Komatsuengine(no series)
Komatsu 6D125-1B-P(S/N 10001-UP)
Maintenance schedule, common problems & OEM parts breakdown
The 6D125-1B-P is a naturally aspirated, mechanically governed inline-six diesel from Komatsu's 6D125-1 engine family, displacing 11.04 L with a 125 mm bore and 150 mm stroke. Rated 116 kW (155 HP) at 2200 rpm, it runs no turbocharger and no ECM — only lamp-based mechanical warning indicators for oil pressure, coolant temperature/level, charge, final drive oil temperature, air pressure, and parking brake. It is the standard engine on Komatsu's older "A"-series articulated motor graders, including the GD605A-5, GD623A-1, GD655A-3, and GD663A-2, built during Komatsu's mechanical-generation grader era spanning roughly the late 1980s into the 1990s. Operating weight across these host graders runs roughly 12.7 to 15 t (28,000 to 33,000 lb) depending on frame size. The same platform also offered a turbocharged S6D125 option rated 134 kW (180 HP) for buyers who needed more output from the same chassis.
The 6D125-1 block was fitted across a range of suffix-coded variants on other Komatsu machines, but the 1B-P calibration is specific to this naturally aspirated grader fitment, distinct from turbocharged and electronically managed versions of the same family used elsewhere. What sets it apart within its own range is simplicity: no turbo, no electronics, and a mechanical injection pump and governor that a field mechanic can service without a laptop or scan tool. That is exactly why it still matters in the used and parts market today. These A-series graders remain in service in municipal, oilfield, and grading fleets, and the engine is valued for straightforward inframe rebuilds — liners, pistons, injection pump recalibration — over chasing electronic fault codes. Buyers and rebuilders should confirm the engine serial matches the grader's chassis plate before ordering parts, since the same block family covers more than one grader model in the range.
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 6D125-1B-P(S/N 10001-UP) specifications
Engine
| Model / family | 6D125-1B-P, a naturally aspirated member of the 6D125-1 mechanical diesel family (no "S" prefix, so no turbocharger — unlike other versions offered on the same grader line). |
| Configuration | Inline 6-cylinder, water-cooled, mechanical (jerk-pump) direct injection diesel. |
| Displacement | 11.04 L (674 cu in). |
| Bore x stroke | 125 x 150 mm (4.92 x 5.91 in). |
| Rated power | 116 kW / 155 HP (157 PS) at 2200 rpm, the naturally aspirated grader-duty rating for this engine. A turbocharged 134 kW / 180 HP (182 PS) rating is offered on the same grader platform under a different engine code — treat as a separate, higher-output configuration, not this engine. |
| Emissions tier | Pre-emissions mechanical diesel. No EPA/Tier or EU Stage certification — this design predates those standards. |
Weights
| Engine dry weight | Not published for this exact engine code in available spec sources. |
| Host grader operating weight | Approximately 13,350-13,620 kg (29,430-30,030 lb) base, rising to about 15,255 kg (33,640 lb) with a rear ripper fitted. This is the host motor grader's weight, not an engine spec, and it varies by grader model and attachment — verify against the specific host machine. |
Dimensions
| Bore x stroke | See Engine group — 125 x 150 mm. |
| Engine envelope (L x W x H) | Not documented for this exact engine code in available spec sources. |
| Machine transport dimensions | Not tied to a single figure for this engine — transport length, width and height depend on the host grader model. Check the specific host machine's spec sheet. |
Performance
| Rated (governed) speed | 2200 rpm. |
| Host transmission | The grader family built around this engine typically uses a powershift transmission with six forward and six reverse speed ranges, single-lever control. This is a machine-level detail, not an engine output figure. |
| Travel speed / gradeability | Not documented for this exact engine/serial range in available sources. Actual km/h figures and gradeability depend on the host grader's final drive and tire setup — verify with your dealer. |
Service capacities (summary)
| Engine oil (crankcase) | Not published for this exact serial range in available sources — check the factory shop manual for the host machine. |
| Cooling system | Not published for this exact serial range in available sources. |
| Fuel tank | Host-machine spec, not an engine spec — varies by grader model. Check the specific host machine's spec sheet. |
| Hydraulic system | Host-machine spec, not an engine spec — varies by grader model. Check the specific host machine's spec sheet. |
Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.
