The 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 model | GX640 |
| Engine type | 4-stroke, OHV, two-cylinders |
| Displacement | 635 cm3, (38.7 cu·in) |
| Max. horsepower | 20.0 PS (14.7 kW; 19.7 HP) at 3,600 rpm |
| Max. torque | 46.1 N·m (4.7 kg·m, 34 ft·lb) at 2,500 rpm |
| Cooling system | Liquid-cooled |
| Air cleaner | Dual-element type |
| Lubricating system | Forced oil by trochoid pump |
| Ignition system | C.D.I.(Capacitor Discharge Ignition) type magneto ignition |
| Fuel system | Horizontal type carburetor with butterfly valve |
| Fuel used | Unleaded gasoline (octane number 86 or higher) |
| Starting system | Electric starter |
| Stoping system | Cut off the power supply to the CDI |
| Governor | Centrifugal governor |
| PTO shaft rotation | Counterclockwise (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 weight | 57 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 bolt | 55 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 nut | 29 Nm; 2.9 kg·m; 21 ft·lb |
| Flywheel bolt | 71 Nm; 7.1 kg·m; 51 ft·lb |
| Timing belt driven pulley bolt | 30 Nm; 3.0 kg·m; 22 ft·lb |
| Oil drain bolt | 34 Nm; 3.4 kg·m; 25 ft·lb |
| Oil filter | 8 Nm; 0.8 kg·m; 6 ft·lb |
| Governot slider shaft | 43 Nm; 4.3 kg·m; 31 ft·lb |
| Tension arm bolt | 40 Nm; 4.0 kg·m; 29 ft·lb |
| PTO shaft bolt | 32 Nm; 3.2 kg·m; 23 ft·lb |
| Oil pressure switch | 8.5 Nm; 0.85 kg·m; 6.1 ft·lb |
| Thermo switch | 12 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
| Engine | |
| 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) |
| Thermostat | |
| Start opening | 82° C (180° F) |
| Fully open | 95° C (203° F) |
| Lift height | 8.0 mm (0.31 in) |
| Thermoswitch | |
| Temperature range for continuity | 110° ± 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 pressure | 294-392 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 clearance | 0.04-0.09 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) |
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