Komatsuengine(no series)

Komatsu 6D125-1C(S/N 10001-UP)

Maintenance schedule, common problems & OEM parts breakdown

Search live 6D125-1C(S/N 10001-UP) parts

The Komatsu 6D125-1C (S/N 10001-Up) is a naturally aspirated, mechanically governed inline-6 diesel: 11.04 L (673.8 cu in) displacement, 125 mm x 150 mm (4.92 in x 5.91 in) bore and stroke, mechanical inline injection pump with an all-speed governor, no electronic controls. It is the factory powerplant for the Komatsu D53A-17 (standard-track) and D53P-17 (wide-track, low-ground-pressure "swamp") crawler dozers, paired with Komatsu's TORQFLO powershift transmission and a torque converter. Operating weight for that host machine runs roughly 12.8-13.6 tonnes (28,200-30,000 lb), varying with track gauge, blade, and ripper fitted. This -17 chassis sits inside Komatsu's broader D53 crawler dozer series, produced roughly across 1993-2002, following the D53A-16/D53P-16 and preceding the D53A-18/D53A-18E/D53P-18E generation. Factory configurations spanned standard or LGP undercarriage, straight or Semi-U dozer blade, a rear ripper, and an optional cable-control winch for logging or pipeline work.

Across the D53 series, Komatsu paired this same-displacement 6D125-1 block architecture with several dash-suffix calibrations tuned for different host machines, plus a separate turbocharged S6D125-1 line built for other chassis and applications entirely; keep those distinct from this naturally aspirated -1C build. By the -18/-18E generation, Komatsu moved the D53A/D53P chassis onto the newer 6D125E-2 engine family, closing out the mechanical 6D125-1's run on this platform. In the used and parts market today, that makes the 6D125-1C a legacy mechanical block: still working in dozers doing production dozing, land-clearing, and forestry, valued for its simplicity (no ECM to fail, no turbocharger to rebuild) and for parts commonality across the wider 6D125-1 family. Buyers still need to confirm the exact dataplate suffix and the absence of an S-prefix before ordering engine-specific parts, since the naturally aspirated and turbocharged lines don't interchange.

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 6D125-1C(S/N 10001-UP) specifications

Engine

Model / platform	Komatsu 6D125-1C (S/N 10001-UP), part of the mechanical 6D125-1 diesel engine family dating from the 1980s
Configuration	Inline 6-cylinder, 4-stroke, water-cooled, naturally aspirated, direct-injection diesel
Displacement	11.04 L (673.8 cu in)
Bore x stroke	125 mm x 150 mm (4.92 in x 5.91 in)
Fuel system	Mechanical injection pump with all-speed governor, typical of this pre-electronic engine family
Gross power	Approx. 116-119 kW (155-160 hp) at 1,850 rpm for this naturally aspirated calibration; exact certified rating for the -1C build specifically is not published in general spec sheets, so treat as a range and confirm against the engine nameplate
Emissions tier	Pre-emissions mechanical diesel; predates EPA Tier and EU Stage certification

Weights

Engine dry weight

Not documented for this exact serial range in available spec sheets — confirm with the shop manual or a Komatsu dealer

Dimensions

Flywheel housing	SAE No. 1/2 flywheel housing
Overall engine envelope (L x W x H)	Not documented for this exact serial range in available spec sheets — confirm with the shop manual or a Komatsu dealer

Performance

Rated speed	1,850 rpm (governed)
Maximum torque	Not documented for this exact configuration; published torque figures found for other 6D125-family calibrations belong to different rated builds and are not applicable here

Service capacities (summary)

Lubricating oil (refill)	Not documented for this exact serial range in available spec sheets — confirm with the shop manual or a Komatsu dealer
Cooling system (engine only)	Not documented for this exact serial range in available spec sheets — confirm with the shop manual or a Komatsu dealer
Fuel tank	Not applicable — this is a bare engine; fuel tank capacity is set by the host machine or generator set it is installed in

