Komatsuengine(no series)
Komatsu 6D125-1AN(S/N 47990-UP)
Maintenance schedule, common problems & OEM parts breakdown
The Komatsu 6D125-1AN (S/N 47990-UP) is a water-cooled, wet-liner inline diesel engine (this arrangement is built to run turbocharged, though aspiration should be confirmed against the block's own arrangement plate) from Komatsu's mechanical-injection 125-class family, built to power Komatsu's A-series motor grader line of that era (GD605A-3, GD623A-1, GD655A-3, GD663A-2). Displacing 11.04 L (673.8 cu in) over a 125 mm x 150 mm (4.92 in x 5.91 in) bore and stroke, it is rated at approximately 134 kW (180 hp) gross and 116 kW (155 hp) net at 2,200 rpm on the grader platform it serves, running a centrifugal-governed inline injection pump with no electronic engine control. Host graders in this power and weight class run approximately 13,350-13,620 kg (29,430-30,030 lb) in base trim, rising to roughly 15,130-15,255 kg (33,360-33,640 lb) with a rear ripper fitted. The 1AN suffix is one of several factory arrangement codes built off the same 6D125-1 case, each calibrated for a different host chassis and accessory drive rather than marking a different engine generation.
Within this 47990-UP production block, Komatsu issued running changes rather than a new model: a revised injection nozzle, oil filter, and fan pulley specification from serial 51975 on, and a cylinder head update noted from serial 68353 on, both handled as in-line spec breaks under the same arrangement number rather than a new nameplate. The mechanical 6D125-1 family sits early in a longer engine lineage: it was succeeded by the 6D125E-2 and turbocharged S6D125E-2 generation, fitted to Komatsu's D65-class dozers through the mid-2000s, and later by the fully electronic, common-rail SAA6D125E series still used in current Komatsu excavators and dozers. Because the 1AN arrangement predates any engine control module, it remains a straightforward rebuild-and-return candidate for owners keeping older A-series graders running, and the shared cylinder head, injection components, and wet liners across the wider 6D125-1 sibling arrangements keep parts support and remanufactured cores available today, provided the arrangement number is matched before ordering.
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.
Komatsu 6D125-1AN(S/N 47990-UP) specifications
Engine
| Base architecture | Water-cooled, 4-stroke inline-6 diesel, direct injection, mechanical (non-electronic) fuel system — the shared 6D125-1 series block also underpins several sibling arrangements; this exact arrangement is fitted as the engine option in the Komatsu GD600-class motor grader line (GD605A/GD623A/GD655A/GD663A-2) |
| Displacement | 11.04 L (673.8 cu in / 11,040 cm3) |
| Bore x Stroke | 125 mm x 150 mm (4.92 in x 5.91 in) |
| Cylinders | 6, inline |
| Aspiration | Turbocharged, per the grader application this arrangement is built for; treat as indicative for this exact serial range and confirm the nameplate/arrangement number, since the plain (no S/SA prefix) designation elsewhere in this engine family denotes naturally aspirated |
| Power rating | Gross approximately 134 kW (180 hp), net approximately 116 kW (155 hp), at 2,200 rpm rated speed, per the grader platform's published engine rating |
| Fuel system / governor | Mechanical inline injection pump with a centrifugal (flyweight) governor — no electronic engine control |
| Emissions tier | Pre-emissions mechanical diesel design; predates EPA and EU nonroad emissions certification tiers, so no formal tier rating applies |
Weights
| Engine dry weight | Not reliably documented for this exact arrangement in accessible public literature — confirm with the applicable shop manual or dealer |
| Host machine operating weight | Approximately 13,350-13,620 kg (29,430-30,030 lb) in base configuration; approximately 15,130-15,255 kg (33,360-33,640 lb) with a ripper fitted, for the grader platform this engine arrangement serves |
| Ground pressure | Not published — this is a wheeled machine (motor grader), so ground pressure figures are not a standard published spec for this class |
Dimensions
| Overall length | Approximately 9.57 m (31.4 ft) |
| Overall height | Approximately 3.07-3.13 m (10.06-10.27 ft), depending on cab/ROPS configuration |
