Honda G300

Honda G300 engineThe Honda G300 is a 272 cc (16.6 cu.in.) single-cylinder air-coolled 4-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 7.0 HP (5.22 kW) at 3600 rpm of maximum horsepower and torque was 14.7 Nm (1.5 kg·m) at 3600 rpm. The dry weight of the Honda G300 with electric starter engine was 25.0 kg (55.1 lbs.), with recoil starter 22 kg (49 lbs.)

General information

Engine Specifications
Model G300
Type 4-stroke, side valve type (L-head), single cylinder
Displacement 272 cm 3 (16.6 cu-in)
Max. horsepower 7.0 HP (5.22 kW) at 3600 rpm
Max. torque 14.7 Nm (1.5 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 285 g/HPh
PTO shaft rotation Counterclockwise (from PTO shaft side)

Cylinder Block and Head

Cylinder block
Compression ratio: 6.5:1
Bore x Stroke 76 X 60 mm (3.0 x 2.4 in)
Sleeve internal diameter 76.000-76.017 mm (2.9921-2.9928 in)
Piston skirt outer diameter 75.97-76.00 mm (2.991-2.992 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 16.005-16.020 mm (0.6301-0.6307 in)
Crankpin outer diameter 29.943-29.959 mm (1.1789-1.1795 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

Engine
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 0.08-0.16 ± 0.02 (0.0031-0.0063 in)
Exhaust valve 0.11-0.19 ± 0.02 (0.0043-0.0075 in)
Oil system
Oil type Honda 4-stroke “SE”
Recommended oil 10W-40
Oil capacity 1.2 L (2.54 US pt.)
Carburetor
Main jet #88
Float height 8.2 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 head 24-26 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 rod 12.6-15.4 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 plug 15-25 Nm; 1.5-2.5 kg·m; 11-18 ft·lb
Others
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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17 Comments

  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.

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