Komatsuengine(no series)

Komatsu 6D125-1DD(S/N 21811-UP)

Maintenance schedule, common problems & OEM parts breakdown

Search live 6D125-1DD(S/N 21811-UP) parts

The Komatsu 6D125-1DD (S/N 21811-UP) is the naturally aspirated member of Komatsu's 6D125-1 inline six-cylinder diesel family: a water-cooled, 11.04-liter block (125 mm bore x 150 mm stroke) with a mechanical inline injection pump and flyweight governor, no electronic engine control. It carries no turbocharger; that's covered by the S6D125-1 sibling in the same block family, with a turbo-aftercooled SA6D125-1 above that, all three sharing one factory shop manual. This block was fitted across Komatsu's mechanical-era motor grader line, roughly the GD605A through GD663A/GD672A range, where it's documented in the 116-134 kW (155-180 hp) power band; the wider naturally aspirated engine family spans as low as about 92 kW (124 hp) depending on host machine and governed speed, so treat any single figure as configuration-dependent. Operating weight across that grader family runs roughly 13,300-15,000 kg (29,300-33,000 lb) depending on model and options, with the GD661A-1 and GD663A-2 the closest documented weight and power matches for this particular engine arrangement.

Within the 6D125-1 family, Komatsu tuned the same basic block three ways, naturally aspirated (this 1DD build), turbocharged, and turbo-aftercooled, to hit different output targets on shared grader platforms without redesigning the core engine. The mechanically governed fuel system and caution-lamp-only monitoring, with no ECM and no stored fault codes, mark this as pre-emissions-era hardware; later Komatsu graders in this size class moved to electronically controlled, Tier-compliant engines with onboard diagnostics. That leaves surviving 6D125-1DD-powered machines most common today in older fleets and in markets without tiered emissions rules, where simple mechanical fuel systems are valued for field serviceability. Parts demand centers on injection pump and governor rebuild kits, cylinder head and liner sets shared across the wider 6D125-1 family, and grader-specific wear items like circle support shoes and the flywheel-to-hydraulic-pump damper coupling.

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 6D125-1DD(S/N 21811-UP) specifications

Engine

Engine family	Naturally aspirated member of the Komatsu 6D125-1 diesel family. This arrangement code (1DD) covers the S/N 21811-UP production break within that family; the factory shop manual for this platform also spans the turbocharged S6D125-1 and turbo-aftercooled SA6D125-1 variants, but this build has no turbocharger.
Configuration	6-cylinder, inline, 4-stroke diesel, water-cooled
Fuel system	Mechanical direct injection with inline injection pump and mechanical (flyweight) governor — no electronic engine control
Bore x stroke	125 mm x 150 mm (4.92 in x 5.91 in)
Displacement	11.04 L (11,040 cc / 674 cu in)
Aspiration	Naturally aspirated (no turbocharger on this arrangement — turbocharged builds of this block carry the S6D125-1 designation instead)
Gross/net power	Not separately published for this exact arrangement/serial break. Across naturally aspirated 6D125-1 family applications, published ratings commonly fall in a broad 92-134 kW (124-180 hp) class at roughly 1,750-2,200 rpm depending on host machine and governed speed — varies by configuration/application. Confirm against the host machine's own engine nameplate or the OEM shop manual for this serial range.
Emissions tier	Predates EPA/EU emissions-tier regulation. Mechanically governed, uncontrolled-emissions design typical of this engine's production era.

Weights

Scope note	This is a bare engine assembly, not a complete machine — no machine-level operating weight applies.
Engine dry weight	Approximately 1,000-1,050 kg (2,205-2,315 lb) reported generally for the 6D125-1 engine family. Not independently confirmed for this specific arrangement/accessory build (alternator and starter options differ within this serial range) — treat as approximate and verify against the OEM engine spec sheet.
Shipping weight	Not documented for this arrangement in available sources
Ground pressure	Not applicable — engine-only assembly, no undercarriage

Dimensions

Scope note	This is a bare engine assembly — machine-level transport length/width/height, wheelbase, track gauge, and ground clearance do not apply.
Overall engine dimensions	Not consistently documented across independent sources for this exact arrangement. Published figures for this engine family vary between sources and are not reliably corroborated — confirm against the OEM engine outline drawing before use.

