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Caterpillar 3126B Engine

The Caterpillar 3126B is a medium-duty, 7.2-liter displacement, inline six-cylinder diesel engine engineered for a wide array of applications including vocational trucks, buses, marine vessels, and industrial equipment. Released as an evolutionary upgrade to the foundational 3126 platform, the "B" designation marked Caterpillar's shift toward highly integrated electronic management systems. This variant introduced a revised three-valve cylinder head design (two intake valves and one exhaust valve) to improve airflow and emission compliance. It bridges the gap between purely mechanical legacy platforms and the later ACERT technology seen in the Caterpillar C7 series. While highly regarded for its power-to-weight ratio and precise fuel delivery, it demands strict adherence to maintenance intervals, specifically regarding engine oil cleanliness and cooling system chemistry.

The 3126B operates on a HEUI (Hydraulic Electronic Unit Injection) system. This design completely eliminates mechanical injection pump linkages. Fuel pressurization is achieved via a dedicated high-pressure hydraulic pump that draws engine lubricating oil and intensifies it up to actuation pressures required by the fuel injectors. An on-board Electronic Control Module (ECM) monitors engine operating parameters and manages electronic solenoids located on each individual injector unit. By digitally controlling the injection timing and duration, the HEUI system optimizes fuel atomization directly into the cylinder across changing engine speeds and loads.

The system utilizes a high-capacity, gear-driven oil pump drawing from a wet-sump oil pan. Standard road configuration oil capacity is 18.0 Liters (4.75 gallons), whereas deep-sump marine setups hold up to 25.0 Liters (6.6 gallons). The engine relies on full-flow filtration through a spin-on element, coupled with an integrated water-cooled oil cooler block. Under standard operating parameters at full throttle, normal operating oil pressure ranges between 40 psi and 60 psi. Because engine oil serves as the hydraulic medium for the fuel system, using the correct viscosity and maintaining a strict 250-hour or 10,000-mile change interval is mandatory to prevent premature hydraulic component failure.

Engine temperature regulation is achieved via a closed-loop liquid cooling system. Coolant circulation is forced by a belt-driven centrifugal jacket water pump, regulated by a conventional capsule-type thermostat. Refill capacity ranges from 13.2 Liters (3.5 gallons) on standard automotive configurations up to 28.0 Liters (7.4 gallons) on heat-exchanger-equipped marine units. Air charge cooling is handled via an Air-to-Air Aftercooler (ATAAC) on land configurations or a sea-water jacketed aftercooler on marine variants to lower air inlet temperatures prior to combustion chamber entry.

General Technical Data

Engine Specifications

Engine Model ............... Caterpillar 3126B
Engine Type ............... 4-cycle inline 6-cylinder turbocharged liquid-cooled diesel
Number of Cylinders ............... 6
Cylinder Arrangement ............... In-line
Engine Bore, in (mm) ............... 4.33 (110.0)
Engine Stroke, in (mm) ............... 5.00 (127.0)
Displacement, cu.in (L) ............... 439 (7.2)
Rated Engine Power (Truck), hp (kW) ............... 190-330 (142-246)
Rated Engine Power (Industrial), hp (kW) ............... 210-330 (157-246)
Rated Engine Power (Marine), hp (kW) ............... 350-450 (260-333)
Rated Engine Speed, rpm ............... 2200-2400
Peak Torque ............... 700-1050 Nm (516-774 lb-ft) at 1400-1600 rpm
Aspiration Type ............... Turbocharged
Operating Speed ............... 2000-2400 rpm
Idle Speed ............... 650-750 rpm
Compression Ratio ............... 17.5:1
Firing Order ............... 1-5-3-6-2-4
Crankshaft Type ............... Forged steel
Piston Type ............... Cast-aluminum alloy
Con Rod Type ............... Forged steel
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 ............... Gear-driven, centrifugal
Air Cleaner Type ............... Dry two-stage
Starting System ............... Electric Starter
Electrical System ............... 12V or 24V
Alternator Rating, Amps ............... 75-120

Fuel System

Fuel System ............... High-Pressure Electronic Unit Injection (HEUI)
Fuel Pump Type ............... Gear-type, transfer
Governor Type ............... Electronic
Injection Pressure, bar ............... 800-1100
Fuel Type ............... Diesel Fuel

Dimensions and Weight

Overall Length, in (mm) ............... 41 (1040)
Overall Width, in (mm) ............... 28.5 (725)
Overall Height, in (mm) ............... 36.5 (925)
Dry Weight, lbs (kg) ............... 1430-1760 (650-800)

