The Honda Honda GX390 RT2/T2/UT2 is a 0.39 l (389 cc, 23.7 cu·in) single-cylinder air-coolled 4-stroke internal combustion small gasoline engine with horizontal shaft, manufactured by Honda Motor for general-purpose applications.
From spring 2010 onwards, Honda producing the updated version of GX390 engines with larger displacement. The new engine has variable ignition timing (digital CDI), increased compression ratio, improved carburetor performance, and a lightweight piston.
The dimensions and outward appearance, however, remained virtually unchanged, in order to guarantee 100% interchangeability with the older GX390-engines. For the same reason, the naming of the engine remained unchanged, but all new engines have a number “2” in model version (T2 or UT2), old versions have a number “1” (GX390 K1).
The Honda GX390 has single cylinder inclined by 25°, overhead valve design, horizontal carburetor with butterfly valve, C.D.I.(Capacitor Discharge Ignition) type magneto ignition system, recoil and starter motor or recoil starter only.
In 2014, Honda introduced “high inclination” version of GX engines. The GX270, GX340 and GX390 are now available for multi-purpose uses including providing power for two-wheel tractors.
The engines have new features:
- A special one-way valve integrated in the cylinder head to improve the oil return flow and guarantee optimal lubrication under inclined conditions;
- A membrane fuel pump, protected by a bracket, and adapted fuel line routing, to ensure an optimal fuel flow;
- A maximum fuel level indicator and inner-vent carburettor fitted to avoid overfilling and to guarantee safe operation under all conditions.
The cylinder bore is 88.0 mm (3.5 in) and the piston stroke is 64.0 mm (2.5 in). Compression ratio rating is 8.2:1. The Honda GX390 produces 11.7 HP (8.0 kW) of maximum horsepower at 3,600 rpm and 9.4 HP (7.0 kW) of continuous rated power. The torque is 26.5 Nm (2.7 kg·m, 19.5 ft·lb) at 2,500 rpm.
General information
| Engine Specifications | |
| Model | GX390 RT2/T2/UT2 |
| Description code | GCBCT, GCBDT, GCBCT |
| Type | 4-stroke, OVH, single cylinder, inclined by 25° |
| Displacement | 389 cm 3 (23.7 cu·in) |
| Max. horsepower | 11.7 HP (8.0 kW) at 3,600 rpm |
| Max. torque (crank PTO) | 26.5 Nm (2.7 kg·m, 19.5 ft·lb) at 2500 rpm |
| Carburetor | Horizontal type, butterfly valve |
| Cooling system | Forced-air |
| Ignition system | C.D.I.(Capacitor Discharge Ignition) type magneto ignition |
| Lubricating system | Forced splash |
| Starting system | Recoil, Recoil and Starter motor |
| Stoping system | Ignition exciter coil circuit open |
| Fuel used | Unleaded gasoline (octane number 86 or higher) |
| Fuel consumption | 3.5 L (0.92 US gal, 0.77 Imp gal) / h |
| PTO shaft rotation | Counterclockwise (from PTO shaft side) |
Cylinder Block and Head
The Honda GX390 has cast iron cylinder sleeve, ball bearing supported crankshaft, improved piston design, heavy duty balancer shaft. The piston has two compression rings and one oil ring. Precision camshaft design offers precise valve timing and optimal valve overlap for better fuel efficiency.
| Cylinder block | ||
| Compression ratio: | 8.2:1 | |
| Bore x Stroke | 88 X 64 mm (3.5 x 2.5 in) | |
| Sleeve internal diameter | 88.000-88.017 mm (3.4646-3.4652 in) | |
| Piston skirt outer diameter | 87.975-87.985 mm (3.4635-3.4640 mm) | |
| Number of piston rings (compression / oil): | 2 / 1 | |
| Ring side clearance | Top | 0.015-0.060 (0.0006-0.0024) | Second | 0.030-0.060 (0.0012-0.0024) |
| Ring end gap | Top | 0.20-0.35 mm (0.0079-0.0138 in) | Second | 0.35-0.50 mm (0.0138-0.0197 in) |
| Oil (side rail) | 0.2-0.7 mm (0.01-0.03 in) | |
| Ring width | Top | 1.160-1.190 mm (0.0457-0.0470 in) |
| Second | 1.160-1.175 mm (0.045-0.0463 in) | |
| Piston pin outer diameter | 19.994-20.000 mm (0.7872-0.7874 in) | |
| Connecting rod small end inner diameter | 20.005-20.020 mm (0.7876-0.7882 in) | |
| Connecting rod big end inner diameter | 36.025-36.039 mm (1.4183-1.4189 in) | |
| Crankpin outer diameter | 35.975-35.985 mm (1.4163-1.4167 in) | |
| Cylinder Head | ||
