________________________________________________________________________________

Cummins 4B3.9-G2 Engine

The Cummins 4B3.9-G2 is an inline, 4-cylinder, 4-stroke, naturally aspirated diesel engine engineered primarily for G-drive generator set applications. Part of the reliable Cummins B-series family, this engine features an integrated cylinder block and cylinder head design that minimizes leakage and reduces total part count by roughly 40% compared to traditional configurations. It utilizes a platform mesh honing design on the cylinder bores alongside optimized piston rings to lower oil consumption and ensure stable mechanical operation under continuous load. Cylinder Bore: 102 mm (4.02 inches). Piston Stroke: 120 mm (4.72 inches). Total Displacement: 3.9 liters (238 cubic inches).

Fuel System Injection Type: Direct injection mechanism for optimized thermal efficiency. Injection Pump: Equipped with a BYC ASIMCO or Weifu mechanical A-style inline fuel injection pump. Governor Assembly: Governed by either a GAC electronic governor system or BYC electronic speed-control unit. Regulation Performance: Maintains a speed-droop regulation tight tolerance of 3% to 5% depending on governor configuration. Average Fuel Consumption: Consumes 6.7 liters per hour at continuous base rated output.

Lubrication System : Full pressure feed lubrication driven by a gear-type oil pump. Filtration: High-efficiency, spin-on full-flow combination bypass oil filter elements. Oil Loss Mitigation: Advanced gasket crimping and precision ring components regulate consumption.

Cooling System Type: Liquid-cooled setup utilizing a belt-driven centrifugal water pump. Integration: Built-in passages within the single-piece casting structural head block to avoid exterior line leaks. Equipment Compatibility: Designed to interface directly with a standard radiator assembly and pusher/puller fan arrangements.

General Technical Data

Engine Specifications

Engine Model ............... Cummins 4B3.9-G2
Engine Type ............... 4-cycle inline 4-cylinder direct injection liquid-cooled diesel
Number of Cylinders ............... 4
Cylinder Arrangement ............... In-line
Engine Bore, in (mm) ............... 4.02 (102.0)
Engine Stroke, in (mm) ............... 4.72 (120.0)
Displacement, cu.in (L) ............... 239 (3.9)
Rated Engine Power, hp (kW) ............... 47-100 (35-74)
Rated Engine Speed, rpm ............... 2000-2500
Peak Torque ............... 230-350 Nm (170-258 lb-ft) at 1400-1800 rpm
Idle Speed ............... 750-850 rpm
Aspiration Type ............... Naturally Aspirated
Operating Speed ............... 2000 rpm
Compression Ratio ............... 16.5:1-17.5:1
Firing Order ............... 1-3-4-2
Cylinder Block Type ............... Cast iron
Cylinder Head Type ............... Cast iron
Lubrication System ............... Full-pressure
Oil Pump Type ............... Gear-driven
Oil Filter Type ............... Full-flow, spin-on
Cooling System ............... Liquid-cooled
Water Pump Type ............... Belt-driven, centrifugal
Air Cleaner Type ............... Dry dual-stage
Starting System ............... Electric Starter
Electrical System ............... 12V or 24V
Alternator Rating, Amps ............... 35-65

Fuel System

Fuel System ............... Direct injection
Injection Pump Type ............... Rotary
Fuel System Control ............... Mechanical
Governor Type ............... Mechanical
Fuel Injector Type ............... Multi-hole
Fuel Type ............... Diesel Fuel

Dimensions and Weight

Overall Length, in (mm) ............... 33.5-37.4 (850-950)
Overall Width, in (mm) ............... 25.6-29.5 (650-750)
Overall Height, in (mm) ............... 35.4-39.4 (900-1000)
Dry Weight, lbs (kg) ............... 730-840 (330-380)

Maintenance Data

Service Information

Thermostat begins opening ............... 82C (180F)
Thermostat fully open ............... 95C (203F)
Cooling system pressure ............... 70-100 kPa (10-15 psi)
Operating oil pressure ............... 300-450 kPa (44-65 psi)
Idle oil pressure ............... 100-150 kPa (15-22 psi)
Intake Valve Clearance, in (mm) ............... 0.254 (0.010)
Exhaust Valve Clearance, in (mm) ............... 0.508 (0.020)

