Honda GX390 RT2/T2/UT2

Honda GX390 RT2/T2/UT2The Honda GX390 RT2/T2/UT2 is a 0.4 L (389 cc, 23.74 cu·in) natural aspirated single-cylinder four-stroke air-cooled internal combustion small gasoline engine with a horizontal shaft, manufactured by Honda Motor Company for general-purpose applications.

An 88.0 mm (3.46 in) cylinder bore and 64.0 mm (2.52 in) piston stroke give the GX390 RT2/T2/UT2 engine a total of 389 cc of displacement. The compression ratio rating is 8.2:1.

The Honda GX390 RT2/T2/UT2 engine produced 11.8 PS (8.7 kW; 11.7 HP) at 3,600 rpm of net power and or 9.5 PS (7.0 kW; 9.4 HP) at 3,600 rpm of continuous rated power and 26.5 N·m (2.7 kg·m, 19.5 ft·lb) at 2,500 rpm of peak torque.

The Honda GX390 RT2/T2/UT2 features:

  • Overhead valve (OHV) design
  • C.D.I.(Capacitor Discharge Ignition) type magneto ignition system
  • Horizontal type carburetor with butterfly valve
  • Centrifugal governor
  • Forced splash lubrication system
  • Forced air cooling system
  • Recoil or recoil and starter motor

General information

Engine Specifications
Engine modelGX390 RT2/T2/UT2
Engine type4-stroke, OHV, single-cylinder
Displacement389 cm3, (23.74 cu·in)
Max. horsepower11.8 PS (8.7 kW; 11.7 HP) at 3,600 rpm
Max. torque26.5 N·m (2.7 kg·m, 19.5 ft·lb) at 2,500 rpm
Cooling systemForced air
Air cleaner Dual element type,
Cyclone type,
Low profile type
Lubricating systemForced splash
Ignition systemC.D.I.(Capacitor Discharge Ignition) type magneto ignition
Fuel systemHorizontal type carburetor with butterfly valve
Fuel consumption3.5 Liters (0.92 US gal, 0.77 Imp gal)/h
Fuel tank capacity6.1 L (1.61 US. gal. 1.34 Imp. gal.)
Fuel usedUnleaded gasoline (octane number 86 or higher)
Starting systemRecoil or recoil and starter motor
Stoping systemIgnition primary circuit open
GovernorCentrifugal governor
PTO shaft rotationCounterclockwise (from PTO shaft side)
Dimensions (L x W X H)407 x 485 x 449 mm (16 x 19.1 x 17.7 in)
Dry weight31.5 kg (69.4 lbs)

Cylinder Block and Head

Cylinder block
Compression ratio:8.2:1
Cylinder bore:88.0 mm (3.46 in)
Piston stroke:64.0 mm (2.52 in)
Cylinder internal diameter (standard):88.000-88.017 mm (3.4646-3.4652 in)
Piston skirt outer diameter (standard):87.975-87.985 mm (3.4636-3.464 in)
Number of piston rings (compression / oil):2/1
Connecting rod big end diameter (standard):36.025-36.039 mm (1.4183-1.4189 in)
Connecting rod small end diameter (standard):20.005-20.020 mm (0.7876-0.7882 in)
Piston pin diameter:19.994-20.000 mm (0.7872-0.7874 in)
Side clearance (Top):0.015-0.060 mm (0.0006-0.0024 in)
Side clearance (Second):0.030-0.060 mm (0.0012-0.0024 in)
Ring end gap (Top):0.200-0.350 mm (0.0079-0.0138 in)
Ring end gap (Second):0.350-0.500 mm (0.0138-0.0197 in)
Ring end gap (Oil):0.200-0.700 mm (0.0079-0.0276 in)
Ring width (Top):1.160-1.190 mm (0.0457-0.0469 in)
Ring width (Second):1.160-1.175 mm (0.0457-0.0463 in)
Crankpin journal diameter:35.975-35.985 mm (1.4163-1.4167 in)
Crankshaft center distance:32 mm (1.26 in)
Cylinder head
Valve arrangement:OHV, gear driven
Valves:2
Valve steam diameter (INTAKE):6.575-6.590 mm (0.2589-0.2594 in)
Valve steam diameter (EXHAUST):6.535-6.550 mm (0.2573-0.2579 in)
Valve spring free length (INTAKE):39.0 mm (1.5354 in)
Valve spring free length (EXHAUST):39.0 mm (1.5354 in)
Camshaft lobe height (INTAKE):32.498-32.698 mm (1.2794-1.2873 in)
Camshaft lobe height (EXHAUST):31.985-32.185 mm (1.2592-1.2671 in)
Camshaft journal diameter :15.966-15.984 mm (0.6286-0.6293 in)

Tightening torque

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

Service Data

Engine
Maximum speed:3,700-4,000 rpm
Idle speed:1,250-1,550 rpm
Cylinder compression:0.51-0.69 MPa (5.2-7.0 kgf/cm2, 74-100 psi) / 600 rpm
Valve clearance
Intake valve clearance:0.15 ± 0.02 mm (0.006 ± 0.001 in)
Exhaust valve clearance:0.20 ± 0.02 mm (0.008 ± 0.001 in)
Oil system
Lubrication system:Forced splash
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.)
Fuel system
Type:Horizontal type carburetor with butterfly valve
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
Ignition system
Type:C.D.I.(Capacitor Discharge Ignition) type magneto ignition
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 (0.0276-0.0315 in)
Spark plug torque:18 N·m (1.8 kg·m, 13.3 ft·lb)
Ignition coil air gap0.2-0.6 mm (0.0079-0.0236 in)

Applications

Manufacturer or BrandModels
Generators
HondaEB5000X, EB6500X
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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We try to use official documentation and verify the published data, but errors may occur. In addition, specifications of machinery and equipment may vary depending on markets and modifications. We do not recommend the use of this data for spare parts or repairs. Use official service manuals or parts catalogs for your product.

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