Caterpillarwheel tractor-scraperwheel tractor
Caterpillar 666
Maintenance schedule, common problems & OEM parts breakdown
The Caterpillar 666 is a twin-engine, tandem-power wheel tractor-scraper built for large-scale earthmoving. The tractor unit runs a turbocharged Caterpillar D346T V8 diesel rated at roughly 450 hp net (up to 560 hp max), while the trailing scraper unit carries its own turbocharged Caterpillar D343T inline-six diesel rated at roughly 335 hp net (up to 420 hp max), combining for close to 980 hp max across both power units. Cat built the 666 from 1962 into the late 1970s, offered alongside the single-engine 660 that shared the same 40-cubic-yard struck / 54-cubic-yard heaped bowl. Loaded operating weight is not consistently documented across surviving sources and varies by configuration, so verify the actual figure against the data plate. Power routes through a nine-speed Caterpillar powershift transmission with a hydraulic retarder. The 666 is push-loaded only, with no self-loading elevator, so it works paired with a dedicated pusher dozer — Cat built a matched tandem D9G pusher for this class.
Partway through the main production run, Caterpillar raised the 666's horsepower, and that higher-output version was later sold as its own model, the 666B, with a distinct serial prefix and different D346/D343 output ratings. Treat the 666B as a related but separate later generation, not a running spec change on the 666 itself. Twin-engine tandem-power scrapers like the 666 fell out of favor industry-wide by the 1980s as Cat and its competitors shifted large-scraper design toward push-pull and elevating configurations. Surviving 666 units today sit mostly in collector, restoration, and niche heavy-haul use rather than active production fleets, which is why documented fluid capacities and torque specs are thin and often serial-range specific. Parts demand on these machines centers on driveline, hitch, and hydraulic wear items rather than electronics, since the 666 predates any onboard diagnostics or fault-code system.
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.
Caterpillar 666 specifications
Engine
| Series / generation covered | Original Cat 666 wheel tractor-scraper, introduced 1962. A later 666B series carries different, higher power ratings and is not reflected in these figures. |
| Tractor engine | Cat D346T, four-cycle turbocharged valve-in-head diesel, V8 |
| Tractor engine power | 336 kW (450 hp) net flywheel @ 1900 rpm; 418 kW (560 hp) maximum rating |
| Tractor engine displacement / cylinders | 19.5 L (1190 cu in), 8 cylinders, bore x stroke 137 x 165 mm (5.4 x 6.5 in) |
| Scraper engine | Cat D343T, four-cycle turbocharged valve-in-head diesel, inline-6 |
| Scraper engine power | 250 kW (335 hp) net flywheel @ 1900 rpm; 313 kW (420 hp) maximum rating |
| Scraper engine displacement / cylinders | 14.6 L (893 cu in), 6 cylinders |
| Combined power (tractor + scraper) | 731 kW (980 hp) maximum rating, twin-engine tandem-power layout |
| Emissions tier | Not applicable - pre-emissions-regulation era production |
Weights
| Operating weight, empty (total unit) | 56,700 kg (125,000 lb) - front axle 10,300 kg (22,700 lb), tractor drive axle 19,550 kg (43,100 lb), scraper axle 26,850 kg (59,200 lb) |
| Loaded weight (approx. 55,300 kg / 122,000 lb average payload, 0.75 load factor) | 112,040 kg (247,000 lb) total - front axle approx 12% (30,000 lb), tractor drive axle approx 34% (84,300 lb), scraper axle approx 54% (132,700 lb) |
| Ground pressure | Not published in factory literature reviewed |
Dimensions
| Overall length | 17.04 m (55'11") |
| Overall width, transport | 4.24 m (13'11") |
| Overall height | 4.37 m (14'4") |
| Scraper unit shipping width (draft arms removed for transport) | 5.31 m (17'5") |
| Bowl inside width | 3.56 m (11'8") |
| Bowl side height | 2.08 m (6'10") |
| Non-stop turning width | 13.87 m (45'6") |
| Tires (tubeless) | Front/steer 18.0-25 (20 PR); tractor drive 37.5-39 (28 PR); scraper 37.5-51 (36 PR) |
Performance
| Top travel speed, loaded | 68 km/h (42 mph) |
| Transmission | Cat powershift with torque-divider drive; 9 forward speeds across three manually selected ranges plus a loading range, 3 reverse speeds; scraper transmission can shift to neutral independently of the tractor |