6D125-1B-P(S/N 10001-UP) fluids & capacities
| System | Capacity | Recommended fluid |
|---|---|---|
| Engine crankcase (oil pan, with filter) | Not published for this exact engine/serial range in available factory literature. Sister engines in the 6D125 family of this displacement generally run in the mid-20s liters (roughly 24-30 L / 6.3-7.9 US gal) with filter change, but treat that as a ballpark only; confirm against the dipstick markings on your specific unit before servicing. | Komatsu Genuine Diesel Engine Oil EO10W30 in cold climates (below about 0C/32F), EO15W40 for normal-to-hot ambient temperatures. Match the API/JASO service class shown on the engine's own lubrication chart. |
| Cooling system (radiator, block, sub-tank) | Not documented for this exact serial range. Total system capacity is chassis-dependent across the grader family this engine powers, so any figure found online should be treated as an estimate; verify against the radiator cap and sub-tank markings. | Komatsu Genuine Super Coolant (AF-NAC), an ethylene-glycol antifreeze/anti-corrosion coolant supplied as a ready-to-use 50/50 premix. Usable year-round; no separate summer/winter grade. |
| Fuel tank | 285 L (75.3 US gal) on the GD623A-1 grader that carries this engine. Other machines sharing the same engine (the larger/heavier grader in this family) size the tank to chassis weight, so treat 285 L as representative of the smaller end of the range rather than universal across every host. | Standard automotive diesel meeting ASTM D975 No. 2-D (or the local equivalent). Use a winter-grade or kerosene-blended diesel in sub-zero climates per the operator's manual cold-weather guidance. |
| Power train case (torque converter, power-shift transmission, differential) | Not published for this exact engine/host combination. Treat any figure found for the wider grader family as unconfirmed until checked against the dipstick on the specific unit. | Komatsu Genuine Powertrain Oil TO10 in cold climates, TO30 for normal/warm ambient temperatures (SAE 30W-class, API CD-equivalent service). |
| Tandem final drive case, left side | Not documented for this specific serial range. Serviced separately from the right-side case, each with its own fill/drain point; check the fill plug level on the unit rather than relying on a published figure. | Komatsu Genuine Axle Oil AXO80 (or AXO85W140 in warmer climates), a GL-4/GL-5-equivalent gear oil. |
| Tandem final drive case, right side | Not documented for this specific serial range. Serviced separately from the left-side case; check the fill plug level on the unit rather than relying on a published figure. | Komatsu Genuine Axle Oil AXO80 (or AXO85W140 in warmer climates), a GL-4/GL-5-equivalent gear oil. |
| Circle and drawbar drive | No standalone oil-fill capacity documented. Graders in this class typically lubricate the circle gear, drawbar ball, and moldboard slide rails through scheduled grease points rather than an oil sump. | Komatsu Genuine Grease G2-LI, a lithium EP multipurpose grease, applied at the scheduled grease-fitting interval. |
| Hydraulic system and tank (blade, circle, steering, lift circuits) | Not documented for this exact serial range. Tank sizing follows chassis size and hydraulic-circuit complexity and varies across the grader family sharing this engine; confirm against the sight gauge on the specific unit. | Komatsu Genuine Hydraulic Oil HO46-HM, an ISO VG46 anti-wear hydraulic fluid. |
| Front axle and steering knuckles | Grease-lubricated only on this grader class; no oil fill is specified for the front axle. | Komatsu Genuine Grease G2-LI at the scheduled interval. No oil substitute. |
| Grease points (pins, bushings, circle gear, drawbar ball, tandem walking beam) | Spec only, no single reservoir. Apply per the periodic grease schedule at each fitting. | Komatsu Genuine Grease G2-LI, a lithium EP (NLGI 2) multipurpose grease rated for roughly -20C to +120C service. |
Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.
