Honda GX640

Honda GX640The Honda GX640 is a 635 cc, (38.7 cu·in) natural aspirated two-cylinders four-stroke liquid-cooled internal combustion small gasoline engine with a horizontal shaft, manufactured by Honda Motor Company for general-purpose applications.

The cylinder bore is 76.0 mm (2.99 in) and the piston stroke is 70.0 mm (2.76 in). The compression ratio rating is 8.0:1.

The Honda GX640 engine produced 20.0 PS (14.7 kW; 19.7 HP) at 3,600 rpm of maximum output power and 46.1 N·m (4.7 kg·m, 34 ft·lb) at 2,500 rpm of peak torque.

The Honda GX640 features:

  • Overhead valve (OHV) design
  • C.D.I.(Capacitor Discharge Ignition)
  • Horizontal type carburetor with butterfly valve
  • Centrifugal governor
  • Forced oil by trochoid pump lubrication system
  • Liquid-cooled cooling system
  • Electric starter

General information

Engine Specifications
Engine modelGX640
Engine type4-stroke, OHV, two-cylinders
Displacement635 cm3, (38.7 cu·in)
Max. horsepower20.0 PS (14.7 kW; 19.7 HP) at 3,600 rpm
Max. torque46.1 N·m (4.7 kg·m, 34 ft·lb) at 2,500 rpm
Cooling systemLiquid-cooled
Air cleanerDual-element type
Lubricating systemForced oil by trochoid pump
Ignition systemC.D.I.(Capacitor Discharge Ignition) type magneto ignition
Fuel systemHorizontal type carburetor with butterfly valve
Fuel usedUnleaded gasoline (octane number 86 or higher)
Starting systemElectric starter
Stoping systemCut off the power supply to the CDI
GovernorCentrifugal governor
PTO shaft rotationCounterclockwise (from PTO shaft side)
Dimensions (L x W X H)449 x 420 x 551 mm (17.7 x 16.5 x 21.7 in)
Dry weight57 kg (125.7 lbs)

Cylinder Block and Head

Cylinder block
Compression ratio:8.0:1
Cylinder bore:76.0 mm (2.99 in)
Piston stroke:70.0 mm (2.76 in)
Cylinder internal diameter (standard):76.000-76.015 mm (2.9921-2.9927 in)
Piston skirt outer diameter (standard):75.970-75.990 mm (2.9909-2.9917 in)
Piston pin diameter:18.994-19.000 mm (0.7478-0.748 in)
Side clearance (Top):0.030-0.060 mm (0.0012-0.0024 in)
Side clearance (Second):0.030-0.060 mm (0.0012-0.0024 in)
Ring end gap (Top):0.250-0.400 mm (0.0098-0.0157 in)
Ring end gap (Second):0.250-0.400 mm (0.0098-0.0157 in)
Ring width (Top):1.175-1.190 mm (0.0463-0.0469 in)
Ring width (Second):1.175-1.190 mm (0.0463-0.0469 in)
Crankshaft main journal diameter:39.976-40.000 mm (1.5739-1.5748 in)
Crankpin journal diameter:37.976-38.000 mm (1.4951-1.4961 in)
Crankshaft center distance:35 mm (1.38 in)
Cylinder head
Valve arrangement:OHV
Valves:2,(4 valves in total)
Valve steam diameter (INTAKE):4.980-4.990 mm (0.1961-0.1965 in)
Valve steam diameter (EXHAUST):4.960-4.970 mm (0.1953-0.1957 in)
Valve spring free length (INTAKE):38.64 mm (1.5213 in)
Valve spring free length (EXHAUST):38.64 mm (1.5213 in)
Camshaft lobe height (INTAKE):30.67-30.95 mm (1.2075-1.2185 in)
Camshaft lobe height (EXHAUST):30.71-30.99 mm (1.2091-1.2201 in)
Camshaft journal diameter :27.959-27.980 mm (1.1007-1.1016 in)

