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Detroit Diesel 12V71 Marine Long Block
Overview
The Detroit Diesel 12V71 marine long block is the heart of one of the most iconic two-stroke diesel engines ever built. Designed for heavy-duty propulsion and auxiliary marine applications, the 12V71 combines twelve cylinders arranged in a V configuration with a robust two-cycle design that delivers exceptional torque, reliability, and crisp throttle response under load. These engines earned their reputation powering commercial vessels, yachts, and workboats where durability and consistent performance were critical.
A long block is the partially assembled engine core that includes all major internal and external rotating components, such as the block, crankshaft, camshafts, pistons, liners, and cylinder heads, up to—but not including—the bolt-on accessories. This configuration provides a strong mechanical foundation while allowing flexibility to reuse or upgrade ancillary components like fuel systems, cooling hardware, and accessories specific to each vessel.
For marine operators, choosing a long block is often the most efficient path to restoring engine reliability without the complexity of a full turnkey engine replacement. It allows the vessel’s powerplant to be rebuilt to factory performance standards while maintaining compatibility with existing onboard systems. When sourced through Diesel Pro Power, the 12V71 marine long block is built using high-quality components and expert assembly practices, helping ensure long service life, dependable operation, and confidence once the engine is back in service.
What the Marine Long Block Includes
A typical Detroit Diesel 12V71 long block assembly contains:
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Cylinder block (V-type, 12 cylinders) – cleaned, machined, and pressure-tested for cracks or warpage.
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Crankshaft assembly – balanced and inspected, with new main bearings installed.
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Connecting rods and pistons – fitted with new piston rings, liners, and retainers for even compression.
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Camshaft and timing gears – precision-timed to factory specification for correct two-stroke sequencing.
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Cylinder heads – rebuilt or new, fitted with valves, guides, springs, seals, injector tubes, and bridges.
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Blower assembly – the positive-displacement air pump that feeds the cylinders with scavenging air.
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Valve train hardware – rocker arms, pushrods, adjusting screws, and bridges for each bank.
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Oil pump and internal lubrication components – ensuring full-pressure oiling throughout the block.
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Gasket sets and seals – installed throughout to prevent internal and external leaks.
This configuration allows a marine mechanic or yard to install accessories (water pumps, injectors, fuel lines, turbo, alternator, governor, etc.) and complete the engine without performing full machining work in-house.
Diesel Pro Power Parts Used to Build the 12V71 Long Block
To assemble a true factory-spec Detroit Diesel 12V71 marine long block, Diesel Pro Power supplies every major internal and external part, including:
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Remanufactured or new engine block, machined and ready for assembly.
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Cylinder liners, piston kits, wrist pins, and retainers.
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Main bearings and thrust washers to ensure crankshaft stability.
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Block breather pads, retainers, and gaskets for crankcase ventilation.
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Complete cylinder head assemblies for all twelve cylinders.
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Valves, guides, seats, springs, and seals.
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Rocker arms, shafts, and bridges precisely matched to head geometry.
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Head bolts and gasket sets for correct torque sealing.
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Roots-type blower assembly with matched rotors, bearings, and seals.
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Optional turbo charger system for marine-turbo 12V71 versions.
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Air intake silencer, blower gaskets, and mounting hardware.
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Fuel injectors, fuel lines, and governor parts (optional for full builds).
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Oil pump assembly, pickup tube, and pressure relief valve.
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Freshwater pump (circulating pump) and raw water pump (sea pump) assemblies.
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Thermostats, gaskets, and coolant transfer tubes for closed cooling.
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Oil cooler core and mounting seals.
Internal Rotating Group
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Crankshaft (balanced and magnaflux-inspected).
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Connecting rods with new bushings and rod bearings.
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Camshaft, gears, idler shafts, and timing cover gaskets.
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Flywheel housing and rear seal assembly.
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Complete overhaul gasket set, including head gaskets, manifold gaskets, oil seals, blower gaskets, and timing cover gaskets.
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O-rings, sealing washers, and mounting hardware for every mating surface.
Why the 12V71 Marine Long Block Is Special
The 12V71 is a proven design built on Detroit Diesel’s legendary two-stroke platform. For marine use, it offers:
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Exceptional power-to-weight ratio – delivering steady torque in propulsion or auxiliary roles.
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Compact design – ideal for engine rooms where space and access are limited.
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Ease of service – the two-stroke layout allows simpler maintenance compared to larger four-stroke diesels.
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Long lifespan – when rebuilt with Diesel Pro Power parts, these engines continue performing for thousands of hours under load.
Why Buy or Build Through Diesel Pro Power
Complete Parts Coverage
Diesel Pro Power offers comprehensive parts support for the Detroit Diesel 12V71 marine engine, covering every internal and external component required for maintenance, overhaul, or full rebuilds. From pistons, liners, and cylinder heads to water pumps, blowers, gaskets, and accessory components, customers can source everything from one trusted supplier—eliminating delays and compatibility issues.
Factory-Quality Fitment
All components supplied by Diesel Pro Power are precision-machined and carefully inspected to meet Detroit Diesel OEM tolerances. Proper clearances and consistent quality are critical on two-stroke engines, and our parts are selected to ensure correct fitment, reliable sealing, and long service life in marine duty applications.
Expert Technical Support
Detroit Diesel 12V71 engines exist in multiple configurations, including naturally aspirated and turbocharged marine setups, as well as left- and right-hand rotation. Diesel Pro Power’s experienced technicians help identify the correct configuration and parts selection, reducing costly mistakes and installation issues during the rebuild process.
Fast, Global Shipping
Downtime is expensive. Diesel Pro Power ships worldwide to over 180 countries, with same-day shipping available on most in-stock items. Our logistics experience ensures parts arrive quickly, securely packaged, and ready for installation—whether the engine is dockside or in a rebuild facility.
Proven Reputation
With more than 40,000 satisfied customers worldwide, Diesel Pro Power has earned a reputation for reliable parts, knowledgeable support, and consistent results. Professionals across marine, industrial, and power generation sectors trust Diesel Pro Power when rebuilding critical Detroit Diesel engines.