Komatsu 6D125-1B-P(S/N 10001-UP) maintenance schedule
| Service interval | Tasks |
|---|---|
| Every 10 h |
|
| Every 250 h |
|
| Every 500 h |
|
| Every 1,000 h |
|
| Every 2,000 h |
|
| Every 4,000 h |
|
Servicing the 6D125-1B-P(S/N 10001-UP) beyond the schedule
Predictive Maintenance & Fluid Analysis
Sample engine oil and torque converter/transmission oil at scheduled changes to catch iron and copper wear trends before a bearing or gear failure shows up as noise or heat. Watch coolant SCA concentration on this wet-lined block — depletion risks cavitation pitting on the liners. With no ECM to throw a fault code, gauge and warning-lamp discipline (oil pressure, coolant temperature, charge, final drive oil temperature, air pressure) is the primary early-warning system on these A-series graders.
Corrective & Common Repairs
Most reported power complaints on this engine trace back to an air leak on the low-pressure fuel lines or a clogged filter, not a failed injection pump or governor — rule those out first before condemning the pump. Drain the fuel/water separator regularly; contaminated fuel is the leading cause of hard starting and intermittent stalling on these mechanical engines. Charging and starter circuit faults are the most common electrical complaints. On the host grader, circle and moldboard wear are the recurring mechanical items to check at every service.
Overhaul & Rebuild Points
Inframe overhaul work centers on cylinder liner and piston/ring wear, plus injection pump and nozzle recalibration once output or exhaust smoke drifts out of spec. Valve clearance and injection timing need resetting by hand — this mechanically governed engine has no self-adjusting valve gear or electronic timing control. Check wet cylinder liners for cavitation pitting tied to coolant additive depletion. At major rebuild, confirm the engine serial still matches the grader's chassis plate, since 6D125-1 variants were fitted across several A-series grader models.
Seasonal & Environment Servicing
This engine has no electronic cold-start assist, so cranking amperage and battery condition matter more in cold weather — verify compression is healthy before blaming a hard start on the engine itself. Check winter-grade fuel and coolant SCA concentration ahead of freezing temperatures to avoid gelling and cavitation damage. In dusty grading or quarry work, shorten air cleaner service intervals and inspect the radiator and oil cooler fins for packed debris, since airflow loss raises operating temperatures on this naturally aspirated engine.
6D125-1B-P(S/N 10001-UP) common service parts
| Part number | Part | Notes | |
|---|---|---|---|
| 07281-00197 | Clamp,(With Pre-Fuel Filter) | Fuel system service | Check fitment → |
| ND092220-0040 | Filter,Fuel | Fuel system service | Check fitment → |
| 09637-02161 | Plate? Instruction,Air Cleaner (Double Element) | Air intake service | Check fitment → |
| 600-181-2500 | Element Ass'Y,Outer | Scheduled service item | Check fitment → |
| 6114-80-7101 | Element Ass'Y | Scheduled service item | Check fitment → |
Always confirm against your machine's serial number — cross-check any part in Heavy Parts AI before ordering.
6D125-1B-P(S/N 10001-UP) fault codes & troubleshooting
| Code | Meaning | Likely cause | What to do |
|---|---|---|---|
| Engine Oil Pressure warning lamp | Engine oil pressure has dropped below the safe minimum for lubrication. | Low oil level, clogged oil filter or strainer, worn oil pump, faulty oil pressure switch, wrong oil viscosity. | Stop the engine immediately. Check oil level and top up, inspect for external leaks, check the filter before restarting. Do not run the engine until the cause is found. |
| Coolant Level warning lamp | Coolant level in the radiator or reserve tank has dropped below the safe minimum. | External coolant leak at hoses, radiator, or water pump seal; blown gasket; loss from prior overheating. | Let the engine cool, check for external leaks, top up coolant to the correct level. Pressure-test the cooling system if level keeps dropping. |
| Engine Coolant Temperature warning lamp | Engine coolant temperature has exceeded the normal operating range. | Low coolant, radiator fins clogged with debris, slipping or broken fan belt, faulty thermostat, water pump failure, sustained overload. | Reduce load and let the engine idle down, then shut off. Once cool, check coolant level and clear radiator debris; inspect fan belt and thermostat. |
| Charge warning lamp | Alternator is not charging the battery; fault in the charging circuit. | Slipping or broken alternator belt, faulty alternator, loose or corroded wiring, poor battery terminal connection. | Check belt tension and condition, inspect wiring and battery terminals, test alternator output. Repair or replace as needed. |
| Final Drive Oil Temperature warning lamp | Oil temperature in the final drive (wheel-end reduction) has risen above normal. | Low final drive oil level, worn bearings or gears generating excess friction heat, prolonged heavy-load operation. | Reduce load and let the unit cool. Check final drive oil level and condition; inspect for internal wear or contamination at the next service. |
| Air Pressure warning lamp | Air pressure in the brake system reservoir has dropped below the safe operating minimum. | Leak in air lines or fittings, faulty compressor or unloader valve, moisture or ice in the air system. | Do not move the machine until pressure builds back up. Check air lines and fittings for leaks, inspect the compressor, and drain moisture from the air tank. |
| Parking Brake indicator lamp | Shows the parking brake is engaged; on some panels it also flags insufficient release pressure. | Normal status when parked; if lit with the brake lever released, points to low brake-release pressure. | Confirm parking brake lever position. If the lamp stays on with the brake released, check the brake-release circuit and pressure before operating. |
Codes and remedies are general guidance for this model family — always confirm with diagnostic tooling and your dealer before major repairs.