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

6D125-1C(S/N 10001-UP) fluids & capacities

System	Capacity	Recommended fluid
Engine crankcase (with filter)	Not separately published for this exact block/serial listing. The 6D125-1 family (same bore/stroke block, related dash-suffix/serial breaks) is commonly cited around 30 L (7.9 US gal) refill including the filter — treat as a ballpark only and confirm against the dipstick or a Komatsu dealer before topping off.	Komatsu Genuine Engine Oil (EO), grade picked by ambient temperature per the standard Komatsu selection chart — SAE 10W, 10W-30, 15W-40 or straight 30 grade, meeting API CD/CE (the period diesel-oil spec for this mechanical, pre-emission engine generation). Field replacements commonly use an equivalent SAE 15W-40 heavy-duty diesel oil meeting API CD/SF.
Cooling system (engine only)	Not published as a stand-alone engine figure for this bare-block listing — Komatsu literature for this generation gives full-machine system capacity, not engine-alone. Complete systems on period D60/D65-class dozers built around this block run roughly 35-65 L (9-17 US gal) depending on host machine and radiator size; treat this as a range that varies by configuration, not a confirmed figure for the engine alone.	Komatsu Supercoolant (AF-NAC) or an equivalent ethylene-glycol antifreeze/corrosion-inhibitor blend, maintained with the engine's own corrosion resistor cartridge on this classic (non long-life) cooling system.
Grease (spec only)	No capacity applies (grease is spec-only).	Komatsu Genuine Grease (multipurpose lithium-base, NLGI No. 2, general-purpose G2-LI type) for external linkage/bracket grease points on the engine and its mounting hardware, standard across Komatsu equipment of this era.

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

Komatsu 6D125-1C(S/N 10001-UP) maintenance schedule

Service interval	Tasks
Every 50 h	Check engine oil level before starting and look for leaks around the pan, filter, and valve cover. Check coolant level and inspect hoses and the corrosion resistor cartridge housing for weeping. Drain water and sediment from the fuel tank and primary fuel filter/sedimenter. Inspect fan belt tension and condition; listen for belt squeal on startup. Check the air cleaner restriction indicator and clean the outer element if it has tripped. Grease exposed engine-mount and linkage points and wipe down fittings, checking for fuel or oil weep.
Every 250 h	Change engine oil and the oil filter. Replace the fuel filter cartridge and bleed the low-pressure fuel system. Check and adjust fan belt tension. Check battery electrolyte level and terminal condition. Inspect the corrosion resistor cartridge and service or replace it if depleted. Check torque on the alternator, starter, and engine mounting hardware.
Every 500 h	Replace the corrosion resistor cartridge on schedule, not just on condition. Check the torque converter/transmission oil level on the host dozer's TORQFLO drivetrain. Clean the fuel tank strainer and breather. Inspect and clean the radiator core fins. Check governor linkage and throttle cable for free movement, since sticking linkage is a common cause of idle hunting on this mechanical engine.
Every 1,000 h	Change the torque converter/transmission oil and filter. Pull, test, and recalibrate the fuel injection nozzles' spray pattern. Check and adjust engine valve clearance. Inspect starter motor and alternator internals (brushes and bearings). Check torque on engine and transmission mounting hardware.
Every 2,000 h	Check and adjust fuel injection pump timing. Re-inspect injection nozzles and replace any that are out of spec. Re-check valve clearance and retorque cylinder head fasteners. Inspect the water pump for bearing and seal wear. Inspect the TORQFLO torque converter and clutch packs for wear. Check the crankshaft pulley/vibration damper for cracking or rubber separation.
Every 4,000 h	Run a compression and blow-by check to assess cylinder liner, piston, and ring wear before committing to an in-frame overhaul. Rebuild or exchange the fuel injection pump. Overhaul the TORQFLO torque converter and transmission clutch packs. Inspect the cylinder head and resurface or replace it if valve seats are worn past spec. Walk the full undercarriage (track, rollers, idlers, sprockets) alongside the engine overhaul window, since wear there tracks closely with engine hours on this dozer class.