| Wheelbase (front axle to tandem center) | Approximately 6.00 m (19.7 ft) |
| Tandem wheelbase | Approximately 1.54 m (5.04 ft) |
| Track gauge | Approximately 2.07 m (6.79 ft) |
| Moldboard (blade) length | Approximately 3.71 m (12.2 ft) |
| Minimum turning radius | Approximately 7.1 m (23.3 ft) |
| Note | Dimensions reflect the grader platform this engine arrangement is built for; a confirmed single-model figure could not be cross-verified beyond the sources above, so treat as representative of the class rather than an exact nameplate figure for every configuration |
Performance
| Rated engine speed | 2,200 rpm |
| Articulation / shoulder reach | Approximately 815 mm (32.1 in) additional shoulder-reach through frame articulation |
| Travel speed (forward/reverse, all gears) | Not reliably documented for this exact model in accessible sources — consult the operator's manual for the geared travel-speed table |
| Gradeability | Not published in accessible sources for this exact arrangement |
Service capacities (summary)
| Engine oil (lubricating system refill) | Not reliably documented for this exact serial range — figures found in general 6D125-1-family literature were internally inconsistent and are not published here; confirm with the applicable shop manual or dealer before service |
| Cooling system (engine only) | Not reliably documented for this exact serial range — confirm with the applicable shop manual or dealer |
| Fuel tank | Not reliably documented for this exact model in accessible sources |
| Hydraulic system | Not reliably documented for this exact model in accessible sources — grader blade/steering hydraulic capacity depends on the specific chassis configuration |
Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.
6D125-1AN(S/N 47990-UP) fluids & capacities
| System | Capacity | Recommended fluid |
|---|---|---|
| Engine crankcase (with oil filter) | Not published in accessible factory literature for this exact serial range. The factory parts catalog for this engine lists both a standard and a large-capacity oil pan option, so refill volume differs by build configuration. Confirm against the host machine's own lubrication chart or dipstick markings. | Komatsu Genuine engine oil (EO): SAE 15W-40 for normal ambient temperatures; SAE 10W-30 for cold-weather starts (down to roughly -20°C/-4°F); SAE 30 acceptable in consistently hot climates. A commercial heavy-duty diesel oil meeting API CD/CE service class is an acceptable equivalent for this mechanically governed engine. |
| Cooling system (engine coolant) | Engine-only (block and head) water capacity is not documented for this serial range in accessible OEM literature. Total system volume, including radiator, is set by the host machine and is not a fixed figure at the engine level. | Komatsu Super Coolant (AF-NAC), supplied as concentrate and mixed 50% with clean water for standard year-round freeze and corrosion protection. Concentrate ratio can be increased, up to about 65%, for extreme cold climates; do not exceed the recommended ratio. |
| Fuel system | Fuel tank capacity is a host-machine specification, not an engine-level figure. It varies by the chassis this engine is installed in (motor grader vs. crawler dozer applications). | Diesel fuel meeting ASTM D975 No. 2-D (or equivalent JIS K2204) for normal operation. No. 1-D, or an equivalent low-cloud-point winter diesel, is recommended in cold weather to prevent fuel waxing. |
Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.
Komatsu 6D125-1AN(S/N 47990-UP) maintenance schedule
| Service interval | Tasks |
|---|---|
| Every 50 h |
|
| Every 250 h |
|
| Every 500 h |
|
| Every 1,000 h |
|
| Every 2,000 h |
|
| Every 4,000 h |
|
| Every 8,000 h |
|
Servicing the 6D125-1AN(S/N 47990-UP) beyond the schedule
Predictive maintenance & fluid analysis
With no ECM or fault-code history on this mechanical 1AN arrangement, scheduled oil sampling at every oil change is the main early-warning tool. Track wear-metal and coolant-in-oil trends, since the wet cylinder liners let cavitation or a seal leak show up in the oil before smoke or a gauge swing does. Watch the corrosion-resistor cartridge and coolant additive concentration alongside the oil sample data; a shift in either one usually points back to the same liner-seal or cooling-side issue, not two unrelated problems.
