Honda G300

Honda G300 engineThe Honda G300 is a 272 cc (16.6 single-cylinder, air-cooled, four-stroke internal combustion small gasoline engine manufactured by Honda Motor Company since 1978 for general-purpose applications.

The Honda G300 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 76.0 mm (3.0 in) and 60.0 mm (2.4 in), respectively. The compression ratio is 6.5:1. The G300 produced 7.1 PS (5.22 KW; 7.0 HP) at 3,600 rpm of maximum horsepower and torque was 14.7 Nm (1.5 kg·m; 10.8 lbf·ft) at 3,600 rpm. The dry weight of the Honda G300 with electric starter engine was 25.0 kg (55.1 lbs.), with recoil starter 22.0 kg (49.0 lbs)

General information

Engine Specifications
Type4-stroke, side valve type (L-head), single cylinder
Displacement272 cm 3 (16.6 cu-in)
Max. horsepower7.1 PS (5.22 KW; 7.0 HP) at 3,600 rpm
Max. torque14.7 Nm (1.5 kg·m; 10.8 lbf·ft) at 3,600 rpm
CarburetorHorizontal, butterfly valve
Cooling systemForced-air
Ignition systemTransistorized magneto or CDI
Lubricating systemSplash type
Starting systemRecoil or electric starter
Stoping systemGround switch
Fuel usedUnleaded gasoline (octane number 86 or higher)
Fuel tank capacity6.0 liters (6.3 US.qts, 5.3 Imp. qts)
Fuel consumption285 g/HPh
PTO shaft rotationCounterclockwise (from PTO shaft side)

Cylinder Block and Head

Cylinder block
Compression ratio:6.5:1
Bore x Stroke76 X 60 mm (3.0 x 2.4 in)
Sleeve internal diameter76.000-76.017 mm (2.9921-2.9928 in)
Piston skirt outer diameter75.97-76.00 mm (2.991-2.992 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 ID16.005-16.020 mm (0.6301-0.6307 in)
Crankpin outer diameter29.943-29.959 mm (1.1789-1.1795 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 speed1,400 ± 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 valve0.08-0.16 ± 0.02 (0.0031-0.0063 in)
Exhaust valve0.11-0.19 ± 0.02 (0.0043-0.0075 in)
Oil system
Oil typeHonda 4-stroke “SE”
Recommended oil10W-40
Oil capacity1.2 liters (1.27 US.qts, 1.06 Imp. qts)
Main jet#88
Float height8.2 mm (0.32 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 head24-26 Nm; 2.4-2.6 kg·m; 17-19 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 rod12.6-15.4 Nm; 1.26-1.54 kg·m; 9.1-11 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 am in need of G300 engine in volume, however, i want to know where and how to get it ( country and price ) regards

  2. hi, just wondering what the size of the shaft is for a pully please..looks like 26mm can anyone confirm.. many thanks

  3. I have one of these G300 engines with a brand new carb and fresh oil regularly. It runs smooth and starts so easy, my 9-year old can start it. I am in Thunder Bay, Ontario, Canada. It’s for sale at $250 CAD.
    The output shaft is 1″ diameter (25.4mm exactly). I also have 2-pulleys included with the motor, one 3″ and the other 3.5″.
    The engine can be heard running. If you are interested and serious, we could facebook video chat and I can show you it cold start and running.

  4. Anyone know what electric starter is used on this motor? Mine is broken and the ID plate is damaged and unreadable.
    I am seeking a replacement 12V starter.

    Any help appreciated.

  5. I have an older G300 Honda 7.0hp and one of the magnets on the inside of the flywheel rusted off. or i should say the bracket holding it on and the magnet is slightly broken. There are 4 total magnets on the inside all at 90 degrees. can this run with 3 magnets? do I need that forth? anyone know?

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