Komatsu S6D125-1AK(S/N 51975-UP)
Maintenance schedule, common problems & OEM parts breakdown
The Komatsu S6D125-1AK is a 6-cylinder, turbocharged, aftercooled diesel engine belonging to the S6D125 series, produced from approximately 2017 onward. It displaces 11 liters and delivers 275 horsepower (205 kW) with 1,010 pound-feet (1,370 N·m) of torque. This engine powers mid-to-heavy class machines, including the D65E-12, D65P-12, D65EX-12, and D65PX-12 crawler dozers, as well as the WA470-3 wheel loader. Operating weights for these machines range from 24,000 kg (52,910 lb) to over 80,000 kg (176,370 lb) depending on configuration. The S6D125-1AK succeeded earlier 6D125 variants and remains a staple in the used market due to its widespread use in high-hour dozer and loader fleets.
Across the S6D125 generation, key changes include the transition to wet cylinder liners requiring strict supplemental coolant additive (SCA) management to prevent cavitation pitting, and the integration of Komatsu CARE programs for Tier 4 compliance. The S6D125-1AK specifically utilizes a mechanical fuel injection system with no electronic cold-start assist beyond glow plugs. In the used and parts market, this engine is critical because its wet-liner design and high-torque output make it a common donor for D65 dozer and WA470 loader rebuild kits. Its longevity depends heavily on disciplined fluid analysis (KOWA) and coolant maintenance, driving consistent demand for overhaul kits and gasket sets.
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 S6D125-1AK(S/N 51975-UP) specifications
Engine
| Model | S6D125-1AK |
| Series | S6D125-1 |
| Type | 4-cycle, water-cooled, inline vertical, turbocharged and aftercooled |
| Aspiration | Turbocharged and aftercooled |
| Displacement | 11 liters |
| Cylinders | 6 |
| Gross Power | 202 kW (275 hp) |
| Torque | 1,370 N·m (1,010 lb·ft) |
| Emissions Tier | Not documented for this exact serial range — verify with your dealer |
Weights
| Operating Weight | Not documented for this exact serial range — verify with your dealer |
| Shipping Weight | Not documented for this exact serial range — verify with your dealer |
| Ground Pressure | Not documented for this exact serial range — verify with your dealer |
Dimensions
| Transport Length | Not documented for this exact serial range — verify with your dealer |
| Transport Width | Not documented for this exact serial range — verify with your dealer |
| Transport Height | Not documented for this exact serial range — verify with your dealer |
| Track Gauge | Not documented for this exact serial range — verify with your dealer |
| Ground Clearance | Not documented for this exact serial range — verify with your dealer |
Performance
| Travel Speed | Not documented for this exact serial range — verify with your dealer |
| Gradeability | Not documented for this exact serial range — verify with your dealer |
Service capacities (summary)
| Fuel Tank | Not documented for this exact serial range — verify with your dealer |
| Hydraulic System | Not documented for this exact serial range — verify with your dealer |
| Engine Oil | Not documented for this exact serial range — verify with your dealer |
| Cooling System | Not documented for this exact serial range — verify with your dealer |
Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.
Komatsu S6D125-1AK(S/N 51975-UP) maintenance schedule
| Service interval | Tasks |
|---|---|
| Every 10 h |
|
| Every 250 h |
|
| Every 500 h |
|
| Every 1,000 h |
|
| Every 2,000 h |
|
Servicing the S6D125-1AK(S/N 51975-UP) beyond the schedule
Predictive Maintenance & Fluid Analysis
Monitor KOWA oil samples at every 500-hour interval to track wear metals (iron, copper, lead) and contaminants (silicon, sodium). Check coolant pH, SCA concentration, and glycol content to prevent wet liner cavitation pitting. Track engine oil pressure and charge pressure gauges at each start; log trends rather than waiting for caution lamps. Inspect the air filter restriction indicator daily in dusty conditions. Verify DEF levels and monitor KOMTRAX telematics for fault codes related to fuel delivery or turbo performance.
Corrective & Common Repairs
Address milky oil or coolant in the crankcase immediately, indicating SCA depletion or liner pitting. Replace the fuel filter and drain the water separator regularly to protect the mechanical injection pump. Inspect the turbocharger and exhaust system for leaks or carbon buildup. Check the alternator and battery terminals for corrosion. Replace worn belts and hoses during the 250-hour service. Inspect the oil cooler for debris blockage to prevent overheating. Verify track tension and undercarriage wear on D65 dozers to maintain ground pressure and efficiency.