Servicing the 6D125-1C(S/N 10001-UP) beyond the schedule

Predictive maintenance & fluid analysis

This mechanical 6D125-1C runs no ECM, so oil analysis at each engine oil change is the best early-warning tool: track iron and copper wear metals for liner and ring wear before it shows up as blow-by. Watch idle stability. Hunting rpm on this inline-pump, all-speed-governor engine points to a fuel filter or governor linkage problem, not electronics. Keep the corrosion resistor cartridge on schedule; neglecting it corrodes the cooling system from the inside. On the host dozer's TORQFLO transmission, sample torque converter oil for the same early-wear signals.

Corrective & common repairs

Owners of this engine family report oil at the crankcase breather more often than a simple gasket leak; trace it to worn piston rings and liner before regasketing. No-crank complaints usually trace to a dirty or failed starting-motor safety relay rather than the starter itself; check that and the wiring before swapping parts. Charging faults are often a bad alternator diode or corroded terminals, not a dead alternator. On the host dozer, idle hunting clears with a fuel filter change and freed governor linkage, not a pump swap.

Overhaul & rebuild points

This naturally aspirated block overhauls simpler than its turbocharged S6D125-1C sibling: no turbocharger or charge-air hardware to rebuild. The top overhaul driver is cylinder liner, piston, and ring wear showing up as breather blow-by; confirm with a compression and blow-by check before committing to a ring-and-liner job. Have the injection pump recalibrated and nozzles tested at the same time, and reset valve clearance. On the host dozer, plan a TORQFLO torque converter and clutch-pack inspection alongside a major engine overhaul, since both wear on similar hour cycles.

Seasonal & environment servicing

The factory parts catalog for this engine lists distinct sandy-and-dusty-terrain and high-altitude arrangement codes, swapping air cleaner and intake hardware for those environments; order the correct build arrangement rather than a generic filter. In cold climates, expect harder cold-starts than a modern engine: this mechanical pump and all-speed governor have no glow-plug or ECM-assisted starting aid, so ether or a block heater is standard practice. In humid or mineral-heavy water regions, keep the corrosion resistor cartridge on schedule, since this classic cooling system has no long-life coolant fallback.

6D125-1C(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.

6D125-1C(S/N 10001-UP) fault codes & troubleshooting

Code	Meaning	Likely cause	What to do
Oil pressure warning lamp (red, engine symbol)	Engine oil pressure has dropped below the safe minimum.	Low oil level, failing oil pump, clogged suction strainer, or worn bearings.	Stop the engine immediately, check oil level and look for leaks before restarting.
Coolant temperature warning lamp / red-zone gauge	Engine coolant temperature has exceeded the normal operating range.	Low coolant level, blocked radiator core, slipping or broken fan belt, or a stuck thermostat.	Let the engine idle down, check coolant level once it is safe to open the system, inspect radiator and belt before returning to work.
Charge warning lamp (alternator/battery symbol)	The alternator is not charging the battery circuit.	Slipping or broken fan/alternator belt, failed alternator, or a loose or corroded wiring connection.	Check belt tension and wiring first, then test alternator output and replace if needed.
Air cleaner restriction indicator	Intake air restriction has reached the service limit.	Dust-clogged air filter element.	Inspect and clean or replace the air filter element, then reset the indicator.

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

6D125-1C(S/N 10001-UP) attachments & work tools

Dozer blade

This engine's host is a mid-size crawler dozer in the Komatsu D60/D65 class (roughly 16 tonne operating class, about 119 kW/160 hp). Factory blade choices for this class run straight blade or Semi-U blade on outside push arms, with a full-U blade offered for high-volume dozing; blade width is around 3.7-4.3 m (12-14 ft) depending on the specific model and blade chosen.

