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Honda GX25 Engine

The Honda GX25 is a four-cycle OHC single cylinder horizontal shaft air-cooled small gasoline engine from GX-series. A 1.5 cubic inches (25 cc) engine has a cylinder bore of 1.38" (35 mm), piston stroke of 1.02" (26 mm), and rated power of 1.1 Hp (0.8 kW) at 7000 rpm. The compression ratio rating is 8.0:1. The Honda GX25 engine is equipped with a diaphragm type carburetor, oil mist lubrication system, transistorized magneto ignition system, and starting system with recoil starter. The operating weight is 6.2 lbs (2.8 kg).

General Technical Data

Engine Specifications

Engine Model ................. Honda GX25
Engine Type ................. 4-cycle 1-cylinder OHC air-cooled gasoline
Number of Cylinders ................. Single
Displacement, cu.in (cc) ................. 1.5 (25.0)
Cylinder Bore, in (mm) ................. 1.38 (35.0)
Piston Stroke, in (mm) ................. 1.02 (26.0)
Rated Engine Power, hp (kW) ................. 1.1 (0.8)
Rated Engine Speed, rpm ................. 7000
Idle Speed, rpm ................. 3100±200
Maximum Torque, Nm (ft-lbs) ................. 1.25 (0.94)
Torque Speed, rpm ................. 5000
Compression Ratio ................. 8.0:1
Lubrication System ................. Oil mist
Cooling System ................. Forced air
Ignition System ................. Transistorized magneto
Starting System ................. Recoil starter
Electrical System Rating, Volts .................. 12

Fuel System

Fuel System Type ................. Carburetor
Carburetor Type ................. Diaphragm
Fuel Consumption, g/HPh ................. 340
Fuel Type ................. Gasoline (86 or higher)
Fuel Tank Capacity, qts (L) ................. 0.58 (0.55)

Dimensions and Weight

Overall Length, in (mm) ................. 7.6 (192)
Overall Width, in (mm) ................. 8.7 (220)
Overall Height, in (mm) ................. 9.1 (230)
Net Weight, lbs (kg) ................. 6.2 (2.8)

Maintenance Data

Engine Oil

Engine Oil Type (Summer) ................. SAE 10W-30
Engine Oil Type (Winter) ................. SAE 10W
API Service Classifications ................. SG or SF
Oil Capacity, qts (L) ................. 0.09 (0.08)

Ignition System

Spark Plug Type ................. NGK CMR8H
Spark Plug Gap, in (mm) ................. 0.0236-0.0276 (0.60-0.70)
Ignition Coil Air Gap, in (mm) ................. 0.014-0.020 (0.35-0.50)
Ignition Timing, deg ................. 30 BTDC

Service Specifications

Cylinder Block

Cylinder Compression, psi (kPa) ................. 131-159 (900-1100)
Cylinder Bore Inside Diameter (Std), in (mm) ................. 1.38 (35.00)
Cylinder Bore Inside Diameter (Wear Limit), in (mm) ................. 1.303 (35.05)
Connecting Rod Small End I.D (Std), in (mm) .................. 0.3141-0.3145 (7.978-7.989)
Piston Skirt Outside Diameter (Std), in (mm) ................. 1.377-1.378 (34.970-34.990)
Top Ring Width, in (mm) .................. 0.038-0.039 (0.970-0.990)
Middle Ring Width, in (mm) .................. 0.038-0.039 (0.970-0.990)
Top Ring Side Clearance, in (mm) .................. 0.0006-0.0022 (0.015-0.056)
Middle Ring Side Clearance, in (mm) .................. 0.0006-0.0022 (0.015-0.056)
Top Ring End Gap, in (mm) .................. 0.004-0.01 (0.10-0.25)
Middle Ring End Gap, in (mm) .................. 0.004-0.01 (0.10-0.25)

Cylinder Head and Valves

Cylinder Head (Max Out of Flatness), in (mm) ................. 0.004 (0.10)
Intake Valve Clearance, in (mm) ................. 0.002-0.004 (0.06-0.10)
Exhaust Valve Clearance, in (mm) ................. 0.003-0.005 (0.09-0.13)
Intake Valve Stem Outside Diameter, in (mm) ................. 0.1366-0.1372 (3.470-3.485)
Exhaust Valve Stem Outside Diameter, in (mm) ................. 0.1352-0.1358 (3.435-3.450)
Intake Valve Guide Inside Diameter, in (mm) ................. 0.1378-0.1385 (3.50-3.52)
Exhaust Valve Guide Inside Diameter, in (mm) ................. 0.1378-0.1385 (3.50-3.52)
Intake Valve Spring Free Length, in (mm) ................. 0.813 (20.66)
Exhaust Valve Spring Free Length, in (mm) ................. 0.813 (20.66)
Intake Cam Height, in (mm) ................. 0.87 (22.1)
Exhaust Cam Height, in (mm) ................. 0.87 (22.1)

Tightening Torque Specs

Lower Crankcase, Nm (ft-lb) ................. 6.4 (4.7)
Fan Cover, Nm (ft-lb) ................. 6.4 (4.7)
Reed Valve Plate, Nm (ft-lb) ................. 3.9 (2.9)
Ignition Coil, Nm (ft-lb) ................. 3.9 (2.9)
Flywheel Nut, Nm (ft-lb) ................. 14.7 (10.8)
Valve Adjusting Lock Nut, Nm (ft-lb) ................. 4.9 (3.6)
Recoil Starter Pulley, Nm (ft-lbs) ................. 6.4 (4.7)
Spark Plug, Nm (ft-lb) ................. 11.8 (8.7)

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