6D125-1B-P(S/N 10001-UP) attachments & work tools
Aspiration and turbocharging
Naturally aspirated arrangement within the 6D125-1 mechanical engine family. No turbocharger or aftercooler is fitted on this specific arrangement. Komatsu's own naming convention marks the turbocharged sibling as S6D125-1 and the turbocharged-and-aftercooled sibling as SA6D125-1, both built on the same base block/architecture but treated as separate factory arrangements.
Governor and fuel system
Mechanical inline fuel-injection pump with an integrated all-speed flyweight governor. No electronic engine control on this series; fuel delivery and speed control are handled entirely mechanically, consistent with the pre-electronic 6D125-1 family.
Host-machine power tier
Used on Komatsu's roughly 13-tonne motor-grader platform (GD663A-2 class), where the factory offered this naturally aspirated arrangement rated near 116 kW (155 hp) alongside a turbocharged sibling rated near 134 kW (180 hp), both around 2200 rpm. Treat exact rating as configuration-dependent and confirm against the nameplate/serial range for a given unit.
All 6D125-1B-P(S/N 10001-UP) assemblies by section
Every catalogued assembly group for the Komatsu 6D125-1B-P(S/N 10001-UP). Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.
Cooling System
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cooling System : Cooling Fan And Mounting
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 6 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 10 |
| 01***40 | Bolt, Mount Master Brake Cylinder | 8 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cooling System : Corrosion Resistor And Piping
| 01***20 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 4 |
| 01***08 | Nut,(For Canvas Canopy?Head Guard) | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cooling System : Water Pump And Mounting
| 01***30 | Bolt,(For France Noise Suppression) | 2 |
| 01***16 | Bolt (With Air Heater & Conditioner) | 6 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 5 |
Cylinder Block
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Camshaft And Tappet
| 01***22 | Bolt | 2 |
| 04***25 | Key, Plain Woodruff | 1 |
| 04***12 | Ring,Snap (For Ripper) | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Crankshaft
| 01***50 | Bolt,(For Strengthened Cover) | 6 |
| 01***15 | Bolt | 1 |
| 01***20 | Bolt | 5 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : 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 : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Cylinder Block Cover
| 01***20 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 1 |
| 01***08 | Nut,(For Canvas Canopy?Head Guard) | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Flywheel And Flywheel Housing
| 01***40 | Bolt,(Dust And Dirt Protection) | 2 |
| 01***45 | Bolt,(For Slag Handling) | 2 |
| 01***50 | Bolt, Radiator To Frame | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Front Cover(#10001-34750)
| 01***25 | Bolt,(Without Auto Priming System) | 6 |
| 01***60 | Screw | 1 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Front Cover(#34751-)
| 01***25 | Bolt,(Without Auto Priming System) | 6 |
| 01***60 | Screw | 1 |
| 01***65 | Bolt,(Except Japan) | 8 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Oil Pan
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 29 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 7 |