| Warpage (limit) | 0.10 mm (0.004 in) | |
| Valve Arrangement: | OHV | |
| Valves: | 2 | |
| Stem outer diameter (standard): | INTAKE | 6.575-6.590 mm (0.2589-0.2594 in) |
| EXHAUST | 6.535-6.550 mm (0.2573-0.2579 in) | |
| Cam height: | INTAKE | 32.498-32.698 mm (1.2794-1.2873 in) |
| EXHAUST | 31.985-32.185 mm (1.2592-1.2671 in) | |
| Camshaft outer diameter: | 15.966-15.984 mm (0.6286-0.6293 in) | |
Tightening torque specs
| Tightening torque specs | |
| Connecting rod bolt (M8x1.25 Special) | 14 Nm; 1.4 kg·m; 10 ft·lb |
| Cylinder head bolt (M10x1.25) | 35 Nm; 3.5 kg·m; 25 ft·lb |
| Flywheel nut (M16x1.5 Special) | 115 Nm; 11.5 kg·m; 83 ft·lb |
| Rocker arm pivot lock nut (M6x0.5) | 10 Nm; 1.0 kg·m; 7 ft·lb |
| Rocker arm pivot bolt (M8x1.25 Special) | 24 Nm; 2.4 kg·m; 17 ft·lb |
| Crankcase cover bolt (M8x1.25) | 24 Nm; 2.4 kg·m; 17 ft·lb |
| Oil level switch joint nut (M10x1.25) | 10 Nm; 1.0 kg·m; 7 ft·lb |
| Fuel filter joint nut (M10x1.25) | 2 Nm; 0.2 kg·m; 1.4 ft·lb |
| Muffler mounting nut (M8x1.25) | 24 Nm; 2.4 kg·m; 17 ft·lb |
| Air cleaner wing nut (M6x1.0) | 9 Nm; 0.9 kg·m; 6.5 ft·lb |
| Oil drain bolt (M12x1.5) | 23 Nm; 2.3 kg·m; 17 ft·lb |
| Fuel tank bolt (M8x1.25) | 24 Nm; 2.4 kg·m; 17 ft·lb |
| Air cleaner mounting nut (M6x1.0) | 10 Nm; 1.0 kg·m; 7 ft·lb |
| Fuel strainer cup (M24x1.0) | 4 Nm; 0.4 kg·m; 2.9 ft·lb |
| Standard torque values | |
| Bolt, nut (5 mm) | 5.5 Nm; 0.55 kg·m; 4.0 ft·lb |
| Bolt, nut (6 mm) | 10 Nm; 1.0 kg·m; 7 ft·lb |
| Bolt, nut (8 mm) | 24 Nm; 2.4 kg·m; 17 ft·lb |
| Bolt, nut (10 mm) | 37.5 Nm; 3.75 kg·m; 27 ft·lb |
| Bolt, nut (12 mm) | 55 Nm; 5.5 kg·m; 40 ft·lb |
Maintenance data
| Engine | |
| Maximum speed | 3850 ± 150 rpm |
| Idle speed | 1400 ± 150 rpm |
| Cylinder compression | 5.2-7.0 kg/cm2 (74-100 psi, 0.51-0.69 MPa) at 600 rpm |
| Valve clearance | |
| Intake valve | 0.15 ± 0.02 (0.006 ± 0.001) |
| Exhaust valve | 0.20 ± 0.02 (0.008 ± 0.001) |
| Carburetor | |
| Main jet |
BE85C B: #100 BE85L A: #95 BE85Q A: #92 BE85Q A: #102 BE89F B: #108 BE89R A: #102 BE89U A: #108 BE94B A: #100 BE94A A: #115 |
| Float height | 13.2 mm (0.52 in) |
| Pilot screw opening |
BE85C B: 1 – 3/4 turns out BE85L A: 2 turns out BE85Q A: 1 – 7/8 turns out BE85Q A: 2 – 1/4 turns out BE89F B: 2 – 1/4 turns out BE89R A: 2 – 1/4 turns out BE89U A: 2 – 1/4 turns out BE94B A: 1 – 3/4 turns out BE94A A: 2 – 1/4 turns out |
| Oil system | |
| Oil type | Honda 4-stroke or an equivalent (SE or later) |
| Recommended oil | SAE 10W-30 |
| Oil capacity | 1.1 L (1.16 US qt, 0.97 Imp qt) |
| Ignition system | |
| Ignition timing | B.T.D.C. 10° / 1,400 rpm |
| Spark plug | NGK: BPR6ES DENSO: W20EPR-U |
| Spark plug gap | 0.7-0.8 mm (0.028-0.031 in) |
| Ignition coil air gap | 0.2-0.6 mm (0.01 – 0.02 in) |
| Starter motor | |
| Brush length | 7.0 mm (0.28 in) limit: 3.5 mm (0.14 in) |
| Mica depth | 1.0 mm (0.04 in) limit: 0.2 (0.01) |
Engine applications
- Pressure washers
- Commercial lawn and garden equipment
- Tillers / cultivators
- Generators
- Construction / industrial equipment
- Agricultural equipment
- Small vehicles
- Water pumps
Author:
1.1 Liters is 1.16 US Quarts NOT Gallons
Fixed! Thanks!
Morning im looking for the following lawnmower spare parts:
1)Honda GX 390 Rebuilt kit(standard) Piston with rings,crank and camshaft,gasket set.
2)Ignition coil Honda GX390
3)Honda GX 390 spark plugs
Good evening,
I am Verónica I am working for Purchasing Department.
Could you please give price for these references?
-0737051 MOTOR HONDA GX390 UT2 QXCU-13CV MOTOR GX390UT2 QC 3 :24 Unidades
-0737006 MOTOR HONDA GX390 SXQ4-13CV MOTOR GX390UT2 QX Q4: 24unidades
Thanks and regards
Good morning. Why is it that non of the spec charts show a shaft length, nor diameter? Thank you.
Can a GX390 UT2 ignition system be converted to an older ignition coil? I recently bought an electric starter kit for a generator (the GX390 is 2016 model with the new coil.) I’ve seen warnings that the 2016 coil won’t work with the earlier model flywheel. If I go to the trouble to install an old coil and oil module, will my pre-2011 flywheel work?
Okay for use in cold? Snowblower engine?