| Gradeability / rimpull | Published as a rimpull-speed-gradeability chart (torque-divider, direct-drive and overdrive curves) rather than a single rated figure; usable rimpull depends on traction and weight on the tractor drive wheels |
| Maximum depth of cut | 483 mm (19 in) |
| Maximum depth of spread | 610 mm (24 in) |
| Maximum hydraulic penetration force at cutting edge | approx. 654 kN (147,000 lb) |
| Apron closure force (cutting edge fully raised, apron open 305 mm / 12 in) | approx. 160 kN (36,000 lb) |
| Hydraulic pump output | 549 L/min (145 GPM) @ 2000 rpm; relief valve setting 13.8 MPa (2000 psi) |
| Retarder | Hydraulic retarder standard, synchronized to brake the scraper axle first; drive wheels can also be braked individually |
Service capacities (summary)
| Fuel tank | Tractor 681 L (180 US gal); scraper 473 L (125 US gal); combined approx. 1154 L (305 US gal) |
| Engine oil (crankcase) | Tractor 57 L (15 US gal); scraper 37 L (9.75 US gal) |
| Cooling system | Tractor 144 L (38 US gal); scraper 117 L (31 US gal) |
| Hydraulic system (steering and scraper circuits combined) | 348 L (92 US gal); retarder circuit approx. 26.5 L (7 US gal) per unit |
Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.
666 fluids & capacities
| System | Capacity | Recommended fluid |
|---|---|---|
| Front (tractor) engine crankcase, with filter — Cat D346T V8 diesel | Not documented for this exact serial range in sources checked — verify with your dealer or the lubrication chart in the OMM | Diesel engine oil per Caterpillar's engine oil series for the period; viscosity chosen by ambient temperature at time of service |
| Rear (scraper unit) engine crankcase, with filter — Cat D343T inline-six diesel | Not documented for this exact serial range in sources checked — verify with your dealer or the lubrication chart in the OMM | Diesel engine oil per Caterpillar's engine oil series for the period; viscosity chosen by ambient temperature at time of service |
| Cooling system, front engine | Not documented for this exact serial range in sources checked | Water-based coolant with corrosion inhibitor per Caterpillar cooling system specification of the period |
| Cooling system, rear engine | Not documented for this exact serial range in sources checked | Water-based coolant with corrosion inhibitor per Caterpillar cooling system specification of the period |
| Fuel tank | Not documented for this exact serial range in sources checked | Diesel fuel, No. 2-D grade typical for the period |
| Powershift transmission (nine-speed, with torque converter) | Not documented for this exact serial range in sources checked | Powershift transmission/torque converter oil per Caterpillar's transmission-oil recommendation for the period |
| Final drives (each side) | Not documented for this exact serial range in sources checked | Gear lubricant per Caterpillar's final-drive oil recommendation for the period |
| Hydraulic system and tank (scraper bowl/apron/ejector circuits) | Not documented for this exact serial range in sources checked | Hydraulic oil per Caterpillar's hydraulic system oil recommendation for the period |
| Grease points (chassis, pins, bushings) | Spec only — no fill volume applies | Multipurpose-type chassis grease per the OMM lubrication chart |
Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.
Caterpillar 666 maintenance schedule
| Service interval | Tasks |
|---|---|
| Every 50 h |
|
| Every 250 h |
|
| Every 500 h |
|
| Every 1,000 h |
|
| Every 2,000 h |
|
| Every 4,000 h |
|
Servicing the 666 beyond the schedule
Predictive Maintenance & Fluid Analysis
The 666 carries two separate power units, so fluid sampling covers the D346T tractor engine and the D343T scraper engine as independent systems, not one combined reading. Pull oil samples from each engine, the powershift transmission, and the hydraulic tank at every scheduled interval, and track turbocharger boost and exhaust temperature on both engines against baseline readings taken when the units were fresh. This mechanical-era machine has no onboard fault-code system, so gauge trending and lab analysis are the only early-warning tools available.