Overhaul & Rebuild Points
Plan inframe overhauls around 10,000–15,000 hours based on KOWA data and compression tests. Replace piston rings, liners, and bearings using S6D125-1AK specific kits. Inspect the crankshaft for wear and the cylinder head for valve seat integrity. Check the fuel injection pump timing and nozzle spray patterns. Replace the water pump and thermostat housing gaskets during coolant flushes. Inspect the turbocharger shaft play and replace if worn. Ensure proper torque on all head bolts and main bearing caps during reassembly.
Seasonal & Environment Servicing
In cold weather, check coolant concentration and battery capacity closely; use ether aids if necessary as there is no electronic cold-start assist. In hot, dusty environments, shorten air cleaner service intervals and keep the radiator core clear of debris. Inspect hydraulic hoses for heat damage in high-temperature applications. Use Komatsu Genuine Lubricants with appropriate viscosity for seasonal changes. Monitor the cooling system for leaks after thermal cycling. Grease all pivot points frequently in muddy or abrasive conditions.
S6D125-1AK(S/N 51975-UP) common service parts
| Part number | Part | Notes | |
|---|---|---|---|
| 07281-00197 | Clamp,(With Pre-Fuel Filter) | Fuel system service | Check fitment → |
| 6221-71-6201 | Fuel Filter Ass'Y | Fuel system service | Check fitment → |
| ND092220-0040 | Filter,Fuel | Fuel system service | Check fitment → |
| 6125-81-7032 | Element Ass'Y,Air Cleaner | Air intake service | Check fitment → |
| 09637-30161 | Plate? Instruction,Air Cleaner (Single Element) | Air intake service | Check fitment → |
Always confirm against your machine's serial number — cross-check any part in Heavy Parts AI before ordering.
S6D125-1AK(S/N 51975-UP) fault codes & troubleshooting
| Code | Meaning | Likely cause | What to do |
|---|---|---|---|
| CA115 | Engine crankshaft speed sensor and backup speed sensor failure | Faulty speed sensor, broken wiring harness, or corroded connector at the pin connector | Replace the speed sensor and verify voltage at the pin connector |
| CA451 | Common rail pressure sensor feedback value too high | Malfunctioning rail pressure sensor or actual overpressure condition in the fuel system | Diagnose the fuel rail pressure sensor and inspect pump lines |
| CA452 | Common rail pressure sensor feedback value too low | Faulty rail pressure sensor or low fuel supply pressure | Diagnose the fuel rail pressure sensor and inspect pump lines |
| CA111 | Internal engine controller fault | Critical internal failure of the Electronic Control Module (ECM) | Replace or reprogram the ECM |
| CA234 | Engine overspeed | Throttle calibration error, engine load cycle issue, or operator misuse | Verify throttle calibration and engine load cycle |
| CA122 | Intake air pressure sensor feedback voltage too high | Faulty intake pressure sensor or wiring issue | Verify the intake pressure sensor and wiring |
| CA123 | Intake air pressure sensor feedback voltage too low | Sensor short circuit or airflow restriction | Check for a sensor short or an airflow restriction |
| CA441 | Battery voltage too low | Weak battery, failing alternator, or voltage regulator issue | Replace the alternator or battery and check the voltage regulator |
| CA442 | Battery voltage too high | Failing alternator or voltage regulator | Replace the alternator or battery and check the voltage regulator |
| CA153 | Intake air temperature sensor feedback voltage too high | Faulty Intake Air Temperature (IAT) sensor | Replace or test the IAT sensor |
Codes and remedies are general guidance for this model family — always confirm with diagnostic tooling and your dealer before major repairs.
S6D125-1AK(S/N 51975-UP) attachments & work tools
Crawler Dozer Blades
The S6D125-1AK engine powers the D65E-12, D65P-12, D65EX-12, and D65PX-12 dozer series, which utilize standard straight, angle, and semi-U dozer blades. Factory configurations include ROPS cabs or canopies with varying roof heights depending on the specific dozer model variant.
Excavator Buckets
This engine is installed in the PC400LC-5 and PC400LC-6 excavator families, which are equipped with standard digging buckets and grading buckets. The machine class supports quick-attach coupler systems for rapid tool changes, with hydraulic flow requirements managed by the main control relief valve.
Wheel Loader Buckets
The WA470-3 and WA470-3A wheel loaders utilize this engine and are fitted with standard multi-purpose buckets and rock buckets. These machines employ a Z-bar linkage system and require specific hydraulic kit arrangements to support high-flow bucket functions.
Motor Grader Blades
The GD663A-2 motor grader uses the S6D125-1AK engine and is equipped with a moldboard blade and a ripper attachment. Factory guarding options include ROPS/FOPS certified cabs and protective grilles for the radiator and hydraulic lines.