| 01***16 | Washer,Radio Controlled Slag Handling | 36 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Oil Pan (For Trimming Dozer)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 29 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 7 |
| 01***16 | Washer,Radio Controlled Slag Handling | 35 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Piston And Connecting Rod
| 02***13 | Pin, Dowel | 2 |
| 04***18 | Ring, Snap | 2 |
| 61***10 | Piston Ass'Y | 6 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Power Take Off (1/2)
| 01***20 | Bolt,(With Auto Emergency Steering) | 6 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 4 |
| 01***14 | Nut,Ring | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Block : Power Take Off (2/2)
| 01***20 | Bolt,(With Auto Emergency Steering) | 6 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 8 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 4 |
Cylinder Head
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Air Cleaner And Mounting
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 2 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
| 01***40 | Bolt, Mount Master Brake Cylinder | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Air Cleaner And Mounting (For Trimming Dozer)
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 2 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
| 01***08 | Nut,(For Straight Tilt Dozer) (D65p) | 6 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Air Intake Manifold And Connection
| 01***16 | Bolt, Mount Washer Tank To Bracket, Bolt | 2 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 15 |
| 01***55 | Bolt, Mount Tube To Switch Pump | 6 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Cylinder Head(#10001-26915)
| 04***20 | Pin,Dowel (Welded) | 2 |
| 61***30 | Sleeve,Holder | 1 |
| 61***20 | Cotter, Valve | 8 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Cylinder Head(#26916-)
| 04***20 | Pin,Dowel (Welded) | 2 |
| 61***30 | Sleeve,Holder | 1 |
| 61***20 | Cotter, Valve | 8 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Exhaust Manifold
| 01***80 | Bolt, Mount Strainer Flange | 18 |
| 01***00 | Bolt (M10-1.50x100) | 18 |
| 07***08 | Plug,(Without Auto Priming System) | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Muffler And Mounting(#10001-27909)
| 01***25 | Bolt,(Sandy? Dusty Terrain Spec.) | 4 |
| 01***40 | Bolt,(Dust And Dirt Protection) | 4 |
| 01***35 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Muffler And Mounting(#27910-)
| 01***25 | Bolt,(Sandy? Dusty Terrain Spec.) | 4 |
| 01***40 | Bolt,(Dust And Dirt Protection) | 4 |
| 01***35 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Rocker Arm And Housing
| 01***08 | Nut, Regular Hexagon (M10-1.50) | 6 |
| 01***08 | Nut,R.H. Thread | 6 |
| 01***16 | Washer,Radio Controlled Slag Handling | 40 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Rocker Arm Housing Cover
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 1 |
| 01***08 | Nut,(For Canvas Canopy?Head Guard) | 1 |
| 01***16 | Washer,Radio Controlled Slag Handling | 25 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Cylinder Head : Water Manifold And Thermostat
| 02***43 | O-Ring,Water Manifold | 10 |
| 07***12 | Washer, Gasket,(Without Thermo Sensor) (K1) | 1 |
| 07***08 | Plug,(Without Auto Priming System) | 1 |
Electrical System
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Alternator (13a)
| 60***50 | Alternator A. (13a) | 1 |
| KD***31 | Bushing,Insulator | 2 |
| KD***80 | Packing | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Alternator (13a) (Sandy Dusty Spec.)