Corrective & Common Repairs
Most field repairs on a 666 center on the tandem-power hitch and driveline that links the scraper's D343T engine into the tractor's transmission — worn U-joints, elongated hitch pins, and drawbar play show up as vibration or uneven pulling power. Apron and ejector hydraulic cylinders take heavy cycling under load and are common leak-and-reseal jobs. Powershift clutch packs and the hydraulic retarder are the other frequent repair items, since both see constant duty cycling working a loaded bowl on grade.
Overhaul & Rebuild Points
The D346T V8 and D343T inline-six are rebuilt independently since they share no major components, so plan separate teardown, cylinder liner, and injector work for each. Both turbochargers are wear items at high hours given the machine's continuous heavy-haul duty cycle. Final drives at each wheel station and the wheel bearings carry the full loaded-bowl weight and warrant close inspection at major overhaul. Apron-ejector linkage and bowl wear plates typically need replacement around the same hours as engine rebuild work.
Seasonal & Environment Servicing
Cold-start prep matters for both engines on a 666: check starting aids and switch to winter-grade fuel and engine oil viscosity on the D346T and D343T ahead of freezing weather. In dusty or abrasive haul-road conditions, shorten air cleaner service intervals on both engines and inspect ejector and apron seals more often, since fine material accelerates cylinder rod and seal wear. Wash accumulated material off the tandem-power hitch and driveline regularly so early wear shows up before it becomes a breakdown.
666 attachments & work tools
Scraper bowl and cutting edge
Bowl rated 40 cu yd (30.6 m3) struck and 54 cu yd (41.3 m3) heaped, with additional volume available when sideboards are fitted. Front edge is a tapered horizontal cutting edge that shears into the cut and fills the bowl as the machine advances.
Apron and ejector system
Loading and discharge use full-hydraulic, double-acting cylinders on all bowl circuits (apron and ejector). The ejector wall sweeps the bowl to discharge the load; no elevating or auger self-loading arrangement is documented for this model.
Tractor-to-scraper hitch
Tractor and bowl unit join through a standard universal-type hitch allowing lateral oscillation of roughly 22 degrees each way, with anti-jackknifing stops. This is a fixed semi-rigid coupling, not the later cushion-hitch or push-pull hitch design used on newer Cat scraper generations; no factory cushion or push-pull hitch option is documented for the 666.
Loading method / pusher pairing
Despite its own tandem-power (twin-engine) drivetrain, the 666 is a push-loaded, open-bowl scraper rather than self-loading, and works with a dedicated pusher tractor in the cut. Caterpillar and dealer Peterson Tractor developed a purpose-built tandem-dozer pusher (twin D9G units on a common frame, later factory-built as the DD9G and DD9H) specifically to load this scraper class.
Operator station / ROPS guarding
Early production used an open operator station with windshield, wiper, and a torsion-flex bucket seat. From about 1972 an optional ROPS (rollover protective structure) cab/canopy became available, offered in different heights across the model's run; verify exact cab configuration against the machine's data plate and serial range.
Windrow breaker accessory
A small V-shaped windrow breaker mounted ahead of the steering axle is fitted to break up and clear windrowed material before it reaches the front wheels.
Tire and axle configuration
Three distinct tire sizes equip the machine: 18x25 on the steering axle, 37.5x39 on the tractor drive axle, and 37.5x51 on the scraper axle. This three-size mixed-tire arrangement is specific to the large tandem-power scraper class the 666 belongs to (shared tractor unit family with the 650 and 660).
Attachment families not documented for this model
No ripper, dozer blade, elevator, or auger self-loading attachment is documented for the 666 in factory or dealer material reviewed; those features belong to smaller or later Cat scraper series. Treat any such option as unconfirmed for this model and verify with a dealer before quoting.
All 666 assemblies by section
Every catalogued assembly group for the Caterpillar 666. Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.