Performance

Scope note	This is a bare engine assembly — machine-level travel speed, drawbar pull, dig/lift/dump performance, and gradeability do not apply; those depend on the host machine, not the engine alone.
Rated (governed) engine speed	Commonly falls in a 1,750-2,200 rpm range across naturally aspirated 6D125-1 family applications. Exact governed speed for this specific arrangement is not independently published — varies by host application.
Maximum torque	Not consistently documented for this specific arrangement in available sources

Service capacities (summary)

Lubricating oil (refill)	Not independently confirmed for this exact arrangement. Similar-era, similarly-configured 6D125-1-powered machines commonly cite engine oil refill capacity in the high-20s to low-30s of liters, but this varies by oil pan and cooler configuration — verify against the host machine's operation and maintenance manual.
Cooling system	Not independently confirmed for this exact arrangement — verify against the host machine's operation and maintenance manual.
Fuel tank	Not applicable to a bare engine — fuel tank capacity is host-machine-specific
Hydraulic system	Not applicable — this engine assembly has no integral hydraulic system; hydraulic capacity is host-machine-specific

Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.

6D125-1DD(S/N 21811-UP) fluids & capacities

System	Capacity	Recommended fluid
Engine crankcase (oil pan, with oil filter)	Not independently published for this exact engine suffix/serial break (6D125-1DD, S/N 21811-UP). Mechanical-governor 6D125-1-family inline-six engines of this displacement are commonly documented in the high-20s to low-30s liters range (roughly 7-8 US gal) with filter change, but the exact refill volume depends on the oil pan fitted for the specific application it was built into. Varies by configuration — confirm against the host machine's own operation and maintenance manual.	Komatsu Genuine engine oil (EO series). Viscosity is selected by ambient starting temperature per Komatsu's standard multigrade chart: lighter grades such as SAE 10W-30 for cooler climates, heavier grades such as SAE 15W-40 for warm climates, with intermediate grades for shoulder seasons. This mechanical fuel-injection-pump engine generation calls for a diesel engine oil meeting API CD/CE class or later CF-4/CI-4 equivalents.
Cooling system (engine coolant)	Not independently documented for this exact engine/serial combination. Whole-machine cooling systems (engine plus radiator) seen across dozer and loader applications built on the related 6D125 engine family run roughly 50-70+ liters (13-19 US gal), but that reflects the host machine's radiator sizing, not the bare engine's water jacket alone. Varies by configuration/host machine — confirm against the specific machine's OMM.	Komatsu genuine permanent-type antifreeze coolant (ethylene-glycol base), mixed to the freeze-point ratio required for the operating region. Do not mix coolant types or brands.
Fuel	No fuel tank on the bare engine assembly. Tank size is set by the host machine and documented in that machine's own operation and maintenance manual.	Diesel fuel meeting ASTM D975 No. 2-D specification (No. 1-D in cold weather) with adequate cetane rating. Low-sulfur diesel is recommended for injection-system longevity.
Grease (engine-mounted points)	Spec only — no refill volume applies. Apply at any external grease fittings on the accessory drive per the standard service interval.	Komatsu Genuine multipurpose lithium EP grease (NLGI No. 2 equivalent) at any external fittings present on this application.
Transmission / powertrain	Not applicable to this unit.	This job covers a bare Komatsu diesel engine assembly, not a complete machine. Transmission/powertrain fluid type and capacity are set by the host machine (dozer, loader, grader, truck, etc.) this engine was installed in — check that machine's own operation and maintenance manual.
Final drives	Not applicable to this unit.	Host-machine component, not part of the bare engine assembly. See the host machine's operation and maintenance manual.
Hydraulic system and tank	Not applicable to this unit.	Host-machine component, not part of the bare engine assembly. See the host machine's operation and maintenance manual.
Axles / differentials	Not applicable to this unit.	Host-machine component, not part of the bare engine assembly. See the host machine's operation and maintenance manual.

Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.