| 60***70 | Alternator A. (13a),(Sandy And Dusty Terrain S.) | 1 |
| KD***31 | Bushing,Insulator | 2 |
| KD***72 | Slip Ring Ass'Y | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Alternator And Mounting
| 01***55 | Bolt, Mount Tube To Switch Pump | 1 |
| 01***25 | Bolt | 1 |
| 01***11 | Nut (Welded) | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Alternator And Mounting (For Anti-Corrosive Alternator)
| 01***55 | Bolt, Mount Tube To Switch Pump | 1 |
| 01***25 | Bolt | 1 |
| 01***11 | Nut (Welded) | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Heater Switch And Heater Signal
| 60***70 | Switch,Air Intake Heater | 1 |
| 60***30 | Signal,Engine Pre Heater | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Starting Motor (7.5kw) (1/2)
| 60***70 | Starting Motor A.,(7.5kw) (See Fig.A6210-B4a4) | 1 |
| KD***20 | Armature Ass'Y | 1 |
| KD***51 | Switch Ass'Y,Safety Relay | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Starting Motor (7.5kw) (2/2)
| KD***60 | Plunger Ass'Y | 1 |
| KD***30 | Holder | 1 |
| KD***20 | Screw | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Starting Motor (7.5kw)(#10001-18199)
| 60***30 | Starting Motor Ass'Y,(7.5kw)(See Fig.0661) | 1 |
| KD***20 | Magnetic Switch Assembly | 1 |
| KD***40 | Spring,Spring | 4 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Starting Motor (7.5kw)(#18200-)
| 60***30 | Starting Motor Ass'Y,(7.5kw) | 1 |
| KD***60 | Shift Lever Assembly | 1 |
| KD***40 | Spring,Spring | 4 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Starting Motor And Mounting
| 01***60 | Bolt,(Shim Thickness?More Than 8mm) | 3 |
| 66***10 | Gasket,Starting Motor | 1 |
| (6***0) | Starting Motor Ass'Y,(7.5kw) (See Fig.0661a) | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Electrical System : Starting Motor And Mounting (For Trimming Dozer)
| 01***40 | Bolt,(With Center Guard) | 3 |
| 60***20 | Clip,(Large Capacity Type) | 1 |
| 66***10 | Gasket,Starting Motor | 1 |
Engine Kit
Komatsu : 6d125-1b-P(S/N 10001-Up) : Engine Kit : Gasket Kit
Fuel System
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Filter And Piping
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 2 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 4 |
| 01***35 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Coupling)(#10001-37407)
| ND***50 | Bolt | 1 |
| ND***70 | Bolt | 2 |
| ND***30 | Coupling Ass'Y | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Coupling)(#37408-)
| ND***10 | Flange,Coupling | 1 |
| ND***50 | Bolt | 1 |
| ND***40 | Bolt | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Feed Pump)
| ND***40 | Washer? 1mm,Plate | 1 |
| ND***00 | Pump Ass'Y,Priming Pump | 1 |
| ND***70 | Washer,Union | 4 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2)(#10001-37407)
| ND***40 | Plug,Screw | 1 |
| ND***50 | Washer? 0.3mm | 0 |
| ND***30 | Governor Ass'Y | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2)(#37408-)
| ND***40 | Plate,Thrust | 1 |
| ND***40 | Plug,Screw | 1 |
| ND***70 | Adapter Ass'Y | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2)(#10001-37407)
| ND***20 | Washer? 1.0mm,Cover Plate | 1 |
| ND***60 | O-Ring | 2 |
| ND***70 | Packing,Rubber Plate | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2)(#37408-)
| ND***20 | Washer? 1.0mm,Cover Plate | 2 |
| ND***90 | Seal,Oil | 1 |
| ND***40 | O-Ring | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Pump) (1/2)
| 61***11 | Injection Pump A | 1 |
| 61***12 | Injection Pump A | 1 |
| 61***13 | Injection Pump A | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Pump) (2/2)
| ND***70 | O-Ring | 6 |
| ND***71 | Rack,Control | 1 |
| ND***10 | Washer | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (1/2)
| 01***75 | Bolt,(See Fig.0142) | 4 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 4 |
| 01***45 | Bolt,(For Power Tilt Mech Angle) | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (2/2)(#10001-11755)
| 01***30 | Bolt | 1 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 2 |
| 01***55 | Bolt, Mount Tube To Switch Pump | 4 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (2/2)(#11755-)
| 01***30 | Bolt | 1 |
| 01***20 | Bolt,(For Quick Shift) (Mitsubishi) | 1 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 1 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Fuel Nozzle And Spill Pipe
| 01***45 | Bolt, Divider Valve Mounting | 12 |
