Komatsu 6D125-1AB-N(S/N 47990-UP)

The 6D125-1AB-N (S/N 47990-UP) is a Komatsu 125-series inline-six diesel engine arrangement, not a standalone machine - an 11.05 L (674 cu in), 125 x 150 mm bore and stroke mechanical diesel rated 116 kW (155.6 hp) at 2200 rpm, run by an inline injection pump and a mechanical flyweight governor with no electronic control module. Komatsu factory-fitted this arrangement across its GD6xx wheeled motor grader line (GD623A-1, GD625A-1, GD655A-3, GD663A-2) and reportedly into contemporary WA3xx-class wheel loaders - always on wheeled chassis, since as a bare engine it carries no undercarriage of its own. Host graders in this power class typically run in the 13.5-14.5 tonne (30,000-32,000 lb) operating-weight range, though exact weight belongs to the host machine and its options, not the engine. This -1 generation mechanical diesel dates to Komatsu's 1980s-into-1990s pre-electronic era. The factory parts book for this serial block covers two build configurations: a standard naturally aspirated engine for sea-level work, and a high-altitude package that adds a turbocharger, turbo oil piping, and a high-altitude cylinder head and block to hold power at elevation.

Across the wider 6D125-1 family, Komatsu split this displacement class by application and configuration: related arrangements on dozer chassis run a lower-output, naturally aspirated tune, while grader and loader arrangements like this one run the higher 116 kW tune, and the catalog carries both mechanical- and electrical-governor variants alongside the sea-level/high-altitude split described above - treat any spec sheet for a different 6D125-1 arrangement as a different configuration, not a direct match. In the used and parts market, this arrangement matters because it sits on wheeled grading and loading equipment that stayed productive long after Komatsu shifted newer machines to electronically controlled, common-rail engines: mechanical simplicity and field-serviceability, with no ECM or sensor network to fail, keep these graders and loaders working in fleets that value predictable, tool-free troubleshooting. Top-end components, the injection pump, and cooling-system parts remain the main wear items driving ongoing parts demand on this block.

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.