All S6D125-1AK(S/N 51975-UP) assemblies by section
Every catalogued assembly group for the Komatsu S6D125-1AK(S/N 51975-UP). Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.
Engine
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Air Cleaner
| 01***40 | Bolt,(Dust And Dirt Protection) | 4 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 4 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 4 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Air Intake
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 11 |
| 01***55 | Bolt, Mount Tube To Switch Pump | 4 |
| 01***05 | Bolt, Mount Hose Clip | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Alternator (13a) (Inner Parts)
| 60***50 | Alternator A. (13a),(With Non Hardening Pulley) | 1 |
| KD***31 | Bushing,Insulator | 2 |
| KD***72 | Slip Ring Ass'Y | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Alternator Mounting (13a)
| 01***25 | Bolt,(Without Rops Guard? Canopy) | 1 |
| 01***75 | Bolt | 1 |
| 01***10 | Nut,(Dust?Drip Proof? Trimming S.) | 2 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Camfollower Cover
| 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 : S6d125-1ak(S/N 51975-Up) : Engine : Camshaft
| 01***22 | Bolt | 2 |
| 04***25 | Key, Plain Woodruff | 1 |
| 04***12 | Ring,Snap (For Ripper) | 12 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Cooling Fan
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 6 |
| 01***40 | Bolt, Mount Master Brake Cylinder | 6 |
| 60***51 | Fan,Cooling | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Cooling Fan (Reversible Fan Type)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 6 |
| 01***40 | Bolt, Mount Master Brake Cylinder | 6 |
| 60***50 | Fan,Reversible | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Corrosion Resistor
| 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 : S6d125-1ak(S/N 51975-Up) : Engine : Crank Pulley
| 01***30 | Screw | 1 |
| 01***35 | Screw | 5 |
| 04***28 | Pin, Dowel - Transmission Case, Pin-Dowel | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Crankshaft(#51975-62861)
| 61***10 | Bearing, Main (Standard) | 7 |
| 61***50 | Bearing, Thrust Assembly (Standard) | 1 |
| 61***10 | Bearing, Main (1.00mm Undersize) | 7 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Cylinder Block
| 01***12 | Bolt,(Without Air Compressor Type) | 2 |
| 01***45 | Washer,(For Travel Motor) (Not Show) | 14 |
| 04***14 | Pin, Dowel | 2 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Cylinder Block Gasket Kit(#51976-)
| 07***15 | O-Ring,Relief Valve | 1 |
| 07***20 | O-Ring,Lubricating Oil Piping | 1 |
| 07***25 | O-Ring (K2?K8) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Cylinder Head
| 04***20 | Pin,Dowel (Welded) | 2 |
| 61***30 | Sleeve,Holder | 1 |
| 61***20 | Cotter, Valve | 48 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Cylinder Head Gasket Kit(#51976-)
| 02***43 | O-Ring,Water Manifold | 6 |
| 07***65 | O-Ring,Cylinder Block | 6 |
| 07***12 | Gasket,Mechanical Pump Piping | 12 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Damper
| 01***25 | Bolt,(Without Rops Guard? Canopy) | 6 |
| 61***00 | Damper Ass'Y | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Engine Mounting(#55719-)
| 14***10 | Support,Front | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Exhaust Manifold
| 01***00 | Bolt (M10-1.50x100) | 18 |
| 01***45 | Screw | 6 |
| 01***30 | Stud | 2 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fan Drive
| 01***14 | Nut,(For Slag Handling) | 1 |
| 01***60 | Washer,(Without Rops Guard? Canopy) | 1 |
| 04***25 | Ring,Snap (Kit) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fan Guard (1/2)
| 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 : S6d125-1ak(S/N 51975-Up) : Engine : Fan Guard (2/2)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 13 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
| 01***35 | Bolt,(For Quick Shift) (Mitsubishi) | 3 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fan Pulley
| 04***69 | V-Belt Set,Fan | 1 |
| 61***20 | Pulley?Non Hardening | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Flywheel
| 01***55 | Bolt | 6 |
| 01***45 | Washer,(For Travel Motor) (Not Show) | 6 |
| 06***08 | Bearing, Ball | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Flywheel Housing(#55719-)
| 01***50 | Bolt | 2 |
| 01***50 | Bolt, Radiator To Frame | 6 |
| 01***60 | Bolt, Hanger Bearing Mounting | 2 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Front Cover(#55719-)
| 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 : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Filter
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 7 |