| 07***65 | O-Ring,Cylinder Block | 6 |
| 07***12 | Gasket,Mechanical Pump Piping | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Fuel System : Water Separator And Mounting
| 01***25 | Bolt,Without Revolution Sensor | 2 |
| 01***20 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
| 01***16 | Washer,(For 35a Alternator) | 2 |
Lubricating Oil System
Komatsu : 6d125-1b-P(S/N 10001-Up) : Lubricating Oil System : Lubricating Oil Cooler
| 01***25 | Bolt,Without Revolution Sensor | 2 |
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 13 |
| 01***65 | Bolt,(Except Japan) | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Lubricating Oil System : Lubricating Oil Filter And Mounting
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 2 |
| 01***30 | Bolt,(For Quick Shift) (Mitsubishi) | 4 |
| 01***35 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Lubricating Oil System : Lubricating Oil Piping
| 01***16 | Bolt, Battery To Frame Ground Wire Attaching | 1 |
| 01***35 | Bolt,(For Quick Shift) (Mitsubishi) | 2 |
| 01***50 | Bolt,(With Service Valve R.H.) | 2 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Lubricating Oil System : Lubricating Oil Pump And Suction Pipe
| 01***25 | Bolt,(Without Auto Priming System) | 4 |
| 01***20 | Bolt,(Large Capacity Type) (Std) | 2 |
| 01***25 | Bolt,(Sandy? Dusty Terrain Spec.) | 5 |
Komatsu : 6d125-1b-P(S/N 10001-Up) : Lubricating Oil System : P.T.O. Lubricating Oil Piping
| 01***25 | Bolt,(For Canvas Canopy?Head Guard) | 2 |
| 07***12 | Gasket,(Without Air Compressor Type) (K2) | 22 |
| 61***60 | Bolt,Joint | 6 |
6D125-1B-P(S/N 10001-UP) serial number reference
The 6D125-1 series dataplate is riveted to the left side of the cylinder block, usually near the fuel-injection pump or valve cover, and reads the full model code plus spec suffix together with the running serial (e.g. 6D125-1B-P, 10001 and up); if the tag is missing, the serial is also stamped into the block casting. There is no 3-character alpha code ahead of the number on this engine family - the letter/number suffix tacked onto "6D125-1" is what identifies the build configuration, and the digits after it are simply the sequential production serial.
| Prefix | Identifies | Notes |
|---|---|---|
| 10001-UP (no 3-character alpha PIN prefix) | 6D125-1B-P build/fitment spec - naturally aspirated, mechanical-injection 6D125-1 family inline-six diesel | The 6D125-1 engine family carries a plain numeric running serial on the dataplate with no 3-letter alpha PIN code ahead of it; the build/fitment configuration is set by the suffix on the model designation itself (A, AB, AB-EW, AF, AM, AN, B-EP, B-P, C, D, K and similar), not by a prefix on the serial. The -1B-P build is fitted to the Komatsu GD663A-2 motor grader. Confirm this and any other grader-family fitment with a dealer parts lookup before ordering, since aftermarket compatibility listings for the wider 6D125-1 family often bundle many suffix codes together under one host machine rather than confirming build-specific fitment. Production years and plant/region are not documented in available sources for this specific build. |
Frequently asked questions
What engine is the Komatsu 6D125-1B-P (S/N 10001-UP)?
It's a naturally aspirated, mechanically governed inline-six diesel from Komatsu's 6D125-1 family: 11.04 L displacement, 125 mm bore x 150 mm stroke, rated 116 kW (155 HP) at 2200 rpm. No turbocharger and no ECM — only lamp-based mechanical warning indicators. It's the standard powerplant on Komatsu's A-series motor graders (GD605A-5, GD623A-1, GD655A-3, GD663A-2).
What's the operating weight of the machine this engine powers?
It varies by grader model in the family — roughly 12.7 t (28,000 lb) on the GD623A-1 up to about 15 t (33,000 lb) on the larger GD605A frame. Check the OEM spec sheet for your exact chassis and configuration, since tire size and counterweights shift the figure too.
What replaced the 6D125-1B-P?
There's no single documented successor for this exact serial range. On the same A-series platform, a turbocharged S6D125 option (134 kW/180 HP) was already available for buyers needing more output from the same chassis. Later Komatsu grader generations moved to turbocharged, electronically governed engines with emissions controls this mechanical engine predates — verify exact succession with a Komatsu dealer for your chassis.