Corrective & common repairs
Most complaints on this arrangement trace to the mechanical fuel side or the cooling system, not electronics. Persistent black smoke under load usually means injection pump timing or delivery drift, not a bad nozzle; pop-test the nozzles and check pump calibration before swapping injectors. A weeping water pump shaft seal is a known wear point on this block; catch it early so it doesn't dilute the corrosion resistor's protection. A corroded oil pressure switch causing a false warning lamp is common on machines that sat outdoors.
Overhaul & rebuild points
Top-end wear on the 1AN centers on ring and liner glazing from air filter neglect on dusty grading sites, plus clogged piston-cooling oil jets that accelerate skirt scuffing. Run a compression or leak-down test before committing to a rebuild scope. At major overhaul, plan on replacing liners, rings, and the injector set together, recheck crankshaft and rod-bearing clearances while the block is open, and recalibrate or exchange the injection pump off-engine rather than adjusting timing by feel on a worn pump.
Seasonal & environment servicing
Cold starts on this pre-electronic engine depend on the cold-start aid fitted to the arrangement (glow plug or intake air heater) plus the correct seasonal oil viscosity and coolant concentrate ratio. Switch to a lower-viscosity engine oil and raise the coolant concentrate ratio for hard winters. Dusty grading sites are the bigger year-round risk: service the air cleaner ahead of the restriction indicator, since ingested dust glazes cylinder bores and clogs piston-cooling jets faster than hours-based wear alone would.
6D125-1AN(S/N 47990-UP) common service parts
| Part number | Part | Notes | |
|---|---|---|---|
| 6221-71-6201 | Fuel Filter Ass'Y | Fuel system service | Check fitment → |
| ND092220-0040 | Filter,Fuel | Fuel system service | Check fitment → |
| 09637-30161 | Plate? Instruction,Air Cleaner (Single Element) | Air intake service | Check fitment → |
| 600-211-5960 | Plate? Caution,Oil Filter | Lubrication service | Check fitment → |
| 6136-51-5120 | Cartridge,Engine Oil Filter | Lubrication service | Check fitment → |
Always confirm against your machine's serial number — cross-check any part in Heavy Parts AI before ordering.
6D125-1AN(S/N 47990-UP) fault codes & troubleshooting
| Code | Meaning | Likely cause | What to do |
|---|---|---|---|
| Engine Oil Pressure Lamp | Engine oil pressure has dropped below the safe minimum sensed by the pressure switch feeding the operator monitor panel. | Low oil level, clogged oil filter, worn oil pump or relief valve, or a failed pressure switch. | Stop the engine immediately, check oil level and condition, then inspect the pressure switch and oil pump before restarting. |
| Engine Coolant Temperature Lamp | Coolant temperature has exceeded the safe operating range as read by the coolant temperature sender. | Low coolant level, blocked radiator core, slipping or broken fan belt, stuck thermostat, or a faulty sender. | Idle down and stop safely, let the engine cool, then check coolant level and radiator/fan belt condition before continuing work. |
| Charge Lamp | Alternator is not charging the battery circuit. | Slipping or broken alternator belt, failed alternator, or a wiring/connector fault. | Check belt tension and alternator output, then inspect wiring and battery connections. |
| Air Cleaner Restriction Indicator | Intake air filter restriction has reached the service limit. | Clogged air filter element from dust ingestion. | Service or replace the air filter element and reset the restriction indicator. |
| Torque Converter Oil Temperature Lamp | Transmission/torque converter oil temperature has risen above the safe limit. | Low transmission fluid, restricted oil cooler, or sustained heavy load in a low gear. | Reduce load and let the converter cool, then check fluid level and cooler for restriction. |
| Fuel Level Lamp | Fuel tank level has reached the reserve mark. | Low fuel, or a faulty fuel level sender/float. | Refuel; if the lamp stays on after refueling, check the sender/float and wiring. |
Codes and remedies are general guidance for this model family — always confirm with diagnostic tooling and your dealer before major repairs.
All 6D125-1AN(S/N 47990-UP) assemblies by section
Every catalogued assembly group for the Komatsu 6D125-1AN(S/N 47990-UP). Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.