Attachments
7d2410 Alternator Arrangement--24 Volt, 50 Ampere
| 1B***04 | Spacer; (6.8x10x9.5-mm Thk) | 2 |
| 3B***02 | Washer,Lock Circuit Breaker Uoc:Bsv,Bsx | 4 |
| 3B***04 | Lockwasher--Sheet Assembly To Angle Assembly | 2 |
4d7150 Alternator Arrangement--24 Volt, 40 Ampere
| 1B***04 | Spacer; (6.8x10x9.5-mm Thk) | 2 |
| 3B***02 | Washer,Lock Circuit Breaker Uoc:Bsv,Bsx | 4 |
| 3B***04 | Lockwasher--Sheet Assembly To Angle Assembly | 2 |
1s2584 Alternator Assembly--24 Volt, 40 Ampere
| 1S***84 | Alternator Assembly | 1 |
7m9606 Alternator Group--24 Volt, 40 Ampere
| 7M***06 | Alternator Group | 1 |
9s3193 Alternator Group--24 Volt, 50 Ampere
| 9S***93 | Alternator Group | 1 |
5s9088 Alternator Assembly--24 Volt, 50 Ampere--Part 2 Of 2--Type 1
| 5S***88 | Alternator Group-Charging | 1 |
5s9088 Alternator Assembly--24 Volt, 50 Ampere--Part 1 Of 2--Type 2
| 5S***88 | Alternator Group-Charging | 1 |
5s9088 Alternator Assembly--24 Volt, 50 Ampere--Part 1 Of 2--Type 1
| 5S***88 | Alternator Group-Charging | 1 |
5s9088 Alternator Assembly--24 Volt, 50 Ampere--Part 2 Of 2--Type 2
| 5S***88 | Alternator Group-Charging | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--Enlarged View Of Terminal Strip--Part 5 Of 5--Type 2
| 4D***20 | Battery, Wiring And Lighting System | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--Enlarged View Of Dash Area--Part 2 Of 5--Type 1
| 4D***20 | Battery, Wiring And Lighting System | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--Rear Half As Viewed From R.H. Side Of Machine--Part 4 Of 5--Type 1
| 4D***20 | Battery, Wiring And Lighting System | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--Front Half As Viewed From R.H. Side Of Machine--Part 3 Of 5--Type 1
| 4D***20 | Battery, Wiring And Lighting System | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--As Viewed From Rear Of Machine--Part 1 Of 5--Type 1
| 4D***20 | Battery, Wiring And Lighting System | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--Enlarged View Of Terminal Strip--Part 5 Of 5--Type 1
| 4D***20 | Battery, Wiring And Lighting System | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--Rear Half As Viewed From R.H. Side Of Machine--Part 4 Of 5--Type 2
| 4D***20 | Battery, Wiring And Lighting System | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--Enlarged View Of Dash Area--Part 2 Of 5--Type 2
| 4D***20 | Battery, Wiring And Lighting System | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--Front Half As Viewed From Right Hand Side Of Machine--Part 3 Of 5--Type 2
| 4D***20 | Battery, Wiring And Lighting System | 1 |
4d7120 Battery, Wiring And Lighting System--24 Volt--As Viewed From Rear Of Machine--Part 1 Of 5--Type 2
| 4D***20 | Battery, Wiring And Lighting System | 1 |
5j5272 Brake Chamber Assembly
| 5J***72 | Chamber As-Brake | 1 |
5d1900 Protection Cover Group--Hydraulic Tank Cap
2s6096 Protection Cover Group--Radiator Cap
2s3025 Protection Cover Group--Oil Filler Cap
Cylinder Head Assembly--L.H.
Cylinder Head Assembly--R.H.