What 6D125-1B-P(S/N 10001-UP) owners discuss
What usually causes high oil consumption or smoking on this engine, and does it always mean a ring/liner job?
Not always. Mechanics who work on this engine say a power-loss or smoke complaint gets blamed on the injection pump first, but the actual cause is often something simpler: a small air leak on the low-pressure fuel feed line, a dirty air filter choking intake, or a leaking injector washing down a cylinder bore. Genuine ring and liner wear does show up on higher-hour, naturally aspirated units - look for oily discharge from the crankcase breather at idle and steady exhaust smoke under load. A compression check or cylinder leak-down test is the standard way to confirm real ring/liner wear before committing to a long-block rebuild. Have your dealer verify with a leak-down test before authorizing major engine work.
Does the mechanical injection pump and governor on this engine need special attention?
This engine runs a mechanical, all-speed governor with no ECM, and owners/mechanics report the pump and governor are rarely the true source of a reported power complaint. Knocking, humming, or buzzing near the pump usually traces to plunger or metering-rod wear rather than governor failure, and oil seepage at the pump fittings is more often a loose connection or worn seal than a sign the pump needs rebuilding. Check fuel feed lines, fittings, and filters before assuming the pump itself is bad.
Any cold-starting quirks specific to this engine?
Being naturally aspirated with a purely mechanical governor, this engine has no electronic cold-start assist. Operators in cold climates lean on starting fluid (ether) or a block heater rather than a glow-plug cycle, since this generation predates glow-plug-assisted Komatsu diesels. Community advice is consistent that ether should only be used briefly during cranking and is not a fix for an engine that's already hard-starting for another reason (weak compression, fuel starvation, tired batteries) - chasing the real cause is safer than dosing it with starting fluid every morning. Have your dealer verify compression and fuel delivery if hard starting persists, since misused starting fluid on a diesel can be a safety hazard.
What electrical or charging issues come up around this engine and its machines?
Most electrical complaints trace back to the starting and charging circuit rather than the engine internals: slow cranking despite a fresh battery, a starter solenoid not getting enough voltage to pull in, or confusion over the alternator's extra terminal (which is a starter-interlock/charge-light wire, not a second charging output). Owners troubleshooting a no-start after fuel filter or injection pump work often find the real culprit is low cranking amperage or a weak battery, not the fuel system they just serviced. Have your dealer verify starter draw and charging output before condemning fuel components.
What maintenance habit do owners say matters most for keeping this engine reliable?
Draining the fuel/water separator daily or every shift is the habit mechanics stress hardest. Water pulled into a mechanical injection pump on this generation of engine is one of the fastest ways to wreck injectors and pump components, and it's a cheap, five-minute check compared to the repair. Regular fuel filter changes and keeping the air filter clean are the other two items cited most often as preventing the vague 'power loss' complaints that otherwise get blamed on the pump or governor.
What driveline and hydraulic behaviors are typical for the graders this engine powers?
These older Komatsu A-series graders pair the engine with a torque-converter powershift transmission and a separate hydraulic circuit for blade and circle functions. When drivability complaints come up on this class of machine, mechanics check torque converter charge pressure and clutch pack condition (worn or twisted plates, degraded piston seals) before condemning the transmission outright. On the hydraulic side, sluggish or hunting blade/circle response is more often a low fluid level, contaminated fluid, or a leaking hose/fitting than a failed pump. Have your dealer verify converter charge pressure and clutch pressures with proper test gauges before authorizing transmission repair.
What should I check when buying a used grader powered by this engine?
Pull the fuel/water separator bowl and look for water or sediment - it's a quick read on how well the machine was cared for. Run a compression or leak-down test and watch the crankcase breather at idle and under load; oily discharge or blue smoke points to ring/liner wear that's costly to fix on a naturally aspirated, mechanically governed engine like this. Check cranking speed and battery/starter condition rather than assuming a slow crank means a fuel problem. Inspect circle and moldboard wear inserts, cutting edge, and end bits for remaining life, and cycle the blade lift and circle functions to feel for hydraulic hesitation. Ask for service records showing regular filter changes - these are mechanically simple, high-hour machines, and their condition comes down almost entirely to how consistently fluids and filters were maintained.
Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.
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