Engine
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Air Cleaner(#51975-)
| 01***40 | Bolt, Mount Master Brake Cylinder | 1 |
| 01***08 | Nut,(For Straight Tilt Dozer) (D65p) | 6 |
| 01***16 | Washer,Radio Controlled Slag Handling | 6 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Air Intake(#51975-)
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 10 |
| 01***55 | Bolt, Mount Tube To Switch Pump | 4 |
| 01***05 | Bolt, Mount Hose Clip | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Alternator (13a) (Inner Parts)(#51975-)
| 60***50 | Alternator A. (13a),(With Non Hardening Pulley) | 1 |
| KD***31 | Bushing,Insulator | 2 |
| KD***72 | Slip Ring Ass'Y | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Alternator Mounting (13a)(#51975-)
| 01***25 | Bolt,(Without Rops Guard? Canopy) | 1 |
| 01***75 | Bolt | 1 |
| 01***10 | Nut,(Dust?Drip Proof? Trimming S.) | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Camfollower Cover(#51975-)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 2 |
| 01***00 | Bolt, Mount Back-Up Alarm | 2 |
| 01***08 | Nut,(For Canvas Canopy?Head Guard) | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Camshaft(#51975-)
| 01***22 | Bolt | 2 |
| 04***25 | Key, Plain Woodruff | 1 |
| 04***12 | Ring,Snap (For Ripper) | 12 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Cooling Fan (Noise Supression Sepc.)(#51975-)
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 12 |
| 60***51 | Fan,Cooling | 1 |
| 61***70 | Spacer | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Corrosion Resistor(#51975-)
| 01***20 | Bolt,(For Quick Shift) (Mitsubishi) | 3 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 2 |
| 01***08 | Nut,(For Canvas Canopy?Head Guard) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Crank Pulley (Non Hardening Pulley)
| 01***30 | Screw | 1 |
| 01***35 | Screw | 5 |
| 04***28 | Pin, Dowel - Transmission Case, Pin-Dowel | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Crankshaft(#47990-63162)
| 61***10 | Bearing, Main (Standard) | 7 |
| 61***50 | Bearing, Thrust Assembly (Standard) | 1 |
| 61***10 | Bearing, Main (1.00mm Undersize) | 7 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Crankshaft(#63163-68350)
| 61***10 | Bearing, Main (Standard) | 7 |
| 61***50 | Bearing, Thrust Assembly (Standard) | 1 |
| 61***10 | Bearing, Main (1.00mm Undersize) | 7 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Crankshaft(#68351-)
| 61***10 | Bearing, Main (Standard) | 7 |
| 61***50 | Bearing, Thrust Assembly (Standard) | 1 |
| 61***10 | Bearing, Main (1.00mm Undersize) | 7 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Cylinder Block Gasket Kit(#51975-)
| 07***15 | O-Ring,Relief Valve | 1 |
| 07***20 | O-Ring,Lubricating Oil Piping | 1 |
| 07***25 | O-Ring (K2?K8) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Cylinder Block(#51975-)
| 01***12 | Bolt,(Without Air Compressor Type) | 2 |
| 01***45 | Washer,(For Travel Motor) (Not Show) | 14 |
| 04***14 | Pin, Dowel | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Cylinder Head Gasket Kit(#55719-)
| 02***43 | O-Ring,Water Manifold | 6 |
| 07***60 | O-Ring,(For Pc750?800?1800) (A2610) | 1 |
| 07***65 | O-Ring,(F4350-55a1 -27 ) | 6 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Cylinder Head(#47990-68353)
| 01***20 | Bolt (M10-1.75x120) | 6 |
| 04***20 | Pin,Dowel (Welded) | 2 |
| 61***30 | Sleeve,Holder | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Cylinder Head(#68354-)
| 01***20 | Bolt (M10-1.75x120) | 6 |
| 04***20 | Pin,Dowel (Welded) | 2 |
| 61***30 | Sleeve,Holder | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Damper
| 01***25 | Bolt,(Without Rops Guard? Canopy) | 6 |
| 61***00 | Damper Ass'Y | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Engine Mounting(#55719-)
| 14***10 | Support,Front | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Exhaust Manifold
| 01***80 | Screw | 18 |
| 07***08 | Plug,(Without Auto Priming System) | 1 |
| 61***20 | Spacer,(For Shipping) | 18 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fan Drive (Noise Supression Spec.)(#51975-)
| 01***14 | Nut,(For Slag Handling) | 1 |
| 01***60 | Washer,(Without Rops Guard? Canopy) | 1 |
| 04***25 | Ring,Snap (Kit) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fan Guard (1/2) (Noise Supression Spec.)(#51975-)
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
| 01***40 | Bolt, Mount Master Brake Cylinder | 2 |
