Caterpillar D337

The Caterpillar D337 is a standalone mechanical marine propulsion and auxiliary diesel engine, not a mobile machine — it has no chassis, undercarriage, or work tools of its own. It is an inline-6, four-stroke, mechanically injected diesel with no electronic control module, part of Caterpillar's pre-1980 D3-family of industrial and marine sixes alongside the D311, D315, D318, D326, D330, D333, D334, and D336. Original production used a Roots-type blower for aspiration and was rated about 168 kW (225 hp) at 1800 rpm; later production switched to a turbocharger, with output climbing through revisions to roughly 224, 239, and finally 257 kW (300, 320, and finally 345 hp), all still at 1800 rpm. Caterpillar's factory literature documents the turbocharged marine build under the D337F designation on the same base block. Dry weight, displacement, and bore/stroke were not carried forward into surviving public documentation, but the engine sits in the same era and physical scale as Caterpillar's other 1950s-60s D3-family industrial/marine sixes. Configurations offered included direct propulsion through a marine reverse/reduction gear, or auxiliary duty driving a generator or pump through an accessory drive, with either raw-water (heat exchanger) or keel-cooled cooling arrangements depending on the installation.

The biggest change across the D337's production run was the aspiration switch from Roots blower to turbocharger, which lifted output and reportedly improved reliability at the same time — owners and mechanics who have run both generations rate the turbocharged build as the stronger used engine of the two. Reduction/reverse gears paired with these engines often came from outside gearbox builders rather than Caterpillar itself, so gear parts support can lag well behind the engine side. None of this changes engine identity by model number alone, which is why serial prefix and induction type matter more than the D337 name when sourcing parts or judging a used unit. In today's market this engine turns up in vintage vessel repowers, stationary auxiliary power, and small industrial drive applications where buyers specifically want a fully mechanical, ECM-free diesel that a shop can service without dealer diagnostic tools. Parts and rebuild support now leans on machine shops experienced with vintage Cat mechanical injection and blower/turbo work rather than a standard dealer counter, so confirming a rebuilder's familiarity with this specific block is as important as confirming the engine's own condition.

Below: full specifications, fluids & capacities, the factory service schedule, common service parts, verified fault codes, what owners discuss, attachment guidance, the complete assembly directory, and a serial-number reference. Complete parts lists with full OEM part numbers, exploded diagrams, quantities, and fitment data are available free in Heavy Parts AI.