Komatsu 6D125-1DD(S/N 21811-UP) maintenance schedule

Service interval	Tasks
Every 10 h	Check engine oil level before starting each shift; top up to the full mark, never above it. Check coolant level at the sight gauge or radiator, and scan hoses, the water pump, and the block for external leaks. Drain water and sediment from the fuel filter/water separator sump. Check the air intake pre-cleaner and restriction indicator; service now if dust load from grading work has already tripped it. Weekly (50h): grease the circle drive, drawbar ball, and blade-lift/tilt linkage points. Weekly (50h): check fan belt tension and condition, and inspect the radiator core for packed dust or debris.
Every 250 h	Change engine oil and the oil filter. Check the injection pump and governor linkage for play or binding. Check battery electrolyte level and terminal condition; clean and re-torque connections. Grease chassis and steering linkage points not covered on the weekly cycle. Check tandem case oil level on each final drive housing.
Every 500 h	Replace the fuel filter/water separator element. Clean the dry-type air cleaner main element, or replace it sooner if the restriction indicator trips. Check coolant condition and corrosion-inhibitor concentration; top off as needed. Check circle and drawbar wear shoes and shims for excess play; adjust as needed. Inspect the alternator and starter mounting and wiring for chafing from grader vibration.
Every 1,000 h	Change the torque-converter/transmission oil and filter feeding the tandem final drives. Change tandem final drive oil in each housing. Check and adjust engine valve clearance. Inspect the flywheel-to-hydraulic-pump damper coupling for spring-tension loss; a knock or rattle from the bellhousing, or hot, dark, overfull damper oil, is the tell. Check engine mounting bolts and the bellhousing connection for looseness.
Every 2,000 h	Drain, flush, and refill the cooling system if coolant hasn't already been replaced. Pull injection nozzles for spray-pattern inspection; recondition or replace as needed. Send an oil sample out for wear-metal and fuel-dilution analysis to benchmark ahead of the next major service. Torque-check the cylinder head and major structural fasteners. Inspect the water pump for shaft play or seal weep.
Every 4,000 h	Pull the cylinder head for valve seat and guide inspection; recondition or replace as worn. Check cylinder bore taper and ovality against wear limits; plan for re-ringing or a liner set if near the limit. Bench-test and recalibrate the mechanical injection pump rather than assume it is still in spec. Inspect circle support shoes and pinion teeth for wear tied to shim drift, and reset shims. Check crankshaft end play and main bearing clearance whenever the engine is opened for other work.

Servicing the 6D125-1DD(S/N 21811-UP) beyond the schedule

Predictive Maintenance & Fluid Analysis

This mechanically governed 1DD build has no ECM and no stored fault codes, so scheduled oil and coolant sampling is the real early-warning tool. Track iron and copper trends from liner and bearing wear, and fuel dilution that signals injection pump or governor linkage wear. A reported power loss on this engine often traces to a small air leak in the low-pressure fuel transfer line rather than a worn pump. Sample tandem case oil on the same window too, since driveline wear metals show up there as well.

Corrective & Common Repairs

Electrical complaints on this pre-ECM engine are almost always in the charging and starting circuit, not engine control. A starter interlock relay runs off alternator output, so a machine that won't crank, or tries to re-engage while the engine runs, usually points to the alternator or wiring, not the starter. A knock or rattle from the bellhousing traces to the flywheel-to-hydraulic-pump damper coupling losing spring tension, which also lets its damper oil run hot, dark, and overfull. Lingering white smoke past warm-up points to an injector or fuel problem.

Overhaul & Rebuild Points

Top-end rebuilds on the 1DD typically open at cylinder liner wear or ring-land carbon buildup, not one dramatic failure. Check bore taper and ovality against wear limits before re-ringing, and check valve seat recession and guide wear at the same interval as head resurfacing. Have the mechanical injection pump bench-tested and recalibrated rather than assumed good on reassembly. On the host grader's chassis side, budget for circle support shoe and pinion inspection, since shim drift accelerates wear, and check tandem case oil whenever the engine comes out for a major.

Seasonal & Environment Servicing

Grading work loads the air intake harder than most jobs, so shorten pre-cleaner and element service on loose or sandy sites. A brief puff of white smoke on cold start is normal for this naturally aspirated mechanical diesel and clears within seconds; smoke that lingers points to an injector or fuel problem, not a seasonal trait. In cold climates, check antifreeze concentration for the expected low temperature and switch to a winter-grade fuel blend ahead of season, since this mechanical injection system has no factory fuel heater.

6D125-1DD(S/N 21811-UP) common service parts

Part number	Part	Notes
07281-00197	Clamp,(With Pre-Fuel Filter)	Fuel system service	Check fitment →
600-311-8291	Cartridge,Fuel Filter	Fuel system service	Check fitment →
6114-71-6200	Fuel Filter Ass'Y	Fuel system service	Check fitment →
6114-71-6210	Head, Fuel Filter	Fuel system service	Check fitment →
ND092220-0040	Filter,Fuel	Fuel system service	Check fitment →

Always confirm against your machine's serial number — cross-check any part in Heavy Parts AI before ordering.