| 01***70 | Bolt, Mount Bracket And Oil Inlet Tube, Bolt | 8 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fan Guard (2/2) (Noise Supression Spec.)(#51975-)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 7 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 8 |
| 01***35 | Bolt,(For Quick Shift) (Mitsubishi) | 3 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fan Pulley (Noise Supression Spec. Pulley Dia 212mm)(#51975-)
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 6 |
| 04***70 | V-Belt Set,(Noise Suppression Spec.) | 1 |
| 61***10 | Pulley?Non Hardening | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Flywheel Housing(#55719-)
| 01***50 | Bolt | 2 |
| 01***50 | Bolt, Radiator To Frame | 6 |
| 01***60 | Bolt, Hanger Bearing Mounting | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Flywheel(#51975-)
| 01***55 | Bolt | 6 |
| 01***45 | Washer,(For Travel Motor) (Not Show) | 6 |
| 06***08 | Bearing, Ball | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Front Cover(#51975-)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 1 |
| 01***65 | Bolt,(Except Japan) | 8 |
| 01***70 | Bolt, Mount Bracket And Oil Inlet Tube, Bolt | 4 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fuel Filter(#51975-)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 7 |
| 01***05 | Bolt, Mount Hose Clip | 1 |
| 01***20 | Screw | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fuel Injection Pump (Coupling) (197ps) (Inner Parts)(#51975-)
| ND***10 | Flange,Coupling | 1 |
| ND***50 | Bolt | 1 |
| ND***40 | Bolt | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fuel Injection Pump (Feed Pump) (197ps) (Inner Parts)(#51975-)
| ND***40 | Washer? 1mm,Plate | 1 |
| ND***70 | Washer,Union | 4 |
| ND***10 | Spring, Check Valve | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fuel Injection Pump (Governor) (1/2) (197ps) (Inner Parts)(#51975-)
| ND***40 | Plate,Thrust | 1 |
| ND***40 | Plug,Screw | 1 |
| ND***50 | Washer? 0.3mm | 0 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fuel Injection Pump (Governor) (2/2) (197ps) (Inner Parts)(#51975-)
| ND***80 | Pin,Stop | 1 |
| ND***70 | Arm,Spring | 1 |
| ND***20 | Washer? 1.0mm,Cover Plate | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fuel Injection Pump (Pump) (1/2) (197ps) (Inner Parts)(#51975-)
| 61***70 | Injection Pump A | 1 |
| ND***80 | Bearing,Center | 1 |
| ND***50 | Ring,Adjust | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fuel Injection Pump (Pump) (2/2) (197ps) (Inner Parts)(#51975-)
| ND***21 | Shim? 2.4mm | 0 |
| ND***91 | Shim? 2.25mm,Element Holder | 0 |
| ND***60 | Element,Pump | 6 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fuel Injection Pump Mounting (197ps)(#51975-)
| 01***75 | Bolt,(See Fig.0142) | 4 |
| 61***10 | Washer,Spring | 4 |
| (6***0) | Injection Pump A.,(See Fig.A4010-B3a1) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Fuel Return(#51975-)
| 01***20 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
| 01***60 | Bolt, Mount Tube To Switch Pump | 1 |
| 01***08 | Nut,(For Canvas Canopy?Head Guard) | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Head Cover
| 01***16 | Washer,Radio Controlled Slag Handling | 24 |
| 61***11 | Cover,Head? No.2?3?4?5?6 | 6 |
| 61***10 | Gasket,Cylinder Head Cover | 6 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Heater Switch(#51975-)
| 01***16 | Bolt, Mount Washer Tank To Bracket, Bolt | 2 |
| 01***55 | Bolt, Mount Tube To Switch Pump | 1 |
| 01***10 | Washer,(For 35a Alternator) | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Idler Gear
| 01***60 | Screw | 1 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 6 |
| 01***30 | Bolt,(With Body Positioner) | 6 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Injection Bracket And Piping(#51975-)
| 01***30 | Bolt | 4 |
| 01***55 | Bolt, Mount Tube To Switch Pump | 4 |
| 01***70 | Bolt, Mount Bracket And Oil Inlet Tube, Bolt | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Injection Nozzle(#51975-)
| 01***45 | Bolt, Divider Valve Mounting | 12 |
| 07***65 | O-Ring,Cylinder Block | 6 |
| 07***12 | Gasket,(Without Air Compressor Type) (K2) | 12 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Injection Pump Drive(#51975-)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 4 |
| 01***14 | Nut (Itl) | 1 |
| 01***60 | Washer,(Without Rops Guard? Canopy) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Muffler(#51975-)
| 01***25 | Bolt,(Sandy? Dusty Terrain Spec.) | 4 |