7s3741 Direct Electric Starting Arrangement--24 Volt
| 0L***51 | Bolt Starting Engine Fuel Tank Bracket To Diesel Intake Manifold | 2 |
| 0L***70 | Bolt (2-1/4") Filler Tube Assembly To Transfer Drive Case | 1 |
| 0S***94 | Bolt (1 In. Long 3/8 In. Dia.); Plate Assembly To Support Assembly | 4 |
9f740 Discharger
| 9F***40 | Discharger Group | 1 |
6d3783 Valve Assembly
| 6D***83 | Double Check Valve Assembly; --Synchro --To Nipple | 1 |
7m2134 Electric Starter Group--24 Volt
| 7M***34 | Electric Starter Group | 1 |
4m4615 Electric Starting Motor Assembly--24 Volt--Part 1 Of 2
| 4M***15 | Electric Starting Motor Assembly | 1 |
4m4615 Electric Starting Motor Assembly--24 Volt--Part 2 Of 2
| 4M***15 | Electric Starting Motor Assembly | 1 |
5d4555 Emergency Relay Valve Assembly
| 5D***55 | Emergency Relay Valve Assembly | 1 |
3d6730 Ether Starting Aid Group
| 3D***30 | Ether Starting Aid Group | 1 |
Flywheel
6d7590 Fuel Line Group
| 6D***90 | Fuel Line Group | 1 |
6d2390 Fuel System Group--Diesel Engine--225 U.S. Gallon--Rapid Filling
| 6D***90 | Fuel System Group | 1 |
7s4273 Glow Plug And Wiring Group
| 7S***73 | Glow Plug And Wiring Group | 1 |
8f9866 Grease Gun--Type 2
8f9866 Grease Gun--Type 1
8m9858 Tool Group
4j2064 Hydraulic Cylinder Group--2 Required
| 4J***64 | Hydraulic Cylinder Group | 1 |
7j596 Hydraulic Pump Assembly
| 7J***96 | Hydraulic Pump Assembly | 1 |
4j507 Hydraulic Pump Assembly--Type 2
| 4J***07 | Hydraulic Pump Assembly | 1 |
3j7645 Hydraulic Pump Assembly
| 3J***45 | Hydraulic Pump Assembly | 1 |
4j507 Hydraulic Pump Assembly--Type 1
| 4J***07 | Hydraulic Pump Assembly | 1 |
6d4205 Low Air Indicator Lamp Assembly
| 6D***05 | Lamp Assembly | 1 |
4s7967 Light Signal Receiver Group--24 Volt
| 4S***67 | Light Signal Receiver Group | 1 |
Name Plates And Transfers
6d8250 Control Arrangement
| 6D***50 | Parking And Emergency Brake Control Arrangement | 1 |
6d8250 Parking And Emergency Brake Control Arrangement
| 6D***10 | Parking And Emergency Brake Control Group | 1 |
| 6D***50 | Parking And Emergency Brake Control Arrangement | 1 |
| 6D***60 | Parking And Emergency Brake Control Group | 1 |
6d8260 Parking And Emergency Brake Control Group--Part 2 Of 3
| 0S***94 | Bolt (1 In. Long 3/8 In. Dia.); Plate Assembly To Support Assembly | 4 |
| 1D***17 | Nut-Plate And Angle Assembly To Hydraulic And Fuel Tanks; Nut Plate And Angle Assembly To Hydraulic And Fuel Tanks | 4 |
| 1S***01 | Union --Inversion Valve To Scraper Supply Hose; -- Inversion Valve To Scraper Supply Hose | 1 |
6d8260 Parking And Emergency Brake Control Group--Part 1 Of 3
| 1D***16 | Nut,Plain,Hexagon Clip Uoc:Bsv,Bsx | 2 |
| 1S***14 | Grommet (1.75 In. I.D.)- To Platform | 1 |
| 2B***95 | Cap Screw--Tube Assembly To Air Inlet Pipe | 2 |
6d8260 Parking And Emergency Brake Control Group--Part 3 Of 3
| 0L***51 | Bolt Starting Engine Fuel Tank Bracket To Diesel Intake Manifold | 1 |
| 0S***15 | Bolt 19 mm (.75 In.) Long Return Air Console To Enclosed Rops | 2 |
| 1B***30 | Nut-- Flame Arrestor Or Adapter To Pipe Assembly | 2 |
