The Honda GX640 is a 0.64 l (635 cc, 38.7 cu-in) two cylinder water-cooled 4-stroke internal combustion small gasoline motor, manufactured by Honda Motor Company for general-purpose applications.
The Honda GX640 engine has an OHV (overhead valve) design with two valves per cylinder; horizontal type carburetor with a butterfly valve; C.D.I.(Capacitor Discharge Ignition) ignition system, a mechanical centrifugal governor and electric starter.
The Honda GX640 develops 20.0 PS (19.7 HP, 14.7 kW) at 3,600 rpm and 46.1 Nm (4.7 kg·m, 34.0 ft·lb) of torque at 2,500 rpm. It is a liquid-cooling motor with a 76.0 mm (3.0 in) cylinder bore and a 70.0 mm (2.8 in) stroke, and 8.0:1 compression ratio.
General information
| Engine Specifications | |
| Model | GX640 |
| Type | 4-stroke, OHV, 2-cylinder |
| Displacement | 635 cm 3 (38.7 cu-in) |
| Max. horsepower | 20.0 PS (19.7 HP, 14.7 kW) at 3,600 rpm |
| Max. torque (crank PTO) | 46.1 Nm (4.7 kg·m, 34.0 ft·lb) at 2,500 rpm |
| Carburetor | Horizontal type, butterfly valve |
| Cooling system | Liquid-cooling |
| Ignition system | Transistorized ignition |
| Lubricating system | Forced oil by trochoid pump |
| Starting system | Electric starter |
| Stoping system | Cut off the power supply to the CDI (Open) |
| Fuel used | Unleaded gasoline (octane number 86 or higher) |
Dimensions and Weight
| GX640 | |
| Length, mm (in) | 449 (17.7) |
| Width, mm (in) | 420 (16.5) |
| Height, mm (in) | 551 (21.7) |
| Dry weight, kg (lb) | 50.0 (110.0) |
| Operating weight, kg (lb) | 52 (115.0) |
Cylinder Block and Head
The GX640 featured a two main bearing crankshaft and OHV (overhead camshaft) cylinder head with 2 valves per cylinder.
| Cylinder block | ||
| Compression ratio: | 8.0:1 | |
| Bore x Stroke | 76 X 70 mm (3.0 x 2.8 in) | |
| Sleeve internal diameter | 76.000-76.015 mm (2.9921-2.9927 in) | |
| Piston skirt outer diameter | 75.97-75.99 mm (2.991-2.992 in) | |
| Number of piston rings (compression / oil): | 2 / 1 | |
| Ring side clearance | Top/second | 0.030-0.060 mm (0.001-0.002 in) |
| Ring end gap | Top/second | 0.25-0.40 mm (0.010-0.016 in) |
| Ring width | Top/second | 1.175-1.190 mm (0.0463-0.469 in)) |
| Crankshaft journal diameter | 39.976-40.000 mm (1.5739-1.5748 in) | |
| Crankpin diameter | 37.976-38.000 mm (1.4951-1.4961 in) | |
| Cylinder Head | ||
| Warpage (limit) | 0.10 mm (0.004 in) | |
| Valve Arrangement: | OHV | |
| Valves: | 4 | |
| Stem outer diameter (standard): | INTAKE | 4.980-4.990 mm (0.1961-0.1965 in) |
| EXHAUST | 4.960-4.970 mm (0.1953-0.1957 in) | |
| Rocker arm sahft outer diameter: | 11.976-11.994 mm (0.4715-0.4722 in) | |
| Rocker arm internal diameter: | 12.000-12.018 mm (0.4724-0.4731 in) | |
| Cam height: | INTAKE | 30.67-30.95 mm (1.207-1.219 in) |
| EXHAUST | 30.71-30.99 mm (1.209-1.220 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 |
Maintenance data
| Engine | |
| Idle speed | 1,400 ± 150 rpm |
| Cylinder compression | 4.0-7.0 kg/cm2 (57-100 psi) at 350 rpm |
| Valve clearance | |
| Intake valve | 0.10-0.14 mm (0.004-0.006 in) |
| Exhaust valve | 0.18-0.22 mm (0.007-0.009 in) |
| Carburetor | |
| Main jet | #98 |
| Float height | 14.0 mm (0.55 in) |
| Pilot screw opening | 1-1/2 turns out |
| Oil system | |
| Oil type | Honda 4-stroke or an equivalent (SE or SF) |
| Recommended oil | SAE 10W-40 |
| Oil capacity (without filter changed) | 2.0 L (2.11 US qt, 1.76 Imp qt) |
| Oil capacity (with filter changed) | 2.3 L (2.43 US qt, 2.02 Imp qt) |
| Oil pressure | 3.0-4.0 kg/cm2 (43-57 psi) at 350 rpm |
| 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 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) |
| Ignition system | |
| Ignition timing | 22 ° B.T.D.C |
| Spark plug |
NGK: BPR5ES-11 NIPPONDENSO: W16EPR-11 |
| Spark plug gap | 1.0-1.1 mm (0.039-0.043 in) |
| Primary side ignition coil resistance | 0.37-0.50 Ω |
| Secondary side ignition coil resistance | 8.0-9.3 kΩ |
| Starter motor | |
| Brush length | 14 mm (0.55 in), limit: 9.0 mm (0.35 in) |
| Mica depth (limit) | 0.2 mm (0.008 in) |
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