6D125-1DD(S/N 21811-UP) fault codes & troubleshooting

Code	Meaning	Likely cause	What to do
ENGINE OIL PRESSURE caution lamp	Engine oil pressure has dropped below the safe minimum	Low oil level, worn oil pump, clogged suction screen or filter, or a failed pressure sender	Stop the engine immediately, check oil level and look for leaks before restarting; do not run the engine on a lit lamp
COOLANT TEMPERATURE caution lamp	Engine coolant temperature has risen above the normal operating range	Low coolant level, clogged radiator core, slipping or broken fan belt, or a stuck thermostat	Idle down and stop in a safe spot, let the engine cool, then check coolant level and radiator fins; do not remove a hot cap
CHARGE caution lamp	Alternator is not charging the battery	Slipping or broken fan/alternator belt, failed alternator, or loose/corroded wiring at the battery or alternator	Check belt tension and battery terminal condition, then test alternator output
AIR CLEANER caution lamp	Intake air filter restriction has reached the service limit	Clogged primary element from dust or debris ingestion, or damaged intake ducting	Service or replace the air filter element and inspect intake hoses/clamps for damage
TORQUE CONVERTER OIL TEMPERATURE caution lamp	Torque converter/transmission oil temperature is too high	Low transmission oil level, clogged oil cooler, or a prolonged high-load stall condition	Reduce load and let the machine idle to cool, then check oil level and inspect the cooler for restriction
FUEL LEVEL warning lamp	Fuel level in the tank is low	Normal fuel consumption approaching empty, or an unexpected fuel leak	Refuel; if the tank empties faster than expected, check fuel lines and tank for leaks
PREHEAT (intake air heater) indicator lamp	Intake air preheating device is active to aid cold starting	Normal cold-weather start sequence, not a fault condition	Wait for the lamp to go out per the cold-start procedure before or while cranking

Codes and remedies are general guidance for this model family — always confirm with diagnostic tooling and your dealer before major repairs.

6D125-1DD(S/N 21811-UP) attachments & work tools

Host equipment class

This is a bare mechanical 6D125-1 family engine, not a complete machine. Parts-fitment records tie the 21811-UP engine block most consistently to Komatsu's mid-to-large motor grader line of the same era (roughly the GD605A through GD672A range). The engine's own parts book does not name one host nameplate, so confirm the exact grader model and options against the machine's own serial plate before ordering work tools.

Front moldboard / cutting blade

The main work tool on this grader class is the front moldboard, roughly 3.7 m (12 ft) long, mounted on the drawbar-circle-moldboard assembly. Moldboard length and depth vary by configuration/series across the family. Bolt-on cutting edges and end bits are the standard wear items, not the moldboard itself.

Circle / drawbar mounting

The moldboard mounts to a rotating circle that is itself carried on the drawbar, not a quick-attach coupler like an excavator or loader. Hydraulic motors turn the circle while separate cylinders handle blade lift, side-shift, and blade tip angle; no aftermarket quick-coupler is standard for the moldboard on this class.

Front-mount scarifier

An optional front scarifier bar attaches to the front lift group ahead of the front axle for light surface ripping and frost-breaking. It shares the grader's existing front lift hydraulic circuit and does not need a separate hydraulic kit.

Mid-mount scarifier

A mid-mount scarifier fits behind the front wheels and ahead of the moldboard, used to loosen compacted or frozen material before grading. Shank spacing and configuration vary by model and options package.

Rear ripper / scarifier

A rear-mounted ripper or scarifier bar bolts to the rear frame and uses tire weight for penetration on harder material. This is typically an optional attachment tied into its own lift/shift hydraulic circuit off the grader's auxiliary valve bank rather than the base machine circuit.

Snow wing / side plow

A hydraulically folding snow wing mounts along the frame for wing plowing and stows flat against the machine when not in use. It is an optional factory or dealer-fit kit that needs its own auxiliary hydraulic circuit and control valve.

Factory guarding and cab options

Factory options documented for this grader class include a ROPS/FOPS-rated operator cab, radiator and screen rock guards, and front push-block or bumper guarding. Exact availability of guarding and cab packages varies by configuration/series and market, and should be checked against the specific machine's option code.