| 01***40 | Bolt,(Dust And Dirt Protection) | 4 |
| 01***45 | Bolt,(For Power Tilt Mech Angle) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Name Plate And Caution (English)(#51975-)
| 09***61 | Plate? Instruction,Air Cleaner (Single Element) | 1 |
| 09***01 | Plate? Instruction,(English) (Except Japan) | 1 |
| 60***60 | Plate? Caution,Oil Filter | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Nozzle Holder (Inner Parts)(#51975-)
| 07***12 | Gasket,Mechanical Pump Piping | 2 |
| 61***11 | Holder Ass'Y | 6 |
| ND***90 | Connector,Inlet | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Oil Cooler (3-Element Type)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 13 |
| 01***65 | Bolt - Return Line And Suction Linehose Clamps | 2 |
| 01***80 | Bolt,(With Corrosion Resistor) | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Oil Filter(#51975-)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 3 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
| 01***35 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Oil Gauge(#51975-)
| 01***16 | Bolt (With Air Heater & Conditioner) | 1 |
| 01***06 | Nut,(With Back-Up Alarm) (Op), Nut; Was 01580-10806 | 1 |
| 01***10 | Washer,See Fig. K4010-12a0 | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Oil Pan And Suction Tube(#51975-)
| 01***20 | Bolt,(Large Capacity Type) (Std) | 1 |
| 01***25 | Bolt,(Sandy? Dusty Terrain Spec.) | 1 |
| 01***35 | Bolt, Mounting - Flywheel Housing End | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Oil Pump
| 01***25 | Bolt,(Rear Attachment Less) | 4 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 3 |
| 01***35 | Bolt,(F4320-52a0 -5 ) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Piston
| 02***13 | Pin, Dowel | 2 |
| 04***18 | Ring, Snap | 12 |
| 61***33 | Piston Ring Ass'Y | 6 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Rocker Arm(#51975-)
| 01***08 | Nut,R.H. Thread | 12 |
| 01***32 | Washer,(With Auto Emergency Steering) | 40 |
| 61***10 | Housing, Rocker Lever | 6 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Starting Motor (7.5kw) (Inner Parts)(#51975-)
| 60***10 | Starting Motor Ass'Y,(7.5kw) | 1 |
| KD***60 | Shift Lever Assembly | 1 |
| KD***40 | Spring,Spring | 4 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Starting Motor Mounting (7.5kw)(#51975-)
| 01***40 | Bolt,(With Center Guard) | 3 |
| 08***19 | Band,(Without Air Conditioner) | 1 |
| 08***10 | Connector, Clip | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Thermostat Housing(#51975-)
| 01***16 | Washer,Radio Controlled Slag Handling | 1 |
| 08***10 | Connector, Clip | 1 |
| 60***10 | Thermostat,(Cold Terrain Spec.) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Water Drain Valve
| 61***12 | Valve,Water Drain | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Water Manifold
| 01***65 | Screw | 10 |
| 02***43 | O-Ring,Water Manifold | 6 |
| 61***11 | Manifold,Water | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Water Pump Gasket Kit(#55719-)
| 07***15 | O-Ring,(For Low Speed) (D40pf) | 1 |
| 07***15 | O-Ring,Scavenge Pump | 1 |
| 61***61 | Gasket (K2?K6) | 1 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Water Pump(#51975-)
| 01***30 | Bolt,(For France Noise Suppression) | 2 |
| 01***16 | Bolt (With Air Heater & Conditioner) | 6 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 6 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Water Separator(#51975-)
| 01***16 | Bolt, Battery To Frame Ground Wire Attaching | 1 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 4 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
Komatsu : 6d125-1an(S/N 47990-Up) : Engine : Water Temperature Sensor (With Corrosion Resistor? For Gauge Panel)(#51975-)
| 07***15 | Plug,(Used When Without Heater) | 1 |
| 07***08 | Plug,(Without Oil Pressure Switch) | 1 |
| 14***80 | Sensor,Thermo (Only For Gauge Panel) | 1 |
6D125-1AN(S/N 47990-UP) serial number reference
The ID plate for this engine is riveted to the block on the left side near the fuel injection pump/valve cover area and lists the model, serial number, and arrangement number; if the plate is gone, the serial is also stamped directly into the block casting. This is a bare engine model, not a machine chassis, so there is no 3-letter alpha PIN code to decode - just read the digits after "6D125-1AN" as the running production serial and compare it against the known breakpoints to see which running spec applies.
