Honda GXR120

Honda GXR120The Honda GXR120 is a 121 cc (7.4 cu-in) single-cylinder air-cooled 4-stroke internal combustion small gasoline engine with horizontal shaft manufactured by Honda Motor Company for commercial heavy-duty applications.

The GXR120 T/RT motor has an OHC (overhead camshaft) design; high compression ratio for better fuel efficiency; mechanical centrifugal governor; forced splash lubrication system; heavy-duty recoil starter and transistorized magneto ignition. The engine can be equipped with a float type, horizontal butterfly valve type or diaphragm type carburetor.

The cylinder bore is 60.0 mm (2.36 in) and the piston stroke is 43.0 mm (1.69 in), compression ratio rating is 8.5:1. The engine produced 3.6 HP (2.7 kW) at 3,600 rpm of maximum horsepower and 2.8 HP (2.1 kW) at 3,600 rpm of continuous rated power. Maximum torque is 7.5 N·m (0.8 kg·m, 5.5 ft·lb) at 2,500 rpm.

General information

Engine Specifications
Model GXR120 T/RT
Type 4-stroke, OHC, single cylinder
Displacement 121 cm 3 (7.4 cu-in)
Max. horsepower 3.6 HP (2.7 kW) at 3,600 rpm
Max. torque (crank PTO) 7.5 N·m (0.8 kg·m, 5.5 ft·lb) at 2,500 rpm
Carburetor Float type / Horizontal butterfly valve type / Diaphragm type
Cooling system Forced-air
Ignition system Transistorized magneto ignition
Lubricating system Forced splash
Starting system Recoil starter
Stoping system Ignition primary circuit ground
Fuel used Unleaded gasoline (octane number 86 or higher)
PTO shaft rotation Counterclockwise (from PTO shaft side)

Cylinder Block and Head

The Honda GXR120 has a special cast iron cylinder sleeve and high carbon steel, dual, ball-bearing crankshaft ensures high durability and reliability. The piston has two compression rings and one oil ring. Cylinder bore and piston stroke are 60.0 mm (2.36 in) and 43.0 mm (1.69 in), respectively. Compression ratio rating is 8.5:1

Cylinder block
Compression ratio: 8.5:1
Bore x Stroke 60 X 43 mm (2.36 x 1.69 in)
Sleeve internal diameter 60.000-60.015 mm (2.3622-2.3628 in)
Piston skirt outer diameter 59.971-59.985 mm (2.3611-2.3616 in)
Number of piston rings (compression / oil): 2 / 1
Ring side clearance Top 0.015-0.054 mm (0.0006-0.0021 in)
Second 0.030-0.069 mm (0.0012-0.0027 in)
Ring end gap Top 0.15-0.30 mm (0.0059-0.0118 in)
Second 0.40-0.55 mm (0.0157-0.0217 in)
Oil (side rail) 0.2-0.7 mm (0.01-0.03 in)
Ring width Top 0.970-0.990 mm (0.0382-0.0390 in)
Second 1.155-1.175 mm (0.0455-0.0463 in)
Connecting rod small end ID 13.005-13.020 mm (0.5120-0.5126 in)
Connecting rod big end ID 26.020-26.033 mm (1.0244-1.0249 in)
Piston pin outer diameter 12.994-13.000 mm (0.5116-0.5118 in)
Crankshaft crank pin outer diameter 25.970-25.980 mm (1.0224-1.0228 in)
Cylinder Head
Warpage (limit) 0.10 mm (0.004 in)
Valve Arrangement: OHV
Valves: 2
Stem outer diameter (standard): INTAKE 3.970-3.985 mm (0.1563-0.1569 in)
EXHAUST 3.935-3.950 mm (0.1549-0.1555 in)
Valve spring free length: 25.8 mm (1.0157 in)
Cam height: INTAKE 36.483 mm (1.4363 in)
EXHAUST 36.483 mm (1.4363 in)

Tightening torque specs

Tightening torque specs
Connecting rod bolt 10 Nm; 1.0 kg·m; 7 ft·lb
Flywheel nut 64 Nm; 6.5 kg·m; 47 ft·lb
Valve adjusting lock nut 7.5 Nm; 0.76 kg·m; 5.5 ft·lb
Oil drain plug bolt 18 Nm; 1.8 kg·m; 13 ft·lb
Crankcase cover bolt 12 Nm; 1.2 kg·m; 9 ft·lb
Muffler nut 12 Nm; 1.2 kg·m; 9 ft·lb

Service Information

Maximum speed 4,100 ± 100 rpm
Idle speed 1,850 ± 150 rpm
Cylinder compression 5.1 kg/cm2 (73 psi) at 700 rpm
Valve clearance
Intake valve clearance 0.15±0.04 mm (0.006±0.002 in)
Exhaust valve clearance 0.20±0.04 mm (0.008±0.002 in)
Oil system
Oil type Honda 4-stroke or an equivalent (SE or higher)
Recommended oil SAE 10W-30
Oil capacity 0.28 L (0.30 US. qt, 0.25 Imp. qt.)
Carburetor (Float type)
Main jet BF33S A: #60
BF33R A: #60
Float height 15.7 mm (0.62 in)
Pilot screw opening BF33S A: 3-1/2 turns out
BF33R A: 3-3/8 turns out
Carburetor (Floatless type)
Main jet #48
Pilot screw opening 2-3/4 turns out
Ignition system
Ignition timing 25° B.T.D.C.
Spark plug NGK: CR5HSB
Spark plug gap 0.7-0.8 mm (0.028-0.031 in)
Primary ignition coil resistance 0.75-0.92 Ω
Secondary ignition coil resistance 6.1-9.3 kΩ
Ignition coil air gap 0.4 ± 0.2 (0.016 ± 0.008)
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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