-in)
Max. horsepower1.1 PS (0.81 kW) at 7,000 rpm
Max. torque1.25 Nm (0.13 kg·m, 0.94 ft·lb) at 5,000 rpm
CarburetorDiaphragm type
Cooling systemForced-air
Ignition systemTransistorized magneto ignition
Lubricating systemOil mist
Starting systemRecoil starter
Stoping systemIgnition primary circuit ground
Fuel usedUnleaded gasoline (octane number 86 or higher)
Fuel tank capacity0.55 L (0.14 US gal, 0.12 lmp gal)
Fuel consumption340 g/kWh (250 g/HPh, 0.55 lb/HPh)
PTO shaft rotationCounterclockwise (from PTO shaft side)

Cylinder Block and Head

Cylinder block
Compression ratio:8.0:1
Bore x Stroke35 X 26 mm (1.4 x 1.0 in)
Sleeve internal diameter35.00 mm (1.378 in)
Piston skirt outer diameter34.970-34.990 mm (1.377-1.378 in)
Connecting rod small end ID7.978-7.989 mm (0.3141-0.3145 in)
Ring end gapTop/second0.10-0.25 mm (0.004-0.010 in)
Ring widthTop/second0.970-0.990 mm (0.038-0.039 in)
Side clearanceTop/second0.015-0.056 mm (0.0006-0.0022 in)
Cylinder Head
Warpage (max)0.2 mm (0.01 in)
Valve Arrangement:OHV
Stem outer diameter (standard):INTAKE3.470-3.485 mm (0.1366-0.1372 in)
EXHAUST3.435-3.450 mm (0.1352-0.1358 in)
Valve guide internal diameter3.50-3.52 mm (0.1378-0.1385 in)
Valve spring free length20.66 mm (0.8134 in)
Cam height:INTAKE22.097 mm (0.8700 in)
EXHAUST22.097 mm (0.8700 in)

Tightening torque specs

Tightening torque specs
Lower crankcase6.4 Nm; 0.7 kg·m; 5.1 ft·lb
Fan cover6.4 Nm; 0.7 kg·m; 5.1 ft·lb
Reed valve plate3.9 Nm; 0.4 kg·m; 2.9 ft·lb
Ignition coil3.9 Nm; 0.4 kg·m; 2.9 ft·lb
Recoil starter pulley6.4 Nm; 0.7 kg·m; 4.6 ft·lb
Flywheel14.7 Nm; 1.5 kg·m; 10.8 ft·lb
Valve adjusting lock nut4.9 Nm; 0.5 kg·m; 3.6 ft·lb
Spark plug11.8 Nm; 1.2 kg·m; 8.7 ft·lb

Maintenance data

Idle speed3100 ± 200 rpm
Cylinder compression9.2-11.2 kg/cm2 (131-159 psi) at 2,000 rpm
Valve clearance
Intake valve0.08 ± 0.02
Exhaust valve0.11 ± 0.02
Main jet #35 (SAU and SAUT types)
#34 (For others)
Oil system
Oil typeHonda 4-stroke or an equivalent (SG, SF)
Recommended oilSAE 10W-30
Oil capacity0.08 L (2.7 US oz, 2.8 lmp oz)
Ignition system
Ignition timing30 ° B.T.D.C (Fixed)
Spark plugNGK: CMR8H
Spark plug gap0.6-0.7 mm (0.024-0.028 in)
Primary ignition coil resistance0.7-0.9 Ω
Secondary ignition coil resistance6.1-9.3 kΩ
Ignition coil air gap0.35-0.5 mm (0.014-0.020 in)
Author: Sergei Wers
About: Co-founder and CEO of a multi-brand auto repair shop (engine repair, chassis repair, body repair and painting) for over 15 years. I have been developing internet sites with technical specifications for more than seven years.

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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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