| Prefix | Identifies | Notes |
|---|---|---|
| 47990-UP | 6D125-1AN base production block | Start point of the S/N range covered by this engine build. Inline-6 diesel, direct injection, mechanical (non-electronic) fuel system, fitted as an engine option in the Komatsu GD663A-2 motor grader (the same GD600-class grader line also takes several sibling 6D125-1-family build suffixes). Aspiration is not confirmed for this exact build suffix: the plain "6D125-1" designation (no S/SA prefix) denotes naturally aspirated elsewhere in this engine family, but a turbo/aftercooled gross-vs-net power split has also been associated with this arrangement in grader-platform literature - confirm against the nameplate/arrangement number before ordering turbo-specific parts. This bare engine model carries no 3-letter alpha PIN prefix ahead of the running serial - Komatsu identifies it by the model/build suffix (1AN) plus a straight numeric production serial. |
| 51975-UP | In-family running change | Revised injection nozzle, oil filter, and a noise-suppression-spec fan pulley (212 mm / 8.3 in dia.) take effect from this point forward within the 47990-up block. Treat as a running spec split inside the same family, not a new model. |
| 68353-UP | Cylinder head running change | Factory parts listing for this exact model shows the cylinder head used from 47990 running through 68353, implying a head spec change after this point. Tied to one parts-catalog listing - confirm with dealer before ordering across this break. |
Frequently asked questions
What engine is the Komatsu 6D125-1AN (S/N 47990-UP)?
It's a wet-liner, water-cooled inline diesel from Komatsu's mechanical-injection 125-class engine family, displacing 11.04 L over a 125 mm x 150 mm (4.92 in x 5.91 in) bore and stroke. On the motor grader platform it powers, it's rated at approximately 134 kW (180 hp) gross and 116 kW (155 hp) net at 2,200 rpm, using a centrifugal-governed inline injection pump with no electronic control. This arrangement runs turbocharged; confirm aspiration off the arrangement plate on the block itself, since the wider 6D125-1 family also includes naturally aspirated variants.
What is the operating weight for a machine using this engine?
This is an engine listing, not a chassis, so weight is set by the host machine. The motor grader platform this arrangement was built for runs approximately 13,350-13,620 kg (29,430-30,030 lb) in base trim, and roughly 15,130-15,255 kg (33,360-33,640 lb) with a rear ripper fitted; exact figures vary by grader configuration.
What replaced the Komatsu 6D125-1AN engine?
The mechanical 6D125-1 arrangement family was succeeded by the 6D125E-2 and turbocharged S6D125E-2 generation, then by the fully electronic, common-rail SAA6D125E series that powers current Komatsu excavators and dozers.
What 6D125-1AN(S/N 47990-UP) owners discuss
The 6D125-1 family has a dozen different suffixes (1A, 1AN, 1AB, 1AF, 1AM, S6D125-1, SA6D125-1...). How do techs actually tell a naturally aspirated unit from a turbo one?