Engine Oil and Coolant

Engine Oil Type .................. SAE 10W-30, 15W-40
API Service Classifications ......................... CI-4, CJ-4, CK-4
Engine Oil Capacity, qts (L) .................. 9.5-10.5 (9.0-10.0)
Coolant Capacity, qts (L) ............... 10.5-12.6 (10.0-12.0)

Service Intervals

Change oil and filter ............... Every 250 hours
Replace fuel filters ............... Every 300 hours
Inspect air filter ............... Every 100 hours
Replace air filter ............... Every 300 hours
Change coolant ............... Every 2000 hours
Valve-lash adjustment ............... Every 1000 hours
Injector inspection ............... Every 1500-2000 hours
Compression test ............... Every 3000 hours
Drain water separator ............... As required
Belt inspection ............... As required

Service Specifications

Cylinder Bore Inside Diameter, in (mm) ............... 4.0161-4.0177 (102.000-102.040)
Crankshaft Journal Outside Diameter, in (mm) ............... 3.2661-3.2681 (82.960-83.010)
Crank Pin Outside Diameter, in (mm) ............... 2.7150-2.7169 (68.960-69.010)
Crankshaft End Play, in (mm) ............... 0.0040-0.0170 (0.102-0.432)
Intake Valve Stem Outside Diameter, in (mm) ............... 0.3134-0.3142 (7.960-7.980)
Exhaust Valve Stem Outside Diameter, in (mm) ............... 0.3126-0.3134 (7.940-7.960)
Intake Valve Guide Inside Diameter (Installed), in (mm) ............... 0.3157-0.3169 (8.020-8.050)
Exhaust Valve Guide Inside Diameter (Installed), in (mm) ............... 0.3157-0.3169 (8.020-8.050)
Intake Valve Stem to Guide Clearance, in (mm) ............... 0.0015-0.0036 (0.038-0.091)
Exhaust Valve Stem to Guide Clearance, in (mm) ............... 0.0025-0.0046 (0.064-0.117)
Intake Valve Spring Free Length, in (mm) ............... 2.150 (54.61)
Exhaust Valve Spring Free Length, in (mm) ............... 2.150 (54.61)
Intake Cam Height, in (mm) ............... 1.831 (46.51)
Exhaust Cam Height, in (mm) ............... 1.819 (46.20)
Piston Pin Outside Diameter, in (mm) ............... 1.5744-1.5752 (39.990-40.010)
Piston Pin to Piston Bore Clearance, in (mm) ............... 0.0002-0.0007 (0.005-0.018)
Second Ring Side Clearance, in (mm) ............... 0.0016-0.0033 (0.040-0.085)
Oil Ring Side Clearance, in (mm) ............... 0.0012-0.0030 (0.030-0.075)
Top Ring End Gap, in (mm) ............... 0.0114-0.0228 (0.290-0.580)
Second Ring End Gap, in (mm) ............... 0.0346-0.0539 (0.880-1.370)
Oil Ring End Gap, in (mm) ............... 0.0110-0.0303 (0.280-0.770)

Tightening Torque Specs

Cylinder head bolts ............... 160-180 Nm (118-133 lb-ft)
Main bearing caps ............... 150-170 Nm (111-125 lb-ft)
Connecting rod bolts ............... 70-85 Nm (52-63 lb-ft)
Flywheel bolts ............... 180-220 Nm (133-162 lb-ft)
Crankshaft pulley bolt ............... 200-240 Nm (148-177 lb-ft)
Glow plugs ............... 10-15 Nm (7-11 lb-ft)
Oil drain plug ............... 30-40 Nm (22-30 lb-ft)
Injector hold-down bolts ............... 25-30 Nm (18-22 lb-ft)

________________________________________________________________________________________

• Cummins Engines Specs
  
• Cummins 4B3.9-G11 Specs
  
• Cummins 4BT3.9-G2 Specs
  
• Cummins 4BTA3.9-G2 Specs
  
• Cummins 4BTA3.9-G11 Specs
  
• Cummins 4BTAA3.3-G11 Specs
  
• Cummins 4BTAA3.9 Specs
  

______________________________________________________________________________________

________________________________________________________________________________________

_______________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________