Blade hydraulic control and mounting

Blade lift, angle and tilt run off dedicated hydraulic cylinders fed from the tractor's own valve bank, not a bolt-on quick coupler. The parts catalog for this engine carries a distinct straight-tilt-dozer build arrangement, confirming factory single-tilt and dual-tilt blade options on this class.

Ripper

Rear-mounted ripper is the standard ground-breaking work tool for this dozer class, offered as a single-shank or multi-shank parallelogram design. It pin-mounts to the tractor's rear frame and runs on its own hydraulic cylinder(s), independent of the blade circuit.

Winch (factory option)

A cable-control winch was a factory-fitted option on this weight class for logging and pipeline work, mounted at the rear in place of or alongside the ripper. Not standard equipment; ordered as a separate rear arrangement.

Factory build/arrangement codes

The parts catalog tied to this engine lists separate factory arrangements for a trimming/grading dozer configuration, sandy-and-dusty-terrain spec, and high-altitude spec (affecting air cleaner and turbocharger hardware). These are factory-ordered build codes, not aftermarket kits, and each swaps specific engine-side components rather than needing a separate hydraulic kit.

Guarding and operator protection

Canopy and head-guard mounting hardware appears as a factory-listed option tied to this engine's fitment, for ROPS/FOPS canopy or debris-guard configurations common on dozers working in rock or forestry conditions. Exact guard style varies by machine order.

All 6D125-1C(S/N 10001-UP) assemblies by section

Every catalogued assembly group for the Komatsu 6D125-1C(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 : 6d125-1c(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
01***95	Bolt,(For 2nd Travel Speed) (Op)	4

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Komatsu : 6d125-1c(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
01***16	Washer,Radio Controlled Slag Handling	1

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Komatsu : 6d125-1c(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
01***35	Bolt,(For Quick Shift) (Mitsubishi)	1

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Cylinder Block

Komatsu : 6d125-1c(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
61***31	Plate,Thrust	1

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Komatsu : 6d125-1c(S/N 10001-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 : 6d125-1c(S/N 10001-Up) : Cylinder Block : Cylinder Block

Komatsu : 6d125-1c(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
01***16	Washer,Radio Controlled Slag Handling	2

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Komatsu : 6d125-1c(S/N 10001-Up) : Cylinder Block : Flywheel And Flywheel Housing

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 : 6d125-1c(S/N 10001-Up) : Cylinder Block : Front Cover

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)	8

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Komatsu : 6d125-1c(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
01***32	Washer,(With Auto Emergency Steering)	1

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Komatsu : 6d125-1c(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
01***32	Washer,(With Auto Emergency Steering)	1

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Komatsu : 6d125-1c(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
61***11	Piston Ass'Y	6

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Cylinder Head

Komatsu : 6d125-1c(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
01***08	Nut,(For Straight Tilt Dozer) (D65p)	6

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Komatsu : 6d125-1c(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
01***16	Washer,Radio Controlled Slag Handling	6

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Komatsu : 6d125-1c(S/N 10001-Up) : Cylinder Head : Air Cleaner And Mounting (High Altitude Spec.)(#30845-)

01***40	Bolt,(Dust And Dirt Protection)	2
01***25	Bolt,(For Canvas Canopy?Head Guard)	2
01***30	Bolt,(For Quick Shift) (Mitsubishi)	6
01***08	Nut,(For Straight Tilt Dozer) (D65p)	6

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Komatsu : 6d125-1c(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
01***60	Bolt, Mount Tube To Switch Pump	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Cylinder Head : Air Intake Manifold And Connection (High Altitude Spec.)(#30845-)

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***10	Bolt, Fan Mounting	8

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Komatsu : 6d125-1c(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
61***40	Seal,Exhaust Valve (K1)	2