6d7210 Parking And Emergency Brake Control Group
| 0L***51 | Bolt Starting Engine Fuel Tank Bracket To Diesel Intake Manifold | 2 |
| 0S***94 | Bolt (1 In. Long 3/8 In. Dia.); Plate Assembly To Support Assembly | 2 |
| 0S***18 | Bolt; (5/16-18x1-In)(Hydraulic Pump) | 2 |
2s2787 Protection Cap Group--Starting Engine Fuel Tank
5d1920 Protection Cover Group--Fuel Tank Cap
8s5101 Radiator Core Protection Grid Group
1s4153 Rain Cap Group--Starting Engine
2d284 Rear Flood Light Group--24 Volt--Type 2
2d284 Rear Flood Light Group--24 Volt--Type 1
4j2575 Relief Control Valve Group
| 4J***75 | Relief Control Valve Group | 1 |
7j6233 Rotochamber Assembly
| 7J***33 | Rotochamber Assembly | 1 |
6d6898 Rotochamber Assembly
| 6D***98 | Rotochamber Assembly | 1 |
5d3306 Rotochamber Assembly
| 5D***06 | Rotochamber Assembly | 1 |
4j5344 Single Lever Bowl And Apron Control Group
| 4J***44 | Single Lever Bowl And Apron Control Group | 1 |
4m1812 Solenoid Switch Assembly
| 4M***12 | Switch As-Solenoid | 1 |
9s7978 Tool Group
Tools (8m9866 N/S)
4d5610 Tractor Control Unit And Cylinder Group--Part 1 Of 2--Type 1--Factory Installation
| 4D***10 | Tractor Control Unit And Cylinder Group | 1 |
4d5610 Traction Control Unit And Cylinder Group--Part 2 Of 2--Type 2--Factory Installation
| 4D***10 | Tractor Control Unit And Cylinder Group | 1 |
4d5610 Traction Control Unit And Cylinder Group--Part 1 Of 2--Type 2--Factory Installation
| 4D***10 | Tractor Control Unit And Cylinder Group | 1 |
4d5610 Traction Control Unit And Cylinder Group--Part 2 Of 2--Type 1--Factory Installation
| 4D***10 | Tractor Control Unit And Cylinder Group | 1 |
6d3782 Emergency Control Valve Assembly
| 6D***82 | Valve As; -- Emergency Control | 1 |
2r618 Double Check Valve Assembly
| 2R***18 | Valve As-Check | 1 |
4d487 Valve Assembly
| 4D***87 | Valve As-Check; -Air Supply-Primary Air Tank-Service Brake Control | 1 |
4j8500 Control Valve Assembly
| 4J***00 | Valve As-Control | 1 |
2r619 Double Check Valve Assembly
| 2R***19 | Valve Group-Check; -Air System-Parking Brake-Scraper Brake-Service Brake Control | 1 |
6d3781 Double Check Valve Assembly
| 6D***81 | Valve Group-Check; -Service Brake Diverter-Retarder Control | 1 |
3d9330 Windshield Wiper Group--24 Volt
6d9750 Windshield Wiper Group--24 Volt
Wiring Diagram--24 Volt System
Diesel Engine
Aftercooler
Air Cleaner--Dry Type
Air Compressor
7s8336 Air Compressor Assembly
| 7S***36 | Air Compressor Assembly | 1 |
2s2765 Air Compressor Assembly
| 2S***65 | Air Compressor Assembly | 1 |
Air Compressor Drive
7m914 Air Filter Service Indicator Group
| 7M***14 | Air Filter Service Indicator Group | 1 |
Breather, Oil Filler And Fumes Disposal
Camshaft
Camshaft Drive
4d6377 Rotochamber Assembly
| 4D***77 | Chamber As | 1 |
Connecting Rod And Piston--8 Required
Crankshaft And Damper
Crankshaft Bearing Replacement Groups
Cylinder Block And Covers--Side View--Part 1 Of 2
Cylinder Block And Covers--End View--Part 2 Of 2
3s9720 Cylinder Block Assembly
| 3S***20 | Cylinder Block Assembly | 1 |