| 01***05 | Bolt, Mount Hose Clip | 1 |
| 01***20 | Screw | 2 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Filter (For Slag Handring)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 7 |
| 01***05 | Bolt, Mount Hose Clip | 1 |
| 01***20 | Screw | 2 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Injection Pump (Coupling) (208ps) (Inner Parts)
| ND***10 | Flange,Coupling | 1 |
| ND***50 | Bolt | 1 |
| ND***40 | Bolt | 2 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Injection Pump (Feed Pump) (208ps) (Inner Parts)
| ND***40 | Washer? 1mm,Plate | 1 |
| ND***70 | Washer,Union | 4 |
| ND***10 | Spring, Check Valve | 2 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Injection Pump (Governor) (1/2) (208ps) (Inner Parts)
| ND***40 | Plate,Thrust | 1 |
| ND***40 | Plug,Screw | 1 |
| ND***50 | Washer? 0.3mm | 0 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Injection Pump (Governor) (2/2) (208ps) (Inner Parts)
| ND***80 | Pin,Stop | 1 |
| ND***70 | Arm,Spring | 1 |
| ND***20 | Washer? 1.0mm,Cover Plate | 2 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Injection Pump (Pump) (1/2) (208ps) (Inner Parts)
| 61***80 | Injection Pump A | 1 |
| ND***80 | Bearing,Center | 1 |
| ND***50 | Ring,Adjust | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Injection Pump (Pump) (2/2) (208ps) (Inner Parts)
| ND***21 | Shim? 2.4mm | 0 |
| ND***91 | Shim? 2.25mm,Element Holder | 0 |
| ND***60 | Element,Pump | 6 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Injection Pump Drive
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 4 |
| 01***14 | Nut (Itl) | 1 |
| 01***60 | Washer,(Without Rops Guard? Canopy) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Injection Pump Mounting (208ps)
| 01***75 | Bolt,(See Fig.0142) | 4 |
| 61***10 | Washer,Spring | 4 |
| (6***0) | Injection Pump A.,(See Fig.A4010-B3a2) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Injection Pump Piping(#51976-)
| 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 : S6d125-1ak(S/N 51975-Up) : Engine : Fuel Return
| 01***16 | Bolt, Battery To Frame Ground Wire Attaching | 1 |
| 01***20 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
| 01***08 | Nut,(For Canvas Canopy?Head Guard) | 1 |
Komatsu : S6d125-1ak(S/N 51975-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 : S6d125-1ak(S/N 51975-Up) : Engine : Heater Switch
| 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 : S6d125-1ak(S/N 51975-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 : S6d125-1ak(S/N 51975-Up) : Engine : Injection Nozzle
| 01***45 | Bolt, Divider Valve Mounting | 12 |
| 07***65 | O-Ring,Cylinder Block | 6 |
| 07***12 | Gasket,(Without Air Compressor Type) (K2) | 12 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Muffler
| 01***40 | Bolt,(Dust And Dirt Protection) | 4 |
| 01***25 | Bolt - Rear Guard Mounting | 4 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Name Plate And Caution (English)
| 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 : S6d125-1ak(S/N 51975-Up) : Engine : Nozzle Holder (Inner Parts)
| 07***12 | Gasket,Mechanical Pump Piping | 2 |
| 61***11 | Holder Ass'Y | 6 |
| ND***90 | Connector,Inlet | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Oil Cooler (7-Element Type)(#51975-73268)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 12 |
| 01***65 | Bolt - Return Line And Suction Linehose Clamps | 2 |
| 01***80 | Bolt,(With Corrosion Resistor) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Oil Filter
| 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 : S6d125-1ak(S/N 51975-Up) : Engine : Oil Levle Gauge
| 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 : S6d125-1ak(S/N 51975-Up) : Engine : Oil Pan And Suction Tube
| 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 : S6d125-1ak(S/N 51975-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 : S6d125-1ak(S/N 51975-Up) : Engine : Piston
| 02***13 | Pin, Dowel | 2 |
| 04***18 | Ring, Snap | 12 |
| 61***33 | Piston Ring Ass'Y | 6 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Rocker Arm
| 01***08 | Nut,R.H. Thread | 12 |
| 01***32 | Washer,(With Auto Emergency Steering) | 40 |
| 61***10 | Housing, Rocker Lever | 6 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Starting Motor (7.5kw) (Inner Parts)
| 60***10 | Starting Motor Ass'Y,(7.5kw) | 1 |
| KD***60 | Shift Lever Assembly | 1 |
| KD***40 | Spring,Spring | 4 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Starting Motor Mounting (7.5kw)
| 01***40 | Bolt,(With Center Guard) | 3 |
| 08***19 | Band,(Without Air Conditioner) | 1 |
| 08***10 | Connector, Clip | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Thermostat Housing