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Komatsu : 6d125-1c(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
61***40	Seal,Exhaust Valve (K1)	2

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Komatsu : 6d125-1c(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
61***20	Spacer,(For Shipping)	18

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Komatsu : 6d125-1c(S/N 10001-Up) : Cylinder Head : Exhaust Manifold And Turbocharger (High Altitude Spec.)(#30845-)

01***00	Bolt	16
01***80	Bolt, Mount Strainer Flange	2
01***45	Screw	4
07***08	Plug,(Without Auto Priming System)	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Cylinder Head : Muffler And Mounting (High Altitude Spec.)(#30845-)

01***40	Bolt,(Dust And Dirt Protection)	4
01***55	Bolt,(For Suction? Reversible Fan)	4
01***30	Bolt,Radio Controlled Slag Handling	4
01***08	Nut, Regular Hexagon (M10-1.50)	4

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Komatsu : 6d125-1c(S/N 10001-Up) : Cylinder Head : Muffler And Mounting(#10001-28436)

01***25	Bolt,(Sandy? Dusty Terrain Spec.)	4
01***40	Bolt,(Dust And Dirt Protection)	4
01***35	Bolt,(For Quick Shift) (Mitsubishi)	1
01***30	Bolt, Mount Pump	2

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Komatsu : 6d125-1c(S/N 10001-Up) : Cylinder Head : Muffler And Mounting(#28437-)

01***25	Bolt,(Sandy? Dusty Terrain Spec.)	4
01***40	Bolt,(Dust And Dirt Protection)	4
01***35	Bolt,(For Quick Shift) (Mitsubishi)	1
01***30	Bolt, Mount Pump	2

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Komatsu : 6d125-1c(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
01***32	Washer,(With Auto Emergency Steering)	40

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Komatsu : 6d125-1c(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
07***50	Cap,Oil Filler	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Cylinder Head : Turbocharger (High Altitude Spec.)

61***00	Turbocharger Ass'Y,(Tv7705)	1
GA***85	V-Band Ass'Y	1
GA***07	Rotor Ass'Y	1
GA***85	V-Band	1

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Komatsu : 6d125-1c(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
07***12	Plug,(Without Corrosion Resistor)	2

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Electrical System

Komatsu : 6d125-1c(S/N 10001-Up) : Electrical System : Alternator (13a)

60***50	Alternator A. (13a)	1
KD***31	Bushing,Insulator	2
KD***80	Packing	1
KD***72	Slip Ring Ass'Y	1

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Komatsu : 6d125-1c(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
KD***90	Wire,Read	1

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Komatsu : 6d125-1c(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
01***26	Washer,(For Additional Rear Lamp)	2

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Komatsu : 6d125-1c(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
01***26	Washer,(For Additional Rear Lamp)	2

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Komatsu : 6d125-1c(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

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Komatsu : 6d125-1c(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
KD***50	Rear Bracket Ass'Y	1

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Komatsu : 6d125-1c(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
KD***01	Washer,(Not Shown)	2

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Komatsu : 6d125-1c(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
KD***50	Brush Assembly	4

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Komatsu : 6d125-1c(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
KD***00	Terminal Assembly	1

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Komatsu : 6d125-1c(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
(6***0)	Starting Motor Ass'Y,(7.5kw) (See Fig.0661)	1

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Komatsu : 6d125-1c(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
(6***0)	Starting Motor Ass'Y,(7.5kw) (See Fig.A6210-B4a4)	1

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Engine Kit

Komatsu : 6d125-1c(S/N 10001-Up) : Engine Kit : Gasket Kit

02***43	O-Ring,Water Manifold	10
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 : 6d125-1c(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
01***16	Washer,Radio Controlled Slag Handling	2

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Filter And Piping (High Altitude Spec.)(#30845-)

