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Caterpillar 3406A Engine

The Caterpillar 3406A represents the foundational first generation of the 3406 engine family, introduced in the 1970s as a replacement for the 1674 and 1693 series. It is a fully mechanical, heavy-duty inline-six diesel engine designed for severe-duty vocational, highway truck, marine, and industrial markets. Characterized by extreme structural durability and complete independence from electrical controls, the 3406A relies on a rugged cast iron block and a gear-driven mechanical powertrain. While it lacks the electronic efficiency and advanced emissions controls of later iterations, its absolute simplicity, rebuildable wet-sleeve design, and straightforward field repairability make it a legendary workhorse in heavy industry.

The 3406A features a completely mechanical direct or pre-combustion chamber fuel injection system, famously utilizing a multi-plunger mechanical fuel injection pump mounted on the left side of the engine block. A mechanical fuel transfer pump draws fuel from the supply tank, routing it through a primary water separator and dual spin-on fuel filters to the injection pump gallery. Driven by the engine timing gears, the injection pump times and pressurizes individual fuel charges, sending them through external, high-pressure steel lines to individual fuel injectors. An integrated, flyweight-style mechanical governor regulates fuel delivery across the RPM band based on throttle position. Acceleration smoke is managed via a mechanical fuel ratio control device that restricts rack travel until the turbocharger develops sufficient intake manifold boost pressure.

The lubrication loop utilizes a full-pressure, wet-sump system driven by a gear-type oil pump mounted internally to the bottom of the engine block. The pump draws engine oil from the pan through a rigid suction screen and forces it directly into an integrated engine oil cooler. After cooling, the fluid passes through a spin-on, full-flow filter assembly equipped with an internal bypass valve to ensure constant lubrication if the filter plugs. Filtered oil feeds into the main block gallery, distributing pressure to the main crankshaft bearings, connecting rod journals, and camshaft bearings. Internal oil jets spray the undersides of the pistons for thermal control, while top-side drilled passages feed the rocker arm assembly and overhead valve train. A dedicated external line feeds high-pressure oil to the turbocharger sleeve bearings, draining back via gravity into the oil pan.

The cooling loop is a pressurized, high-capacity system forced by a gear-driven centrifugal water pump located on the front timing gear housing. The water pump forces coolant directly into the engine block jacket, where it flows upward through the cylinder head decks to absorb combustion heat. The flow path splits to circulate fluid through the integrated engine oil cooler and, on configurations with charge-air cooling, through a jacket-water aftercooler. Temperature management is governed by dual mechanical thermostats positioned in a housing at the top front of the cylinder head. During cold startup, the thermostats remain closed, forcing coolant through a bypass tube back to the inlet side of the water pump. When operating temperature is achieved, the thermostats open to redirect the hot fluid out to the radiator or marine heat exchanger.

General Technical Data

Engine Specifications

Engine Model ............... Caterpillar 3406A
Engine Type ............... 4-cycle inline 6-cylinder direct injection liquid-cooled diesel
Number of Cylinders ............... 6
Cylinder Arrangement ............... In-line
Engine Bore, in (mm) ............... 5.39 (137.0)
Engine Stroke, in (mm) ............... 6.49 (165.0)
Displacement, cu.in (L) ............... 893 (14.6)
Rated Engine Power (Industrial), hp (kW) ............... 250-425 (185-300)
Rated Engine Power (Marine), hp (kW) ............... 285-365 (213-272)
Rated Engine Speed, rpm ............... 1800-2100
Peak Torque ............... 1600-1900 Nm (1180-1400 lb-ft) at 1200-1400 rpm
Idle Speed ............... 600-650 rpm
Aspiration Type ............... Turbocharged and Air-to-Air Aftercooled (ATAAC)
Operating Speed ............... 2000 rpm
Compression Ratio ............... 14.5:1-15.5:1
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 dual-stage
Starting System ............... Electric Starter
Electrical System ............... 12V or 24V
Alternator Rating, Amps ............... 45-100

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) ............... 59.1-65.0 (1500-1650)
Overall Width, in (mm) ............... 35.4-41.3 (900-1050)
Overall Height, in (mm) ............... 43.3-49.2 (1100-1250)
Dry Weight, lbs (kg) ............... 2860-3190 (1300-1450)

