Honda GX390 RT2/T2/UT2

Honda GX390 RT2/T2/UT2The 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
ModelGX390 RT2/T2/UT2
Description codeGCBCT, GCBDT, GCBCT
Type4-stroke, OVH, single cylinder, inclined by 25°
Displacement389 cm 3 (23.7 cu·in)
Max. horsepower11.7 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
CarburetorHorizontal type, butterfly valve
Cooling systemForced-air
Ignition systemC.D.I.(Capacitor Discharge Ignition) type magneto ignition
Lubricating systemForced splash
Starting systemRecoil, Recoil and Starter motor
Stoping systemIgnition exciter coil circuit open
Fuel usedUnleaded gasoline (octane number 86 or higher)
Fuel consumption3.5 L (0.92 US gal, 0.77 Imp gal) / h
PTO shaft rotationCounterclockwise (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 Stroke88 X 64 mm (3.5 x 2.5 in)
Sleeve internal diameter88.000-88.017 mm (3.4646-3.4652 in)
Piston skirt outer diameter87.975-87.985 mm (3.4635-3.4640 mm)
Number of piston rings (compression / oil):2 / 1
Ring side clearanceTop0.015-0.060 (0.0006-0.0024)
Second0.030-0.060 (0.0012-0.0024)
Ring end gapTop0.20-0.35 mm (0.0079-0.0138 in)
Second0.35-0.50 mm (0.0138-0.0197 in)
Oil (side rail)0.2-0.7 mm (0.01-0.03 in)
Ring widthTop1.160-1.190 mm (0.0457-0.0470 in)
Second1.160-1.175 mm (0.045-0.0463 in)
Piston pin outer diameter19.994-20.000 mm (0.7872-0.7874 in)
Connecting rod small end inner diameter20.005-20.020 mm (0.7876-0.7882 in)
Connecting rod big end inner diameter36.025-36.039 mm (1.4183-1.4189 in)
Crankpin outer diameter35.975-35.985 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):INTAKE6.575-6.590 mm (0.2589-0.2594 in)
EXHAUST6.535-6.550 mm (0.2573-0.2579 in)
Cam height:INTAKE32.498-32.698 mm (1.2794-1.2873 in)
EXHAUST31.985-32.185 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 speed3850 ± 150 rpm
Idle speed1400 ± 150 rpm
Cylinder compression5.2-7.0 kg/cm2 (74-100 psi, 0.51-0.69 MPa) at 600 rpm
Valve clearance
Intake valve0.15 ± 0.02 (0.006 ± 0.001)
Exhaust valve0.20 ± 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 height13.2 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 typeHonda 4-stroke or an equivalent (SE or later)
Recommended oilSAE 10W-30
Oil capacity1.1 L (1.16 US qt, 0.97 Imp qt)
Ignition system
Ignition timingB.T.D.C. 10° / 1,400 rpm
Spark plugNGK: BPR6ES
DENSO: W20EPR-U
Spark plug gap0.7-0.8 mm (0.028-0.031 in)
Ignition coil air gap0.2-0.6 mm (0.01 – 0.02 in)
Starter motor
Brush length7.0 mm (0.28 in) limit: 3.5 mm (0.14 in)
Mica depth1.0 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
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