01***35	Bolt,(For Quick Shift) (Mitsubishi)	2
01***16	Washer,Radio Controlled Slag Handling	2
07***12	Gasket,Mechanical Pump Piping	2
07***12	Gasket,(Without Air Compressor Type) (K1)	4

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Coupling)(#10001-11478)

ND***50	Bolt	1
ND***70	Bolt	2
ND***30	Coupling Ass'Y	1
ND***60	Bolt	6

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Coupling)(#11479-)

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

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Feed Pump)(#10001-11478)

ND***40	Washer? 1mm,Plate	1
ND***00	Pump Ass'Y,Priming Pump	1
ND***70	Washer,Union	4
ND***10	Piston? 21.988mm,Blue	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Feed Pump)(#11479-)

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 : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2)(#10001-11478)

ND***40	Plug,Screw	1
ND***50	Washer? 0.3mm	0
ND***90	Lever Ass'Y,Control	1
ND***20	Screw,Adjusting	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2)(#11479-)

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

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2)(#10001-11478)

ND***20	Washer? 1.0mm,Cover Plate	2
ND***60	O-Ring	2
ND***70	Packing,Rubber Plate	1
ND***10	Seal,Oil	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2)(#11479-)

ND***20	Washer? 1.0mm,Cover Plate	2
ND***40	O-Ring	1
ND***10	Seal,Oil	1
ND***50	Packing,Rubber Plate	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Pump) (1/2)(#10001-11478)

61***21	Injection Pump A	1
61***22	Injection Pump A	1
ND***50	Ring,Adjust	1
ND***80	O-Ring	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Pump) (1/2)(#11479-)

61***20	Injection Pump A	1
ND***50	Ring,Adjust	1
ND***80	O-Ring	1
ND***40	Shim? 0.16mm	0

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Pump) (2/2)(#10001-11478)

ND***70	O-Ring	6
ND***50	Seat,Spring? Lower	6
ND***11	Spring,Plunger	6
ND***20	Shim? 1.9mm,Element Holder	0

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump (Pump) (2/2)(#11479-)

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 : 6d125-1c(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
01***70	Bolt, Mount Bracket And Oil Inlet Tube, Bolt	2

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (1/2) (High Altitude Spec.)(#30845-)

01***75	Bolt,(See Fig.0142)	4
01***25	Bolt,(For Canvas Canopy?Head Guard)	4
01***70	Bolt, Mount Bracket And Oil Inlet Tube, Bolt	2
01***80	Bolt,(Not Shown)	2

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (2/2) (High Altitude Spec.)(#30845-)

01***30	Bolt	4
01***20	Bolt,(For Quick Shift) (Mitsubishi)	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	1
01***55	Bolt, Mount Tube To Switch Pump	4

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (2/2)(#10001-11478)

01***30	Bolt	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	2
01***55	Bolt, Mount Tube To Switch Pump	4
01***19	Washer,Spring (With Extinguisher) (D31p)	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Fuel Injection Pump And Piping (2/2)(#11479-)

01***30	Bolt	4
01***20	Bolt,(For Quick Shift) (Mitsubishi)	1
01***25	Bolt,(For Canvas Canopy?Head Guard)	1
01***55	Bolt, Mount Tube To Switch Pump	4

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Komatsu : 6d125-1c(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
07***12	Gasket,(Without Air Compressor Type) (K2)	12

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Komatsu : 6d125-1c(S/N 10001-Up) : Fuel System : Water Separator And Mounting (High Altitude Spec.)(#30845-)

01***25	Bolt,Without Revolution Sensor	2
01***16	Washer,(For 35a Alternator)	2
07***60	O-Ring,(For Pc750?800?1800) (A2610)	1
07***12	Gasket,(Without Air Compressor Type) (K1)	2

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Lubricating Oil System

Komatsu : 6d125-1c(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
01***80	Bolt,(Not Shown)	2

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Komatsu : 6d125-1c(S/N 10001-Up) : Lubricating Oil System : Lubricating Oil Cooler (High Altitude Spec.)(#30845-)

