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
ModelGXR120 T/RT
Type4-stroke, OHC, single cylinder
Displacement121 cm 3 (7.4 cu-in)
Max. horsepower3.6 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
CarburetorFloat type / Horizontal butterfly valve type / Diaphragm type
Cooling systemForced-air
Ignition systemTransistorized magneto ignition
Lubricating systemForced splash
Starting systemRecoil starter
Stoping systemIgnition primary circuit ground
Fuel usedUnleaded gasoline (octane number 86 or higher)
PTO shaft rotationCounterclockwise (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 Stroke60 X 43 mm (2.36 x 1.69 in)
Sleeve internal diameter60.000-60.015 mm (2.3622-2.3628 in)
Piston skirt outer diameter59.971-59.985 mm (2.3611-2.3616 in)
Number of piston rings (compression / oil):2 / 1
Ring side clearanceTop0.015-0.054 mm (0.0006-0.0021 in)
Second0.030-0.069 mm (0.0012-0.0027 in)
Ring end gapTop0.15-0.30 mm (0.0059-0.0118 in)
Second0.40-0.55 mm (0.0157-0.0217 in)
Oil (side rail)0.2-0.7 mm (0.01-0.03 in)
Ring widthTop0.970-0.990 mm (0.0382-0.0390 in)
Second1.155-1.175 mm (0.0455-0.0463 in)
Connecting rod small end ID13.005-13.020 mm (0.5120-0.5126 in)
Connecting rod big end ID26.020-26.033 mm (1.0244-1.0249 in)
Piston pin outer diameter12.994-13.000 mm (0.5116-0.5118 in)
Crankshaft crank pin outer diameter25.970-25.980 mm (1.0224-1.0228 in)
Cylinder Head
Warpage (limit)0.10 mm (0.004 in)
Valve Arrangement:OHV
Stem outer diameter (standard):INTAKE3.970-3.985 mm (0.1563-0.1569 in)
EXHAUST3.935-3.950 mm (0.1549-0.1555 in)
Valve spring free length:25.8 mm (1.0157 in)
Cam height:INTAKE36.483 mm (1.4363 in)
EXHAUST36.483 mm (1.4363 in)

Tightening torque specs

Tightening torque specs
Connecting rod bolt10 Nm; 1.0 kg·m; 7 ft·lb
Flywheel nut64 Nm; 6.5 kg·m; 47 ft·lb
Valve adjusting lock nut7.5 Nm; 0.76 kg·m; 5.5 ft·lb
Oil drain plug bolt18 Nm; 1.8 kg·m; 13 ft·lb
Crankcase cover bolt12 Nm; 1.2 kg·m; 9 ft·lb
Muffler nut12 Nm; 1.2 kg·m; 9 ft·lb

Service Information

Maximum speed4,100 ± 100 rpm
Idle speed1,850 ± 150 rpm
Cylinder compression5.1 kg/cm2 (73 psi) at 700 rpm
Valve clearance
Intake valve clearance0.15±0.04 mm (0.006±0.002 in)
Exhaust valve clearance0.20±0.04 mm (0.008±0.002 in)
Oil system
Oil typeHonda 4-stroke or an equivalent (SE or higher)
Recommended oilSAE 10W-30
Oil capacity0.28 L (0.30 US. qt, 0.25 Imp. qt.)
Carburetor (Float type)
Main jet BF33S A: #60
BF33R A: #60
Float height15.7 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 opening2-3/4 turns out
Ignition system
Ignition timing25° B.T.D.C.
Spark plugNGK: CR5HSB
Spark plug gap0.7-0.8 mm (0.028-0.031 in)
Primary ignition coil resistance0.75-0.92 Ω
Secondary ignition coil resistance6.1-9.3 kΩ
Ignition coil air gap0.4 ± 0.2 (0.016 ± 0.008)
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