All 6D125-1DD(S/N 21811-UP) assemblies by section

Every catalogued assembly group for the Komatsu 6D125-1DD(S/N 21811-UP). Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.

Cooling System

Komatsu : 6d125-1dd(S/N 21811-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
01***95	Bolt,(For 2nd Travel Speed) (Op)	4

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Komatsu : 6d125-1dd(S/N 21811-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
01***16	Washer,Radio Controlled Slag Handling	1

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Komatsu : 6d125-1dd(S/N 21811-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)	6
01***35	Bolt,(For Quick Shift) (Mitsubishi)	1

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Cylinder Block

Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Block : Camshaft

01***22	Bolt	2
04***25	Key, Plain Woodruff	1
04***12	Ring,Snap (For Ripper)	12
61***12	Camshaft Assembly	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Block : Crankshaft

01***50	Bolt,(For Strengthened Cover)	6
01***15	Bolt	1
01***20	Bolt	5
04***28	Pin, Dowel - Transmission Case, Pin-Dowel	1

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Komatsu : 6d125-1dd(S/N 21811-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
04***20	Pin,Dowel (Welded)	12

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Komatsu : 6d125-1dd(S/N 21811-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
01***16	Washer,Radio Controlled Slag Handling	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Block : Flywheel And Flywheel Housing(#21811-32669)

01***40	Bolt,(Dust And Dirt Protection)	2
01***50	Bolt, Radiator To Frame	6
01***65	Bolt - Battery Box	2
01***40	Bolt,R.H. Outside And L.H. Inside, Bolt	8

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Block : Flywheel And Flywheel Housing(#32670-)

01***40	Bolt,(Dust And Dirt Protection)	2
01***50	Bolt, Radiator To Frame	6
01***65	Bolt - Battery Box	2
01***40	Bolt,R.H. Outside And L.H. Inside, Bolt	4

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Block : Front Cover And Front Support(#21811-34801)

01***25	Bolt,(Without Auto Priming System)	6
01***60	Screw	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	1
01***65	Bolt,(Except Japan)	7

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Block : Front Cover And Front Support(#34802-)

01***25	Bolt,(Without Auto Priming System)	6
01***30	Bolt,(For France Noise Suppression)	6
01***60	Screw	1
01***65	Bolt,(Except Japan)	7

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Komatsu : 6d125-1dd(S/N 21811-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
01***32	Washer,(With Auto Emergency Steering)	2

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Block : Piston And Connecting Rod

02***13	Pin, Dowel	2
04***18	Ring, Snap	12
61***32	Piston Ring Ass'Y	6
61***33	Piston Ring Ass'Y	6

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Cylinder Head

Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Head : Air Cleaner And Mounting

01***25	Bolt,(For Canvas Canopy?Head Guard)	2
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

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Komatsu : 6d125-1dd(S/N 21811-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	5
01***60	Bolt, Mount Tube To Switch Pump	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Head : Cylinder Head(#21811-26697)

04***20	Pin,Dowel (Welded)	2
61***30	Sleeve,Holder	1
61***20	Cotter, Valve	48
61***40	Seal,Exhaust Valve (K1)	12

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Head : Cylinder Head(#26698-)

04***20	Pin,Dowel (Welded)	2
61***30	Sleeve,Holder	1
61***20	Cotter, Valve	48
61***40	Seal,Exhaust Valve (K1)	12

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Head : Exhaust Manifold

01***80	Bolt, Mount Strainer Flange	18
07***08	Plug,(Without Auto Priming System)	1
61***20	Spacer,(For Shipping)	18
61***20	Manifold,Exhaust? Center	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Head : Muffler And Mounting

01***25	Bolt,(Sandy? Dusty Terrain Spec.)	4
01***40	Bolt,(Dust And Dirt Protection)	4
01***30	Bolt, Mount Pump	2
01***45	Screw	2

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Head : Rocker Arm And Housing

01***08	Nut,R.H. Thread	12
01***16	Washer,Radio Controlled Slag Handling	40
01***32	Washer,(With Auto Emergency Steering)	40
61***10	Housing, Rocker Lever	6

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Head : Rocker Arm Housing Cover

01***25	Bolt,(For Canvas Canopy?Head Guard)	1
01***00	Bolt, Mount Back-Up Alarm	2
01***08	Nut,(For Canvas Canopy?Head Guard)	1
01***16	Washer,Radio Controlled Slag Handling	27