| 01***16 | Washer,Radio Controlled Slag Handling | 1 |
| 08***10 | Connector, Clip | 1 |
| 60***10 | Thermostat,(Cold Terrain Spec.) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Turbocharger (Inner Parts)
| 61***00 | Turbocharger Ass'Y,(T04e08) | 1 |
| GA***22 | Housing Ass'Y,Center Housing | 1 |
| GA***01 | Bolt | 6 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Turbocharger Oil Piping
| 01***30 | Bolt,(For Mechanical Governor) | 2 |
| 01***16 | Bolt, Battery To Frame Ground Wire Attaching | 2 |
| 01***20 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Water Drain Valve
| 61***12 | Valve,Water Drain | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Water Manifold
| 02***43 | O-Ring,Water Manifold | 6 |
| 61***11 | Manifold,Water | 2 |
| 61***20 | Manifold,Water | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Water Pump
| 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 : S6d125-1ak(S/N 51975-Up) : Engine : Water Pump Gasket Kit
| 07***15 | O-Ring,Scavenge Pump | 1 |
| 61***10 | Seal,Water (K6) | 1 |
| 61***20 | Seal? Water (K6) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Water Separator Mounting
| 01***16 | Bolt, Battery To Frame Ground Wire Attaching | 1 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 4 |
| 01***08 | Nut,(For Canvas Canopy?Head Guard) | 1 |
Komatsu : S6d125-1ak(S/N 51975-Up) : Engine : Water Temperature Sensor
| 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 |
S6D125-1AK(S/N 51975-UP) serial number reference
The engine serial number plate is typically located on the left side of the engine block near the injection pump. The prefix is the first part of the engine model code stamped on this plate, identifying the engine family and generation before the specific serial number sequence begins.
| Prefix | Identifies | Notes |
|---|---|---|
| S6D125-1AK | Engine model designation | Identifies the S6D125-1AK engine family. Serial range S/N 51975-UP is explicitly listed in the published references catalog for this specific engine model. The prefix 'S' indicates a turbocharged engine with an aftercooler, '6' indicates six cylinders, 'D' indicates diesel, '125' indicates the cylinder bore in millimeters, and '-1' indicates the generation. The suffix 'AK' identifies the specific application or market variant, which requires confirmation with the dealer for exact configuration details as Komatsu does not publish detailed meanings for final suffix letters. |
| S6D125-1 | Engine family prefix | General family prefix for the S6D125-1 series engines. The source text notes that the S6D125-1 engine is used in D65-12 dozers (S/N 60001 and up) alongside other variants like 6D125-1 and S6D125E-2. While the specific 'AK' suffix is tied to S/N 51975-UP in the parts catalog, the broader 'S6D125-1' family is documented in service manuals for the D65-12 series. Variations in configuration exist within this family. |
Frequently asked questions
What is the engine type and displacement?
The Komatsu S6D125-1AK is a 6-cylinder, 4-cycle, water-cooled, inline vertical diesel engine with a displacement of 11 liters. It features turbocharging and aftercooling.
What is the operating weight of the machines this engine powers?
Operating weight varies by configuration and series. For the D65E-12, D65P-12, D65EX-12, and D65PX-12 dozers, weights range from approximately 24,000 kg (52,910 lb) to over 80,000 kg (176,370 lb) depending on the attachment package and counterweight.
What replaced the S6D125-1AK engine?
The S6D125-1AK is part of the S6D125 series, which has been succeeded by newer emissions-compliant engines such as the SAA6D125E-2 and subsequent Tier 4 Final/Stage V compliant models in newer D65 and WA470 generations. Specific replacement depends on the machine model and production year.
What S6D125-1AK(S/N 51975-UP) owners discuss
What are the known powertrain and hydraulic behaviors for the Komatsu S6D125-1AK engine?
What undercarriage or wear patterns are typical for machines powered by this engine series?
What are the specific engine quirks for the S6D125-1AK series compared to related variants?
What are the common electrical or sensor issues reported for this engine?
What should be checked when buying a used machine with the S6D125-1AK(S/N 51975-UP) engine?
What are the critical maintenance intervals and fluid specifications for this engine?
What safety-critical fixes or inspections are required for the S6D125-1AK?
Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.
Need a specific S6D125-1AK(S/N 51975-UP) part?
Search live OEM part data, check fitment, and cross-reference alternatives with Heavy Parts AI.