Detroit 671 Diesel Engine Overview

The Detroit 671 diesel engine stands as a cornerstone of two-stroke diesel technology. As part of the iconic Series 71 family from Detroit Diesel, the 6-cylinder 671 configuration—often casually referred to as the "6-71" or just "671"—earned a place in both industrial and marine engine history. With a 426 cubic inch displacement, roots-type blower, and reliable two-stroke operation, this engine powered everything from fishing boats to generator sets. This article explores the engine's history, specifications, blower function, applications, rebuild options, and ongoing maintenance.

History and Evolution of the Detroit 671

Origins of the Series 71 Two-Stroke Design

Detroit Diesel launched the Series 71 engine line in the late 1930s to provide compact, powerful engines for military and industrial use. Each cylinder displaced 71 cubic inches, which gave rise to the model naming conventions: 3-71, 4-71, 6-71, and so on. The Detroit 671 specifically refers to the 6-cylinder version with inline cylinder arrangement.

Two-stroke design was chosen for its simplicity, compactness, and power density. Unlike four-stroke engines that require separate strokes for intake and exhaust, the two-stroke Series 71 design handled both with help from a mechanically driven blower. The result was a compact, torquey engine that was easy to maintain and could run reliably for thousands of hours in harsh conditions.

Transition from 6-71 to "671" Nomenclature

Originally labeled "6-71" to indicate six cylinders at 71 cubic inches each, the name eventually morphed into "671," especially within the marine community. The shorthand "671 Detroit" became common among boaters, mechanics, and industrial operators alike.

While technically referring to the same engine, the 671 and 6-71 designations can be used interchangeably. You'll find rebuild kits, parts catalogs, and engine blocks using either label, depending on the source. The name Detroit Diesel 671 remains a hallmark in heavy-duty diesel applications.

Technical Specifications of the Detroit 671

2-Stroke Cycle, Displacement, and Cylinder Configuration

The Detroit 671 features a two-stroke cycle with six inline cylinders, each displacing 71 cubic inches. The total displacement is 426 cubic inches (7.0 liters). Bore measures 4.25 inches, and stroke is 5 inches.

Constructed with a cast-iron block and cylinder head, the engine uses unit injectors operated by a gear-driven camshaft. A gear train also drives the Roots-type blower and various accessory drives. The engine is naturally aspirated through its blower but can also be turbocharged.

Roots Blower Functionality and Requirements

A key component of any 671 engine is the Roots-type blower. This mechanically driven air pump supplies the fresh air needed for combustion and scavenging. Since the 671 is a two-stroke diesel, it does not have dedicated intake and exhaust strokes. Instead, the blower ensures that every combustion cycle receives adequate oxygen while purging the cylinder of exhaust gases.

The blower mounts directly to the front of the engine and is powered by the crankshaft. Most 671 blowers operate at a drive ratio of around 1:1.2 to engine speed, using two lobed rotors that spin in opposite directions. These rotors do not compress air internally but force it through the intake ports under slight pressure, typically around 6 to 8 psi in naturally aspirated configurations.

Horsepower, Torque, and Turbocharging Options

In its naturally aspirated form, the Detroit 671 produces around 230 horsepower at 2,100 rpm and 440 lb-ft of torque at 1,600 rpm. The turbocharged version, commonly referred to as the 671T or 6-71T, boosts output to 300–350 horsepower, depending on fuel settings and boost levels.

Turbochargers mount directly above the blower, feeding pressurized air into the inlet. Intercooling is rare on older marine models but can be added for additional power and thermal control. The combination of turbo and blower increases cylinder filling efficiency, resulting in higher torque curves.

Blower Deep Dive: Maintenance and Parts

Why the Roots Blower is Essential to 671 Operation

Without the blower, the 671 cannot run. The blower not only fills the cylinders with fresh air but also scavenges exhaust gases out. Proper timing between blower operation and exhaust port closure is crucial. A worn or misaligned blower leads to smoking, poor fuel economy, and hard starts.

The blower’s internal clearance is measured in thousandths of an inch. Excessive wear leads to air leakage and poor scavenging. Proper lubrication, regular inspection, and timely replacement of seals and bearings are necessary to maintain engine performance.