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Komatsu : 6d125-1dd(S/N 21811-Up) : Cylinder Head : Water Manifold And Thermostat

02***43	O-Ring,Water Manifold	6
07***12	Washer, Gasket,(Without Thermo Sensor) (K1)	1
07***08	Plug,(Without Auto Priming System)	1
07***12	Plug,(Without Corrosion Resistor)	2

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Electrical System

Komatsu : 6d125-1dd(S/N 21811-Up) : Electrical System : Alternator (13a)

60***50	Alternator A. (13a),(With Non Hardening Pulley)	1
KD***31	Bushing,Insulator	2
KD***72	Slip Ring Ass'Y	1
KD***90	Wire,Read	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Electrical System : Alternator (35a)

60***60	Alternator A. (35a),(With Non Hardening Pulley)	1
KD***57	Bearing,Ball? Front	1
KD***80	Collar	1
KD***70	Rear Bracket Ass'Y,Rear	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Electrical System : Alternator And Mounting (13a)

01***55	Bolt, Mount Tube To Switch Pump	1
01***25	Bolt	1
01***11	Nut (Welded)	1
01***26	Washer,(For Additional Rear Lamp)	2

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Komatsu : 6d125-1dd(S/N 21811-Up) : Electrical System : Alternator And Mounting (35a)

01***30	Bolt,(For Quick Shift) (Mitsubishi)	1
01***30	Bolt	1
01***23	Washer,(A)(W/After Cooler)(S6d155-4b)	2
01***32	Washer,(Not Used With Emergency Str.)	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Electrical System : Starting Motor (11kw)(#38123-)

60***12	Starting Motor Ass'Y,(11kw)	1
SD***X9	Switch Ass'Y,Engage	1
SD***X0	Bearing,Ball	1
SD***X0	Bolt	2

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Komatsu : 6d125-1dd(S/N 21811-Up) : Electrical System : Starting Motor (7.5kw)

60***30	Starting Motor Ass'Y,(7.5kw)	1
KD***60	Shift Lever Assembly	1
KD***40	Spring,Spring	4
KD***00	Terminal Assembly	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Electrical System : Starting Motor And Mounting (11kw)(#38123-)

01***40	Bolt,(With Center Guard)	3
66***10	Gasket,Starting Motor	1
(6***2)	Starting Motor Ass'Y,(11kw)(See Fig.0662a)	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Electrical System : Starting Motor And Mounting (7.5kw)

01***40	Bolt,(With Center Guard)	3
66***10	Gasket,Starting Motor	1
(6***0)	Starting Motor Ass'Y,(7.5kw) (See Fig.0661a)	1

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Engine Kit

Komatsu : 6d125-1dd(S/N 21811-Up) : Engine Kit : Gasket Kit

02***43	O-Ring,Water Manifold	6
07***15	O-Ring,(For Low Speed) (D40pf)	1
07***25	O-Ring,Injection Pump Mounting	1
07***30	Ring, Seal - Between Sections	1

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Fuel System

Komatsu : 6d125-1dd(S/N 21811-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
01***16	Washer,Radio Controlled Slag Handling	2

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Coupling)(#21965-)

ND***30	Block,Coupling	2
ND***10	Flange,Coupling	1
ND***50	Bolt	1
ND***50	Coupling,Cross	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Feed Pump)(#21965-)

ND***40	Washer? 1mm,Plate	1
ND***70	Washer,Union	4
ND***10	Spring, Check Valve	2
ND***10	Pin,Roller	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2)(#21965-29456)

ND***40	Plate,Thrust	1
ND***40	Plug,Screw	1
ND***50	Washer? 0.3mm	0
ND***70	Spring,Inner	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2)(#29457-)

ND***40	Plate,Thrust	1
ND***80	Pin,Stop	1
ND***40	Plug,Screw	1
ND***10	Adapter	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2)(#21965-29456)

ND***20	Washer? 1.0mm,Cover Plate	2
ND***10	Seal,Oil	1
ND***50	Packing,Rubber Plate	1
ND***80	Plate,Cover	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2)(#29457-)

ND***20	Washer? 1.0mm,Cover Plate	2
ND***90	Seal,Oil	1
ND***40	O-Ring	1
ND***10	Seal,Oil	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Pump) (1/2)(#21965-29456)

