Honda G400

Honda G400 engineThe Honda G400 is a 406 cc (24.8 single-cylinder air-coolled 4-stroke internal combustion small gasoline engine manufactured by Honda Motor Company since 1978 for general-purpose applications.

The Honda G400 engine has a slide valve (L-head) design and camshaft driven by a single row chain driven by the crankshaft. The engine has an electric or recoil starter, horizontal carburetor with butterfly valve and transistorized magneto or contact point breaker (CDI) ignition system.

Cylinder bore and piston stroke are 86.0 mm (3.4 in) and 70.0 mm (2.8 in), respectively. Compression ratio is 6.5:1 for G400 and 6.8:1 for G400K1. The G400 produced 10.0 HP (7.45 kW) at 3600 rpm of maximum horsepower and torque was 22.5 Nm (2.3 kg·m) at 3600 rpm. The dry weight of the Honda G400 with electric starter engine was 33 kg (72.7 lbs.).

General information

Engine Specifications
Model G400
Type 4-stroke, side valve type (L-head), single cylinder
Displacement 406 cm 3 (24.8 cu-in)
Max. horsepower 10.0 HP (7.45 kW) at 3600 rpm
Max. torque 22.5 Nm (2.3 kg·m) at 3600 rpm
Carburetor Horizontal, butterfly valve
Cooling system Forced-air
Ignition system Transistorized magneto or CDI
Lubricating system Splash system
Starting system Recoil or electric starter
Stoping system Ground switch
Fuel used Unleaded gasoline (octane number 86 or higher)
Fuel tank capacity 6.0 L
Fuel consumption 270 g/HPh
PTO shaft rotation Counterclockwise (from PTO shaft side)

Cylinder Block and Head

Cylinder block
Compression ratio: G400 – 6.5:1
G400K1 – 6.8:1
Bore x Stroke 86 X 70 mm (3.4 x 2.8 in)
Sleeve internal diameter 86.00-86.02 mm (3.3858-3.3866 in)
Piston skirt outer diameter 85.97-86.00 mm (3.385-3.386 in)
Number of piston rings (compression / oil): 2 / 1
Ring side clearance Top 0.02-0.06 mm (0.0008-0.0024 in)
Second/oil 0.01-0.05 mm (0.0004-0.0020 in)
Ring end gap Top/second 0.2-0.4 mm (0.008-0.016 in)
Oil 0.2-0.4 mm (0.008-0.016 in)
Ring width Top/second 1.97-1.99 mm (0.0776-0.783 in)
Oil 3.97-3.99 mm (0.1563-0.1571 in)
Connecting rod small end ID 19.005-19.020 mm (0.7482-07488 in)
Crankshaft crankpin outer diameter 36.943-36.959 mm (1.4544-1.1551 in)
Cylinder Head
Warpage (limit) 0.10 mm (0.004 in)
Valve Arrangement: Side valve type
Valves: 2
Stem outer diameter (standard): INTAKE 6.955-6.970 mm (0.2738-0.2744 in)
EXHAUST 6.910-6.925 mm (0.2720-0.2726 in)
Guide internal diameter INTAKE / EXHAUST 7.00-7.015 mm (0.2756-0.2762 in)
Spring free length 42.7 mm (1.6811 in)
Cam height: INTAKE 38.46-38.72 mm (1.5142-1.5244 in)
EXHAUST 38.49-38.75 mm (1.5154-1.5256 in)

Maintenance data

Idle speed 1400 ± 150 rpm
Cylinder compression 5.3 kg/cm2 (75 psi)
Adjust timing ignition
Ignition timing 20° B.T.D.C. – for magneto ignition
The CDI type does not require adjustment.
Adjust valve clearance
Intake valve 0.05-0.11 ± 0.02 (0.0020-0.0043 in)
Exhaust valve 0.09-0.15 ± 0.02 (0.0035-0.0059 in)
Oil system
Oil type Honda 4-stroke “SE”
Recommended oil 10W-40
Oil capacity 1.2 L (2.54 US pt.)
Main jet #102
Float height 8.6 mm (0.34 in)
Ignition system
Spark plug NGK: B4H, BR4HS for magneto ignition
NGK: BP4HS-10, BPR4HS-10 for CDI ignition
Spark plug gap 0.6-0.7 mm (0.024-0.028 in), for magneto ignition
0.9-0.10 mm (0.035-0.039 in), for CDI ignition

Tightening torque specs

Tightening torque specs
Fuel valve (12 mm) 12-15 Nm; 1.2-1.5 kg·m; 8.7-10.8 ft·lb
Flywheel (16 mm) 110-120 Nm; 11.0-12.0 kg·m; 80-87 ft·lb
Cylinder head Serial № before 129741: 43-47 Nm; 4.3-4.7 kg·m; 31-34 ft·lb
Serial № after 129742: 32-38 Nm; 3.2-3.8 kg·m; 23-27 ft·lb
Tappet cover (6 mm) 8-12 Nm; 0.8-1.2 kg·m; 5.8-8.7 ft·lb
Crankcase cover (8 mm) 20-24 Nm; 2.0-2.4 kg·m; 15-17 ft·lb
Connecting rod Serial № before 1085859: 21.6-26.4 Nm; 2.16-2.64 kg·m; 16-19 ft·lb
Serial № after 1085860: 24-28 Nm; 2.4-2.8 kg·m; 17-20 ft·lb
Fuel tank (8 mm) 20-28 Nm; 2.0-2.8 kg·m; 15-20 ft·lb
Oil drain plug 15-25 Nm; 1.5-2.5 kg·m; 11-18 ft·lb
5 mm bolts, nuts 4-7 Nm; 0.4-0.7 kg·m; 2.9-5.1 ft·lb
6 mm bolts, nuts 8-12 Nm; 0.8-1.2 kg·m; 5.8-8.7 ft·lb
8 mm bolts, nuts 20-28 Nm; 2.0-2.8 kg·m; 14.5-20.3 ft·lb
10 mm bolts, nuts 35-40 Nm; 3.5-4.0 kg·m; 25-29 ft·lb
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  1. i’m in need of an exciter coil for the g400 or g300 if anyone has some parts laying around. apparently this has been discontinued for some time.

  2. Can this engine really run on Unleaded?
    I have one form the 80’s and was under the impression in required leadded fuel.

    • From owners manual: “Use any automotive gasoline”, but in the manual, for the G100 (the same G-series) we can see: “Unleaded gasoline (or regular grade)”.

  3. Hi can I convert this engine to start electronicly and where can I get the kit and a video maybe to show me how it is done. Thanks Bill

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