Honda GX640

Honda GX640The 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
Type4-stroke, OHV, 2-cylinder
Displacement635 cm 3 (38.7 cu-in)
Max. horsepower20.0 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
CarburetorHorizontal type, butterfly valve
Cooling systemLiquid-cooling
Ignition systemTransistorized ignition
Lubricating systemForced oil by trochoid pump
Starting systemElectric starter
Stoping systemCut off the power supply to the CDI (Open)
Fuel usedUnleaded gasoline (octane number 86 or higher)

Dimensions and Weight

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 Stroke76 X 70 mm (3.0 x 2.8 in)
Sleeve internal diameter76.000-76.015 mm (2.9921-2.9927 in)
Piston skirt outer diameter75.97-75.99 mm (2.991-2.992 in)
Number of piston rings (compression / oil):2 / 1
Ring side clearanceTop/second0.030-0.060 mm (0.001-0.002 in)
Ring end gapTop/second0.25-0.40 mm (0.010-0.016 in)
Ring widthTop/second1.175-1.190 mm (0.0463-0.469 in))
Crankshaft journal diameter39.976-40.000 mm (1.5739-1.5748 in)
Crankpin diameter37.976-38.000 mm (1.4951-1.4961 in)
Cylinder Head
Warpage (limit)0.10 mm (0.004 in)
Valve Arrangement:OHV
Stem outer diameter (standard):INTAKE4.980-4.990 mm (0.1961-0.1965 in)
EXHAUST4.960-4.970 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:INTAKE30.67-30.95 mm (1.207-1.219 in)
EXHAUST30.71-30.99 mm (1.209-1.220 in)
Camshaft journal diameter27.959-27.980 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

Maintenance data

Idle speed1,400 ± 150 rpm
Cylinder compression4.0-7.0 kg/cm2 (57-100 psi) at 350 rpm
Valve clearance
Intake valve0.10-0.14 mm (0.004-0.006 in)
Exhaust valve0.18-0.22 mm (0.007-0.009 in)
Main jet#98
Float height14.0 mm (0.55 in)
Pilot screw opening1-1/2 turns out
Oil system
Oil typeHonda 4-stroke or an equivalent (SE or SF)
Recommended oilSAE 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 pressure3.0-4.0 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)
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 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)
Ignition system
Ignition timing22 ° B.T.D.C
Spark plug NGK: BPR5ES-11
Spark plug gap1.0-1.1 mm (0.039-0.043 in)
Primary side ignition coil resistance0.37-0.50 Ω
Secondary side ignition coil resistance8.0-9.3 kΩ
Starter motor
Brush length14 mm (0.55 in), limit: 9.0 mm (0.35 in)
Mica depth (limit)0.2 mm (0.008 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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