61***60	Injection Pump A	1
ND***50	Ring,Adjust	1
ND***80	O-Ring	1
ND***20	Retainer,Bearing	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Pump) (1/2)(#29457-)

61***20	Injection Pump A	1
61***21	Injection Pump A	1
ND***50	Ring,Adjust	1
ND***80	O-Ring	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Pump) (2/2)(#21965-29456)

ND***70	O-Ring	6
ND***71	Rack,Control	1
ND***50	Seat,Spring? Lower	6
ND***20	Shim? 1.9mm,Element Holder	0

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump (Pump) (2/2)(#29457-)

ND***30	Sleeve,Control	6
ND***20	Shim? 0.3mm	0
ND***70	O-Ring	6
ND***20	Bolt,With Washer	3

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump And Piping(#21965-29456)

01***75	Bolt,(See Fig.0142)	4
01***30	Bolt	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	6
01***45	Bolt,(For Power Tilt Mech Angle)	2

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Injection Pump And Piping(#29457-)

01***75	Bolt,(See Fig.0142)	4
01***30	Bolt	1
01***20	Bolt,(For Quick Shift) (Mitsubishi)	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	5

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Nozzle And Spill Pipe(#21811-29456)

01***45	Bolt, Divider Valve Mounting	12
07***65	O-Ring,Cylinder Block	6
07***12	Gasket,Mechanical Pump Piping	2
07***12	Gasket,(Without Air Compressor Type) (K2)	12

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Fuel Nozzle And Spill Pipe(#29457-)

01***45	Bolt, Divider Valve Mounting	12
07***65	O-Ring,Cylinder Block	6
07***12	Gasket,Mechanical Pump Piping	2
07***12	Gasket,(Without Air Compressor Type) (K2)	12

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Water Separator (For 13a Alternator)

07***60	O-Ring,(For Pc750?800?1800) (A2610)	1
60***00	Water Separator A.,(4d105-5h? 4d105-5t)	1
60***60	Plug,Drain (4d105-5h? 4d105-5t)	1
60***10	Cover	1

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Komatsu : 6d125-1dd(S/N 21811-Up) : Fuel System : Water Separator And Mounting (For 35a Alternator)

01***25	Bolt,Without Revolution Sensor	2
01***20	Bolt,(For Quick Shift) (Mitsubishi)	2
01***16	Washer,(For 35a Alternator)	2
07***60	O-Ring,(For Pc750?800?1800) (A2610)	1

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Lubricating Oil System

Komatsu : 6d125-1dd(S/N 21811-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
01***80	Bolt,(Not Shown)	2

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Komatsu : 6d125-1dd(S/N 21811-Up) : Lubricating Oil System : Lubricating Oil Filter

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
01***16	Washer,Radio Controlled Slag Handling	4

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Komatsu : 6d125-1dd(S/N 21811-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
01***85	Bolt,(For 2nd Travel Speed) (Op)	1

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Komatsu : 6d125-1dd(S/N 21811-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
01***35	Bolt, Mounting - Flywheel Housing End	1

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Frequently asked questions

What engine does the Komatsu 6D125-1DD (S/N 21811-UP) use?

It is the naturally aspirated member of Komatsu's 6D125-1 diesel family: a water-cooled, inline six-cylinder, 11.04-liter block (125 mm bore x 150 mm stroke) with a mechanical inline injection pump and flyweight governor, no turbocharger and no electronic engine control. The same block is also built as the turbocharged S6D125-1 and turbo-aftercooled SA6D125-1, sharing one factory manual family with this arrangement.

What's the operating weight of the machine this engine powers?

As a bare engine it has no weight rating of its own. It was fitted across Komatsu's mechanical-era motor grader line (roughly the GD605A through GD663A/GD672A range), where operating weights run about 13,300-15,000 kg (29,300-33,000 lb) depending on model and options. The closest documented power and weight matches in that family are the GD661A-1 and GD663A-2.

What replaced the 6D125-1DD?

There is no direct successor under the same naturally aspirated 6D125-1 code. Komatsu's later graders in this size class moved to electronically controlled, emissions-compliant engines with onboard diagnostics, replacing the mechanically governed 6D125-1 family across most regulated markets. Where emissions rules do not apply, machines carrying this engine remain common in service and in the used market.

What 6D125-1DD(S/N 21811-UP) owners discuss

How does this engine perform under load in real grading work, and is the power rating consistent across sources?