01***25	Bolt,Without Revolution Sensor	2
01***25	Bolt,(For Canvas Canopy?Head Guard)	12
01***30	Bolt,(For Quick Shift) (Mitsubishi)	2
01***65	Bolt,(Except Japan)	2

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Komatsu : 6d125-1c(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
01***16	Washer,Radio Controlled Slag Handling	4

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Komatsu : 6d125-1c(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
01***85	Bolt,(For 2nd Travel Speed) (Op)	1

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Komatsu : 6d125-1c(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
01***35	Bolt, Mounting - Flywheel Housing End	1

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Komatsu : 6d125-1c(S/N 10001-Up) : Lubricating Oil System : Turbocharger Lubricating Oil Piping (High Altitude Spec.)(#30845-)

01***20	Bolt,(For Quick Shift) (Mitsubishi)	5
01***30	Bolt,(For Quick Shift) (Mitsubishi)	2
01***08	Nut,(For Canvas Canopy?Head Guard)	1
01***16	Washer,Radio Controlled Slag Handling	4

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6D125-1C(S/N 10001-UP) serial number reference

The engine dataplate sits on the left side of the block, on or near the valve cover; if the tag is missing or unreadable the serial is also stamped directly into the block casting. On the plate, "6D125-1C" is the full engine model — the "C" is a factory build/fitment suffix, not a letter prefix ahead of the serial. The digits following S/N are the sequential production serial, which for this engine starts at 10001 with no alpha characters ahead of it.

Prefix	Identifies	Notes
10001-UP	6D125-1C (base and only listed spec)	Naturally aspirated inline-six diesel, no turbocharger. Fitted to the Komatsu D53A-17 and D53P-17 crawler dozer (chassis S/N 80001-UP). A later production option on that same chassis used the newer 6D125E-2 non-turbo engine instead — confirm the exact chassis-serial break with your dealer if pinning down an engine-option changeover. Confirm exact fitment with your dealer.

Frequently asked questions

What engine does the Komatsu 6D125-1C (S/N 10001-Up) use?

It is the engine itself: a Komatsu 6D125-1C, naturally aspirated inline-6 diesel, 11.04 L (673.8 cu in) displacement, 125 mm x 150 mm (4.92 in x 5.91 in) bore and stroke, with a mechanical inline injection pump, an all-speed governor, and no electronic controls. Gross power runs roughly 116-119 kW (155-160 hp) at 1,850 rpm, though the exact certified figure varies by source and configuration, so confirm it against the engine nameplate. Do not confuse it with the turbocharged S6D125-1C, a different engine built for different host machines.

What is the operating weight of the machine this engine powers?

The 6D125-1C is factory-fitted to the Komatsu D53A-17 (standard-track) and D53P-17 (wide-track, low-ground-pressure "swamp") crawler dozers. Operating weight runs roughly 12.8-13.6 tonnes (28,200-30,000 lb) depending on track gauge, blade, and ripper fitted; treat this as a range rather than a fixed figure, since it varies by how the dozer was ordered.

What replaced the 6D125-1C in later production?

Komatsu carried the D53A/D53P-17 chassis forward into the -18 and -18E generation (D53A-18, D53A-18E, D53P-18E), which moved to the newer 6D125E-2 engine family in place of this naturally aspirated 6D125-1 block. That shift lines up with Komatsu's broader move away from this mechanical engine generation across its D50-D65 dozer class toward the 6D125E-2-based lineup used on later machines.

What 6D125-1C(S/N 10001-UP) owners discuss

What powertrain behavior is normal on machines running this engine, and what isn't?