Go by the prefix, not the dash suffix. A bare 6D125 designation is naturally aspirated. An S prefix (S6D125) means turbocharged; SA (SA6D125) means turbocharged with an aftercooler. The dash-number and trailing letters (1AN, 1AB, 1AF, 1AM, etc.) are factory arrangement codes tracking injection pump calibration and accessory drive for the specific machine the engine was built for, not boost level or generation. Parts and fitment charts often lump 6D125, S6D125 and SA6D125 together under one grader family because they share a case and many internals, which is exactly where the turbo/NA mix-ups start. Confirm aspiration from the arrangement plate on the block itself, not from a parts-fitment listing.
This engine has no ECM. What do owners actually rely on to catch trouble early?
Scheduled oil sampling carries more weight here than on a common-rail engine, since there is no fault-code history to fall back on. Pull a sample at every oil change and watch the wear-metal and fuel/coolant dilution trend over time rather than reacting to a single result. On a wet-liner block like this, a jump in coolant-related elements or a sudden viscosity shift is often the first sign of liner seal or injector trouble, well before it shows up as smoke or a gauge swing. Sampling frequency for mechanical diesels of this type is commonly tied to the oil change interval; confirm the exact interval for your specific arrangement with your dealer or oil supplier.
Is regular injection pump and nozzle work normal on this engine, or a sign something is wrong?
Periodic nozzle pop-testing and pump recalibration is routine maintenance on a mechanical inline pump like this, not a red flag on its own. What experienced operators flag is chasing black or white smoke by swapping injectors without first checking pump timing and delivery — the two interact, and a governor or timing issue can look identical to a bad injector. A compression or cylinder leak-down test is the standard first step before condemning either. Injection timing and fuel delivery calibration directly affect engine speed control and can be safety-relevant if the governor linkage is involved — have your dealer or an approved diesel fuel shop verify timing and governor settings rather than adjusting by feel.
Any cooling system quirks specific to this engine family that owners complain about?
Two items come up repeatedly: water pump shaft seal weeping over time, which is a wear item across the whole 6D125 family and not unique to one arrangement, and neglected coolant additive maintenance on the wet cylinder liners. Because the liners sit directly against coolant, skipping supplemental coolant additive top-offs lets cavitation pit the liner exterior, which later shows up as coolant-in-oil or oil-in-coolant contamination that looks like a head gasket failure but is not. Keep coolant chemistry on the service schedule, not just the fluid level.
What wear pattern shows up most as these engines rack up hours?
Rising oil consumption tied to ring and liner wear, usually accelerated by air filter neglect on dusty grader sites — ingested dust glazes the bores faster than normal blow-by wear alone would. Plugged piston-cooling oil jets are the other common culprit; once clogged, piston skirts scuff and both oil consumption and blow-by climb together. Community consensus is to run a compression or leak-down test before assuming a top-end rebuild is needed, since the repair scope differs a lot between worn rings/liners and an injector dribbling fuel and washing down one cylinder.
The engine is all-mechanical. What electrical or sensor-type gremlins actually show up on these?
Nothing resembling modern sensor fault codes. The recurring complaints are a coolant temperature sender or gauge giving false readings, an oil pressure switch corroding and triggering a false warning light, a tired alternator under-charging, and cold-start aids (glow plug or intake air heater, depending on arrangement) failing in cold climates. Starter and battery cable corrosion is also common on older units that sat outdoors. None of this touches fuel delivery — if smoke or power complaints show up alongside electrical symptoms, treat them as two separate problems rather than one root cause.
What should I check before buying a used 6D125-1AN or a grader powered by one?
Pull the dipstick and coolant cap and check for cross-contamination — milky oil or an oil sheen in the coolant points to liner seal or gasket trouble. Run the engine idle-to-full-throttle and watch smoke color at each stage: persistent black smoke under load suggests injection pump or air filtration problems, brief white smoke on a cold start is normal, white smoke that lingers is not. Ask for maintenance and oil-sample records rather than trusting the hour meter alone, since these gauges are often swapped or disconnected on older machines. Check the water pump weep hole for staining, and confirm the arrangement/plate number matches the injection pump calibration on file, since the 6D125-1 family has many arrangement variants that are not directly interchangeable. Have your dealer or an independent diesel mechanic run a compression or leak-down test and verify injection timing before you commit to the purchase.
Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.
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