This is a naturally aspirated, mechanically governed engine. Expect steady power delivery with no turbo lag, but a lower ceiling than the turbocharged (S-prefix) or turbo-aftercooled (SA-prefix) versions of the same block. Published output for naturally aspirated members of this engine family spans a wide range, roughly 92-134 kW (124-180 hp), depending on the host machine's governed speed and pump calibration. Treat any single number quoted online with caution and check the engine nameplate on the actual machine. A common field lesson: a reported power loss on this engine generation is often a small air leak in the low-pressure fuel transfer line, not a worn injection pump. Check the transfer line before pulling the pump.

What wear patterns show up most on the machines this engine powers?

This block was fitted across a family of mechanical-era Komatsu motor graders. On that machine type, the circle and moldboard get the closest attention. Uneven wear on the circle support shoes or pinion teeth usually traces to a circle that has drifted out of shim adjustment. Packed material buildup in the circle track accelerates that wear. Tandem case oil condition and cutting-edge or end-bit wear are the other routine watch items on this machine class. Wear rates vary by ground conditions and material worked, so judge by inspection and oil sampling rather than a fixed hour figure.

Are the naturally aspirated, turbo, and turbo-aftercooled versions of this block really different, or mostly the same engine?

The short block is shared, but the top end and output differ. This DD arrangement is naturally aspirated with no turbocharger. The same base 6-cylinder block is also built as a turbocharged version and, further up the range, a turbocharged-and-aftercooled version. All three share one factory shop manual family, which is why they get blended together in casual listings. Power, cooling demand, and top-end hardware like the turbo and aftercooler plumbing differ between the three. When comparing specs or parts online, confirm which prefix you are actually reading about.

What electrical faults come up most with this engine's charging and starting circuit?

This engine predates electronic engine management. There is no ECM and no sensor network to fault-code, so electrical complaints are almost always in the charging and starting circuit, not engine control. The wiring scheme on this era of engine runs a starter safety interlock relay off the alternator's output. When the alternator charges normally, that circuit grounds out and locks the starter from engaging while the engine runs. If a machine tries to re-engage the starter while running, or will not crank at all, check the alternator and its interlock wiring before condemning the starter motor. Because this affects whether the starter can engage against a running engine, have your dealer verify the interlock circuit before returning the machine to service.

Is a puff of white smoke and rough running on a cold start something to worry about?

A brief puff of white smoke and a slightly rough few seconds on a cold start is normal for this generation of naturally aspirated mechanical diesel, especially in cool weather. It should clear within seconds as the engine warms and combustion stabilizes. White smoke that lingers well past warm-up, or hard starting that does not improve once batteries and the starting aid check out good, points to a fuel or injector problem rather than a normal cold-start trait: think weak injector spray pattern, water or contamination in the fuel, or timing drift. Have your dealer or a diesel fuel shop pop-test the injectors before condemning the injection pump.

Is there a drivetrain-adjacent wear item near the engine that gets missed?

The coupling connecting the engine flywheel to the machine's engine-driven hydraulic pump is a commonly overlooked wear item on this era of machine. It typically uses a damper-disc style coupling with internal springs. On higher-hour machines those springs lose tension, which shows up as a knock or rattle from the bellhousing area and can let the coupling's damper oil, where fitted, run hot, dark, and overfull. This is a host-machine hydraulic-drive item, not an engine-internal fault, but it is worth ruling out before chasing an engine noise complaint into the block itself.

What should a buyer check specifically before purchasing a machine fitted with this engine variant?

Confirm the engine data plate reads this exact arrangement code with a serial in the 21811-and-up range. Several visually similar suffix codes exist on the same base block, each covering a different parts and configuration break, so do not assume interchangeability from the bare model number alone. Because this is a naturally aspirated mechanical engine with no electronic diagnostics, a pre-purchase check leans on classic methods: a compression or leak-down test, exhaust color checked across the full rev range and not just at idle, an oil sample for wear metals and fuel dilution, and a check of the injection pump and governor linkage for play. On the host machine, walk the circle and moldboard for shim wear and check tandem case oil condition. Full service and oil-sample history is the strongest tell on a mechanically governed engine like this one, since there is no fault-code history to fall back on. Have your dealer or an independent diesel shop verify compression, injector condition, and injection timing before final purchase.

Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.

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