Maintenance Data

Service Information

Thermostat begins opening ............... 82C (180F)
Thermostat fully open ............... 95C (203F)
Cooling system pressure ............... 70-90 kPa (10-13 psi)
Operating oil pressure ............... 300-450 kPa (44-65 psi)
Idle oil pressure ............... 100-150 kPa (15-22 psi)
Intake Valve Clearance, in (mm) ............... 0.015 (0.38)
Exhaust Valve Clearance, in (mm) ............... 0.025 (0.64)

Engine Oil and Coolant

Engine Oil Type .................. SAE 10W-30, 15W-40
API Service Classifications ......................... CH-4, CG-4, CI-4, or higher
Engine Oil Capacity, qts (L) .................. 18.9-26.2 (18.0-25.0)
Coolant Capacity, qts (L) ............... 21.0-42.0 (20.0-40.0)

Service Intervals

Change oil and filter ............... Every 250 hours
Replace fuel filters ............... Every 500 hours
Inspect air filter ............... Every 100 hours
Change coolant ............... Every 2 years
Valve-lash adjustment ............... Every 1000 hours
Injector inspection ............... Every 1000-1500 hours
Compression test ............... Every 2000 hours
Drain water separator ............... As required
Belt inspection ............... As required

Service Specifications

Cylinder Bore Inside Diameter, in (mm) ................ 4.3307-4.3323 (110.000-110.040)
Crankshaft Journal Outside Diameter, in (mm) ................ 3.4642-3.4650 (87.990-88.010)
Crank Pin Outside Diameter, in (mm) ................ 2.9524-2.9531 (74.990-75.010)
Crankshaft End Play, in (mm) ................ 0.0030-0.0130 (0.070-0.320)
Intake Valve Stem Outside Diameter, in (mm) ................ 0.3126-0.3134 (7.940-7.960)
Exhaust Valve Stem Outside Diameter, in (mm) ................ 0.3118-0.3126 (7.920-7.940)
Intake Valve Guide Inside Diameter (Installed), in (mm) ................ 0.3146-0.3161 (7.990-8.030)
Exhaust Valve Guide Inside Diameter (Installed), in (mm) ................ 0.3146-0.3161 (7.990-8.030)
Intake Valve Stem to Guide Clearance, in (mm) ................ 0.0012-0.0035 (0.030-0.090)
Exhaust Valve Stem to Guide Clearance, in (mm) ................ 0.0020-0.0043 (0.050-0.110)
Intake Valve Spring Free Length, in (mm) ................ 2.441 (62.00)
Exhaust Valve Spring Free Length, in (mm) ................ 2.441 (62.00)
Intake Cam Height, in (mm) ................ 2.6322 (66.857)
Exhaust Cam Height, in (mm) ................ 2.5855 (65.671)
Piston Pin Outside Diameter, in (mm) ................ 1.5745-1.5748 (39.993-40.000)
Piston Pin to Piston Bore Clearance, in (mm) ................ 0.0003-0.0011 (0.008-0.027)
Second Ring Side Clearance, in (mm) ................ 0.0024-0.0043 (0.060-0.110)
Oil Ring Side Clearance, in (mm) ................ 0.0016-0.0035 (0.040-0.090)
Top Ring End Gap, in (mm) ................ 0.0138-0.0217 (0.350-0.550)
Second Ring End Gap, in (mm) ................ 0.0236-0.0315 (0.600-0.800)
Oil Ring End Gap, in (mm) ................ 0.0098-0.0217 (0.250-0.550)

Tightening Torque Specs

Cylinder Head Bolts ............... 170-190 Nm (125-140 lb-ft)
Main Bearing Bolts ............... 200-230 Nm (148-170 lb-ft)
Connecting Rod Bolts ............... 120-140 Nm (89-103 lb-ft)
Flywheel Bolts ............... 180-210 Nm (133-155 lb-ft)
Crankshaft Pulley Bolts ............... 350-400 Nm (258-295 lb-ft)
Oil Drain Plug ............... 40-55 Nm (30-41 lb-ft)
Injector Hold-Downs ............... 25-30 Nm (18-22 lb-ft)

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• Caterpillar Engines Specs
  
• Cat 3126 DITA Specs
  
• Cat 3126E Specs
  
• Cat 3176 Specs
  
• Cat 3176B Specs
  
• Cat 3176C Specs
  

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