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Cummins 4BTA3.9-G2 Engine

The Cummins 4BTA3.9-G2 is a rugged, 4-stroke, heavy-duty industrial diesel engine configured as an in-line 4-cylinder unit. It belongs to the reliable Cummins B-series engine family and is engineered primarily as a G-Drive engine to power stationary and mobile generator sets. The engine design minimizes complexity by utilizing an integrated cylinder block and head configuration, which eliminates many potential water and oil leakage paths and reduces the total component count by roughly 40% compared to equivalent traditional models. Aspiration is managed via a Holset turbocharger equipped with an integral wastegate valve paired with a jacket water-cooled aftercooler (intercooler) to deliver excellent transient response and high thermal efficiency under severe continuous loads.

The fuel architecture utilizes a direct-injection design relying on a heavy-duty mechanical inline fuel injection pump (BYC PB or Weifu PW series) mated to an electronic governor assembly. This governor configuration enforces stable speed control with a regulation threshold of less than or equal to 3% to maintain strict frequency tolerances for electrical power generation. Primary filtration is managed through a high-efficiency spin-on threaded fuel filter canister, which is paired with an inline water separator to isolate fuel-borne contaminants before the fuel enters the high-pressure lines.

The lubrication circuit relies on a positive displacement gear-driven oil pump that draws from a heavy-stamped steel oil pan. The system uses a full-flow spin-on oil filter combined with an integrated oil cooler core assembly to manage oil temperature during sustained operations. Cylinder walls feature a platform mesh honing design that stabilizes oil film retention and reduces structural oil loss across the piston rings.

Thermal regulation uses a liquid cooling circuit driven by an engine-mounted water pump to circulate coolant throughout the block, head, and jacketed aftercooler assembly. The engine's operating temperature is regulated by an internal thermostat operating within a functional control range of 82 to 95C. The cooling circuit allows a maximum permitted coolant temperature of 100C under prime power output and up to 104C under standby power output before critical thermal thresholds are reached.

General Technical Data

Engine Specifications

Engine Model ............... Cummins 4BTA3.9-G2
Engine Type ............... 4-cycle inline 4-cylinder direct injection turbocharged 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) ............... 67-120 (50-90)
Rated Engine Speed, rpm ............... 2200-2500
Peak Torque ............... 300-450 Nm (220-332 lb-ft) at 1400-1600 rpm
Idle Speed ............... 750-850 rpm
Aspiration Type ............... Turbocharged and aftercooled
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 ............... 35-75

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) ............... 33.5-39.4 (850-1000)
Overall Width, in (mm) ............... 25.6-31.5 (650-800)
Overall Height, in (mm) ............... 35.4-39.4 (900-1000)
Dry Weight, lbs (kg) ............... 750-840 (340-380)

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) .................. 9.5-12.6 (9.0-12.0)
Coolant Capacity, qts (L) ............... 12.6-16.8 (12.0-16.0)

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)

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

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