The host dozer for this engine pairs it with Komatsu's TORQFLO powershift transmission and a torque converter, not a direct-drive gearbox. Expect the usual torque-converter traits: some rpm rise before the machine actually moves under load, and stall speed that drops off if the converter oil is low or overheated. Separately, because this is a mechanical inline injection pump with an all-speed governor and no electronic control, a mild idle or light-load speed hunt (rpm creeping up and down before settling) shows up in the field on this engine family. Usual causes are a restricted fuel filter, water or air in the low-pressure fuel circuit, or a sticking governor linkage, not a bad pump. Clear the fuel system and check linkage freedom first. If hunting persists after that, have your dealer verify the governor and injection pump.

What wear pattern should owners expect on the undercarriage of this dozer class?

On the D53/D60/D65-class crawler dozer this engine is built for, undercarriage (track chain, rollers, idlers, sprockets) is consistently the highest-wear component group, more so than the engine itself. Track tension matters more than people expect: running tracks too tight loads the bushing-to-sprocket-tooth contact and the roller-to-link contact and accelerates wear on both. Life to replacement varies widely by ground condition, rock content, and how consistently tension was maintained, so treat any single hours figure you're quoted as a starting point, not a guarantee, and inspect the actual undercarriage rather than trusting the meter.

Is this engine turbocharged, and why does that matter for troubleshooting?

No. The plain 6D125-1C covered here is naturally aspirated. Komatsu used an S-prefix (S6D125-1C) to denote the turbocharged version of the same block family, fitted to different host machines. Parts listings and generic engine write-ups blend the two often enough that owners get confused mid-repair, chasing turbo-specific symptoms or parts on a naturally aspirated block. Before ordering parts or diagnosing a boost-related complaint, confirm the dataplate reads plain 6D125 with no leading S.

What electrical issues come up most on this engine generation?

There's no ECM or sensor package on this mechanical engine, so electrical complaints are old-school: starting and charging. A no-crank or single-click no-start on this generation of Komatsu machine is commonly traced to a dirty or failed starting-motor safety relay rather than the starter itself, since that relay has to see alternator output drop before it'll pass voltage to the solenoid. Charging complaints are frequently a bad internal alternator diode or corroded/loose terminals rather than a dead alternator. Clean and check connections and the safety relay before swapping the starter or alternator outright. If the fault persists, have your dealer verify the charging and starting circuit.

Is there a maintenance item on this engine's cooling system that gets missed?

Yes. This engine's cooling system carries a dedicated corrosion-resistor cartridge, a filter-style conditioner that buffers the coolant against internal corrosion, standard on Komatsu's classic coolant system from this era (as opposed to later long-life coolant formulations that don't need one). It's easy to overlook on a used machine since it doesn't affect running temperature until it's badly neglected. Confirm it's been serviced on the interval your service literature calls for, and ask your dealer whether a coolant-type change on this engine still requires the cartridge.

What's behind reports of oil coming out of the breather on this engine family?

Owners of this engine family report oil weeping or blowing out of the crankcase breather as a recurring complaint. In documented cases this wasn't fixed by re-gasketing alone; the root cause traced to worn piston rings, liner, and piston needing replacement, i.e. genuine blow-by rather than a simple seal leak. If you see a wet breather tube or oil pooling near the valve cover, check compression and blow-by before spending on gaskets. Have your dealer or a diesel shop confirm before committing to a ring-and-liner job.

What should I check before buying a used 6D125-1C engine or the dozer it's fitted to?

Confirm the dataplate reads plain 6D125-1C, not S6D125-1C, and note the serial before assuming parts interchange with a sibling arrangement. Check for oil at the breather or valve-cover area (see the blow-by question above), and check idle stability for governor hunting. Pull the radiator cap and ask whether the corrosion-resistor cartridge has been serviced on schedule; neglected coolant on this system corrodes from the inside. Walk the full undercarriage and blade/ripper for wear and impact damage, since abuse there usually means abuse everywhere else on the machine. Ask for maintenance records if any exist. Before finalizing, have your dealer or an independent diesel shop run a compression and blow-by check and inspect the charging/starting circuit.

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

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