Tightening torque specs

Tightening torque specs
Cylinder head bolt55 Nm; 5.5 kg·m; 40 ft·lb
Crankcase – cylinder block (M8)22 Nm; 2.2 kg·m; 16 ft·lb
Crankcase – cylinder block (M10)44 Nm; 4.4 kg·m; 32 ft·lb
Connecting rod cup nut29 Nm; 2.9 kg·m; 21 ft·lb
Flywheel bolt71 Nm; 7.1 kg·m; 51 ft·lb
Timing belt driven pulley bolt30 Nm; 3.0 kg·m; 22 ft·lb
Oil drain bolt34 Nm; 3.4 kg·m; 25 ft·lb
Oil filter8 Nm; 0.8 kg·m; 6 ft·lb
Governot slider shaft43 Nm; 4.3 kg·m; 31 ft·lb
Tension arm bolt40 Nm; 4.0 kg·m; 29 ft·lb
PTO shaft bolt32 Nm; 3.2 kg·m; 23 ft·lb
Oil pressure switch8.5 Nm; 0.85 kg·m; 6.1 ft·lb
Thermo switch12 Nm; 1.2 kg·m; 9 ft·lb
Standard torque values
Bolt, nut (5 mm)5.5 Nm; 0.55 kg·m; 4.0 ft·lb
Bolt, nut (6 mm)10 Nm; 1.0 kg·m; 7 ft·lb
Bolt, nut (8 mm)24 Nm; 2.4 kg·m; 17 ft·lb
Bolt, nut (10 mm)37.5 Nm; 3.75 kg·m; 27 ft·lb
Bolt, nut (12 mm)55 Nm; 5.5 kg·m; 40 ft·lb

Service Data

Maximum speed: 3,000 rpm type: 3,050-3,150 rpm
3,600 rpm type: 3,550-3,750 rpm
Idle speed:1,300-1,400 rpm
Cylinder compression:392-685 kPa, (4.0-7.0 kgf/cm2), 56.9 to 99.6 psi / 350
Valve clearance
Intake valve clearance:0.10-0.14 mm (0.0039-0.0055 in)
Exhaust valve clearance:0.18-0.22 mm (0.0071-0.0087 in)
Cooling system
Coolant capacity (radiator and engine)3.5 L (3.70 US qt, 3.08 Imp qt)
Coolant capacity (reserve tank)0.4 L (0.42 US qt, 0.35 Imp qt)
Coolant capacity (total system)3.9 L (4.12 US qt, 3.43 Imp qt)
Start opening82° C (180° F)
Fully open95° C (203° F)
Lift height8.0 mm (0.31 in)
Temperature range for continuity110° ± 2° C (230° ± 36° C)
Oil system
Lubrication system:Forced oil by trochoid pump
Oil type:Honda 4-stroke or an equivalent (SE or SF)
Recommended oil:SAE 10W-30
Oil capacity: Without oil firter replacement: 2.0 L (2.11 US. qt, 1.76 Imp. qt.)
With oil firter replacement: 2.3 L (2.43 US. qt, 2.02 Imp. qt.)
Oil pressure294-392 kPa (3.0-4.0 kg/cm2, 43-57 psi) at 350 rpm
Oil pump
Tip clearance (standard)0.14 mm (0.006 in)
Outer rotor clearance (standard)0.15-0.21 mm (0.006-0.008 in)
Pump end clearance0.04-0.09 mm (0.0016-0.0035 in)
Fuel system
Type:Horizontal type carburetor with butterfly valve
Main jet:#98
Float height:14.0 mm (0.5512 in)
Pilot screw opening:1-1/2 turns out
Ignition system
Type:C.D.I.(Capacitor Discharge Ignition) type magneto ignition
Ignition timing:22° B.T.D.C.
Spark plug:NGK: BPR5ES-11 NIPPONDENSO: W16EPR-11
Spark plug gap:1.0-1.1 mm (0.0394-0.0433 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 official documentation and verify the published data, but errors may occur. In addition, specifications of machinery and equipment may vary depending on markets and modifications. We do not recommend the use of this data for spare parts or repairs. Use official service manuals or parts catalogs for your product.

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