Maintenance Data

Service Information

Thermostat begins opening ............... 82C (180F)
Thermostat fully open ............... 99C (210F)
Cooling system pressure ............... 70-100 kPa (10-15 psi)
Operating oil pressure ............... 345-550 kPa (50-80 psi)
Idle oil pressure ............... 140-275 kPa (15-30 psi)
Intake Valve Clearance, in (mm) ............... 0.015-0.017 (0.38-0.43)
Exhaust Valve Clearance, in (mm) ............... 0.025-0.027 (0.64-0.69)

Engine Oil and Coolant

Engine Oil Type .................. SAE 10W-30, 15W-40
API Service Classifications ......................... CD/CE, CH-4 or CI-4
Engine Oil Capacity, qts (L) .................. 36.7-44.1 (35.0-42.0)
Coolant Capacity, qts (L) ............... 31.5-47.3 (30.0-45.0)

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) ................ 5.400-5.402 (137.16-137.21)
Crankshaft Journal Outside Diameter, in (mm) ................ 4.7500-4.7512 (120.650-120.680)
Crank Pin Outside Diameter, in (mm) ................ 3.5000-3.5012 (88.900-88.930)
Crankshaft End Play, in (mm) ................ 0.005-0.021 (0.13-0.53)
Intake Valve Stem Outside Diameter, in (mm) ................ 0.3715-0.3725 (9.436-9.462)
Exhaust Valve Stem Outside Diameter, in (mm) ................ 0.3705-0.3715 (9.411-9.436)
Intake Valve Guide Inside Diameter (Installed), in (mm) ................ 0.3735-0.3745 (9.487-9.512)
Exhaust Valve Guide Inside Diameter (Installed), in (mm) ................ 0.3735-0.3745 (9.487-9.512)
Intake Valve Stem to Guide Clearance, in (mm) ................ 0.0010-0.0030 (0.025-0.076)
Exhaust Valve Stem to Guide Clearance, in (mm) ................ 0.0020-0.0040 (0.051-0.102)
Intake Valve Spring Free Length, in (mm) ................ 2.49 (63.2)
Exhaust Valve Spring Free Length, in (mm) ................ 2.49 (63.2)
Intake Cam Height, in (mm) ................ 2.285-2.297 (58.04-58.34)
Exhaust Cam Height, in (mm) ................ 2.245-2.257 (57.02-57.32)
Piston Pin Outside Diameter, in (mm) ................ 2.0000-2.0004 (50.800-50.810)
Piston Pin to Piston Bore Clearance, in (mm) ................ 0.0004-0.0012 (0.010-0.030)
Top Ring Side Clearance, in (mm) ................ 0.0035-0.0055 (0.089-0.140)
Second Ring Side Clearance, in (mm) ................ 0.0025-0.0045 (0.064-0.114)
Oil Ring Side Clearance, in (mm) ................ 0.0015-0.0035 (0.038-0.089)
Top Ring End Gap, in (mm) ................ 0.0200-0.0340 (0.508-0.864)
Second Ring End Gap, in (mm) ................ 0.0180-0.0300 (0.457-0.762)
Oil Ring End Gap, in (mm) ................ 0.0150-0.0270 (0.381-0.686)

Tightening Torque Specs

Cylinder head bolts ................ 270-310 Nm (199-229 lb-ft)
Main bearing caps ................ 400-450 Nm (295-332 lb-ft)
Connecting rods ................ 155-175 Nm (114-129 lb-ft)
Flywheel bolts ................ 380-420 Nm (280-310 lb-ft)
Crankshaft pulley bolt ................ 450-500 Nm (332-369 lb-ft)
Oil drain plug ................ 50-70 Nm (37-52 lb-ft)
Injector hold-downs ................ 35-45 Nm (26-33 lb-ft)

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• Caterpillar Engines Specs
  
• Cat 3306B Specs
  
• Cat 3306C Specs
  
• Cat 3306TA Specs
  
• Cat 3406 DITA Specs
  
• Cat 3406B Specs
  

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