The 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
| Engine | |
| 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 DENSO: U16FSR-UB |
| 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) |
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