Rato R390

Rato R390The Rato R390 is a 389 cc (23.7 cu-in) single-cylinder air-cooled 4-stroke internal combustion general-purpose gasoline engine with horizontal shaft manufactured by Chongqing RATO Holding (Group) CO., LTD.

The Rato R390 engine has an (OHV) overhead-valve design; flat suction valve carburetor; recoil starter or electric starter, transistorized magneto ignition (TCI) system and centrifugal mechanical governor system.

The R390 motor has an 88.0 mm (3.50 in) cylinder bore and 64.0 mm (2.50 in) piston stroke with a compression ratio of 8.0:1. It produced 13.0 HP of horsepower at 3,600 rpm and 25 N·m (2.54 kg·m, 18.43 ft·lb) of torque at 2,500 rpm.

General information

Engine Specifications
Type4-stroke, OHV, single cylinder
Displacement389 cm 3 (23.7 cu-in)
Max. horsepower13.0 HP at 3,600 rpm
Max. torque (crank PTO)25 N·m (2.54 kg·m, 18.43 ft·lb) at 2,500 rpm
CarburetorFlat suction valve carburetor
Cooling systemForced-air
Ignition systemTransistorized magneto ignition (TCI)
Lubricating systemSplash
Starting systemRecoil / electric 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 Rato R390 engine has cast-iron cylinder liner, dual ball bearings and steel forged and hardened crankshaft.

Cylinder block
Compression ratio:8.0:1
Bore x Stroke88 X 64 mm (3.5 x 2.5 in)
Sleeve internal diameter88.000-88.017 mm (3.4645-3.4652 in)
Piston skirt outer diameter87.965-87.985 mm (3.4631-3.4640 in)
Number of piston rings (compression / oil):2 / 1
Ring side clearanceTop/Second0.015-0.045 mm (0.0006-0.0018 in)
Ring end gapTop/Second0.20-0.40 mm (0.008-0.016 in)
Oil0.20-0.70 mm (0.008-0.027 in)
Connecting rod small end ID20.005 mm (0.7876 in)
Connecting rod big end ID36.025 mm (1.4183 in)
Crankshaft crank pin outer diameter35.985 mm (1.4167 in)
Cylinder Head
Warpage (limit)0.10 mm (0.004 in)
Valve Arrangement:OHV
Stem outer diameter (standard):INTAKE6.590 mm (0.2594 in)
EXHAUST6.550 mm (0.2579 in)
Cam height:INTAKE33.6-33.8 mm (1.3228-1.3307 in)
EXHAUST32.90-33.10 mm (1.2952-1.3031 in)
Valve spring free length:39.0 mm (1.5354 in)
Camshaft journal diameter:15.984 mm (0.6293 in)

Tightening torque specs

Tightening torque specs
Connecting rod bolt15 Nm; 1.5 kg·m; 11 ft·lb
Oil dran plug22 Nm; 2.2 kg·m; 16 ft·lb
Crankcase cover28 Nm; 2.8 kg·m; 21 ft·lb
Flywheel subassembly75 Nm; 7.5 kg·m; 55 ft·lb
Cylinder head subassembly42 Nm; 4.2 kg·m; 31 ft·lb
Rocker shaft bolt24 Nm; 2.4 kg·m; 18 ft·lb
Cylinder head cover10 Nm; 1.0 kg·m; 7 ft·lb
Spark plug28 Nm; 2.8 kg·m; 21 ft·lb
Ignition coil10 Nm; 1.0 kg·m; 7 ft·lb

Service Information

Maximum speed3750-3810 rpm
Cylinder compression14.0 kg/cm2 at 1400 rpm
Valve clearance
Intake valve clearance0.15±0.02 mm (0.006±0.001 in)
Exhaust valve clearance0.20±0.02 mm (0.008±0.001 in)
Oil system
Oil type4-stroke or an equivalent (SJ)
Recommended oilSAE 10W-30, 15W-40
Oil capacity1.1 L
Main jet0.92
Float height13.2 mm (0.52 in)
Pilot screw opening1-7/8 turns out
Ignition system
Spark plugNGK: BPR6ES
Spark plug gap0.7-0.8 mm (0.028-0.031 in)
Primary ignition coil resistance0.6-0.9 Ω
Secondary ignition coil resistance5.6-6.9 kΩ
Ignition coil air gap0.4 ± 0.2 (0.016 ± 0.008)
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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  1. I have this motor on my generator Rato R390 is a 389 cc, it is a Pull start unit, does your company have a “electric start (12 volt) conversion kit available to purchase?
    Rato R390 is a 389 cc

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