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
Type4-stroke, side valve type (L-head), single cylinder
Displacement406 cm 3 (24.8 cu-in)
Max. horsepower10.0 HP (7.45 kW) at 3600 rpm
Max. torque22.5 Nm (2.3 kg·m) at 3600 rpm
CarburetorHorizontal, butterfly valve
Cooling systemForced-air
Ignition systemTransistorized magneto or CDI
Lubricating systemSplash system
Starting systemRecoil or electric starter
Stoping systemGround switch
Fuel usedUnleaded gasoline (octane number 86 or higher)
Fuel tank capacity6.0 L
Fuel consumption270 g/HPh
PTO shaft rotationCounterclockwise (from PTO shaft side)

Cylinder Block and Head

Cylinder block
Compression ratio: G400 – 6.5:1
G400K1 – 6.8:1
Bore x Stroke86 X 70 mm (3.4 x 2.8 in)
Sleeve internal diameter86.00-86.02 mm (3.3858-3.3866 in)
Piston skirt outer diameter85.97-86.00 mm (3.385-3.386 in)
Number of piston rings (compression / oil):2 / 1
Ring side clearanceTop0.02-0.06 mm (0.0008-0.0024 in)
Second/oil0.01-0.05 mm (0.0004-0.0020 in)
Ring end gapTop/second0.2-0.4 mm (0.008-0.016 in)
Oil0.2-0.4 mm (0.008-0.016 in)
Ring widthTop/second1.97-1.99 mm (0.0776-0.783 in)
Oil3.97-3.99 mm (0.1563-0.1571 in)
Connecting rod small end ID19.005-19.020 mm (0.7482-07488 in)
Crankshaft crankpin outer diameter36.943-36.959 mm (1.4544-1.1551 in)
Cylinder Head
Warpage (limit)0.10 mm (0.004 in)
Valve Arrangement:Side valve type
Stem outer diameter (standard):INTAKE6.955-6.970 mm (0.2738-0.2744 in)
EXHAUST6.910-6.925 mm (0.2720-0.2726 in)
Guide internal diameterINTAKE / EXHAUST7.00-7.015 mm (0.2756-0.2762 in)
Spring free length42.7 mm (1.6811 in)
Cam height:INTAKE38.46-38.72 mm (1.5142-1.5244 in)
EXHAUST38.49-38.75 mm (1.5154-1.5256 in)

Maintenance data

Idle speed1400 ± 150 rpm
Cylinder compression5.3 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 valve0.05-0.11 ± 0.02 (0.0020-0.0043 in)
Exhaust valve0.09-0.15 ± 0.02 (0.0035-0.0059 in)
Oil system
Oil typeHonda 4-stroke “SE”
Recommended oil10W-40
Oil capacity1.2 L (2.54 US pt.)
Main jet#102
Float height8.6 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 plug15-25 Nm; 1.5-2.5 kg·m; 11-18 ft·lb
5 mm bolts, nuts4-7 Nm; 0.4-0.7 kg·m; 2.9-5.1 ft·lb
6 mm bolts, nuts8-12 Nm; 0.8-1.2 kg·m; 5.8-8.7 ft·lb
8 mm bolts, nuts20-28 Nm; 2.0-2.8 kg·m; 14.5-20.3 ft·lb
10 mm bolts, nuts35-40 Nm; 3.5-4.0 kg·m; 25-29 ft·lb
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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  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

  4. What was the G400 normally used for? I’m just wondering if it would be possible to apply it to a generator? Any idea what this engine is most suitable for?

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