Blower Rebuild Kits and Cost Considerations

Rebuilding the Detroit 671 blower typically includes new rotors (if required), shaft seals, bearings, timing gears, and a fresh gasket set. Costs for a quality rebuild kit range between $1,200 and $1,800. Labor costs vary depending on access and mechanical skill.

When sourcing kits, make sure they include OEM-grade parts or high-quality aftermarket components. Rotor-to-case clearance and gear lash must be measured precisely. Rebuilds often require specialized tools like feeler gauges and micrometers.

Marine and Industrial Applications of the 671

Marine Installations and Performance Considerations

In the marine world, the Detroit 671 was the backbone of commercial fishing boats, tugboats, and pleasure craft. Marine variants featured heat exchangers for cooling, raw-water pumps, and salt-resistant exhaust manifolds.

Typical cruise RPM is around 1,800, with peak power at 2,100. Trolling speeds may sit closer to 1,000–1,200 rpm. Most installations use a 40 GPM raw-water pump and dual thermostats for thermal balance. Marine configurations also included special oil pans and mounts for below-deck installation.

Industrial and Generator Set Uses

Industrial users favor the 671 for generator sets, water pumps, and skid-mounted powerplants. These engines are often housed in metal enclosures with belt-driven accessories, radiator-cooled systems, and electric start.

Standby-rated generator models can deliver up to 230 kW at 1,800 rpm. Belt layouts support alternators, coolant pumps, and compressors. These units are prized for their cold-start capability and resistance to dust, heat, and vibration.

Standby-rated generator models can deliver up to 230 kW at 1,800 rpm. Belt layouts support alternators, coolant pumps, and compressors. These units are prized for their cold-start capability and resistance to dust, heat, and vibration.

Parts, Rebuild Kits, and Availability

Common Rebuild Kit Components for the 671

A full rebuild kit for the Detroit 671 typically includes:

Additional components like governors, water pumps, and accessory drives may need replacement depending on wear. Matching serial numbers ensures compatibility.

Where to Find Detroit 671 Engines and Parts for Sale

Detroit 671 engines remain available from a variety of sources:

Detroit 671 engines remain available from a variety of sources:

Recommended Oil Type and Change Intervals

The Detroit 671 runs best on SAE 30W or 40W diesel-rated oil (API CI-4 or higher). Marine applications often require straight 40W for better high-temp stability. Oil changes should occur every 250 to 500 hours of operation. On-highway versions may go 5,000 to 7,500 miles between changes, depending on load and duty cycle.

Oil sump capacity is approximately 15 quarts. Filters should be replaced with every oil change. Many operators also install bypass oil filtration systems to extend oil life and reduce engine wear.

Injector, Valve Lash, and Fuel System Checks

Injector tips should be inspected for carbon buildup every 500 hours. If power drops or fuel consumption spikes, poor injector spray may be to blame. Mechanical tappet valve lash should be adjusted every 1,000 hours to factory specs (usually .016 to .018 inches cold).

Fuel filters and water separators must be replaced every 250 hours. Two-stroke operation relies on precise fuel timing, so it’s critical to verify that injection timing gears are aligned and functioning.

Cooling System Care (Water Pump, Heat Exchanger)

For marine models, the freshwater pump should be inspected every 500 hours. Impellers wear over time and may cause overheating if ignored. Raw-water pumps should have impellers replaced seasonally, and heat exchangers must be pressure-tested annually.

Use a 50/50 mix of ethylene glycol-based coolant for corrosion prevention. Always maintain the correct pressure cap rating and monitor coolant color for signs of oil contamination or cavitation.

Final Thoughts

The Detroit Diesel 671 remains a benchmark in diesel engine design. Its two-stroke operation, reliable blower system, and widespread industrial and marine applications have cemented its legacy. With careful maintenance and quality parts, these engines can easily exceed 20,000 hours of operation.

Whether you’re restoring a vintage fishing boat, running an off-grid generator, or sourcing parts for an industrial unit, the Detroit 671 offers a dependable and serviceable platform. Explore trusted sources for rebuild kits, OEM components, and used engines to keep your 671 running strong for years to come.

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