Cylinder Head And Valve Mechanism--L.H. Front View
Cylinder Head And Valve Mechanism--R.H. Front View
Engine Support And Lifting Eye
Exhaust By-Pass
Fan And Shroud (2s3226 N/S)
| 2S***26 | Fan And Shroud | 1 |
3s4132 Fan Assembly--Reversible Blade
| 3S***32 | Fan Assembly | 1 |
Fan Belt Tightener
8h7204 Primary Fuel Filter Group
| 8H***04 | Filter Group-Fuel | 1 |
Flywheel
Fuel Filter
Fuel Injection Valves And Lines
7m3853 Fuel Priming Pump Group
| 7M***53 | Fuel Priming Pump Group | 1 |
Fuel Pump And Governor Drive
Fuel Pump Housing And Rack Limiter (7m1068 N/S)
| 7M***68 | Fuel Pump Housing And Rack Limiter | 1 |
Fuel Transfer Pump
5s4234 Fuel Transfer Pump Group
| 5S***34 | Fuel Transfer Pump Group | 1 |
Gasket Kits
| 2S***57 | Diesel Engine Supplementary | 0 |
| 9M***94 | Gasoline Starting Engine Clutch Mechanism Control | 0 |
| 9M***95 | Water Pump | 0 |
Governor--End View - Part 2 Of 2
Governor--Side View - Part 1 Of 2
Governor Control--As Viewed From Right Hand Side Of Machine
Manifolds
Oil Filter
Oil Lines--Part 1 Of 2
Oil Lines--Part 2 Of 2
2s1826 Oil Pump Assembly
| 2S***26 | Oil Pump Assembly | 1 |
3s859 Pressure Ratio Regulator Assembly
| 3S***59 | Pressure Ratio Regulator Assembly | 1 |
8m1601 Rack Limiter Group
| 8M***01 | Rack Limiter Group | 1 |
Radiator (2s3228 N/S)--As Viewed From Rear Of Engine - Part 1 Of 2
| 2S***28 | Radiator | 1 |
Radiator (2s3228 N/S)--As Viewed From Right Hand Side Of Engine - Part 2 Of 2
| 2S***28 | Radiator | 1 |
Rear Power Take-Off Drive
3s3583 Service Meter And Tachometer Drive Group
| 3S***83 | Service Meter And Tachometer Drive Group | 1 |
5d3384 Throttle Valve Assembly
| 5D***84 | Throttle Valve Assembly | 1 |
Torque Converter And Engine Oil Cooler
Torque Spring (2s3215 N/S)
| 2S***15 | Torque Spring | 1 |
Transmission And Steering Pump Drive
2s879 Turbocharger Assembly
| 2S***79 | Turbocharger Assembly | 1 |
4d7695 Control Valve Assembly
| 4D***95 | Valve Group-Air; (Air Control) | 1 |
Water Lines
Water Pump Belt Tightener (2s6074 N/S)
| 2S***74 | Water Pump Belt Tightener | 1 |
7m1080 Water Pump Group
| 7M***80 | Water Pump Group | 1 |
Starting System
3s1814 Air Cleaner And Exhaust System Group
| 3S***14 | Air Cleaner And Exhaust System Group | 1 |
Battery, Wiring And Lighting System (4d6930 N/S)--24 Volt--Rear Half As Viewed From R.H. Side Of Machine--Part 4 Of 5
| 4D***30 | Battery Wiring And Lighting System | 1 |
Battery, Wiring And Lighting System (4d6930 N/S)--24 Volt--Enlarged View Of Dash Area--Part 2 Of 5
| 4D***30 | Battery Wiring And Lighting System | 1 |
Battery, Wiring And Lighting System (4d6930 N/S)--24 Volt--Enlarged View Of Terminal Strip--Part 5 Of 5
| 4D***30 | Battery Wiring And Lighting System | 1 |
Battery, Wiring And Lighting System (4d6930 N/S)--24 Volt--Front Half As Viewed From R.H. Side Of Machine--Part 3 Of 5
| 4D***30 | Battery Wiring And Lighting System | 1 |
Battery, Wiring And Lighting System (4d6930 N/S)--24 Volt--As Viewed From Rear Of Machine--Part 1 Of 5
| 4D***30 | Battery Wiring And Lighting System | 1 |
Camshaft And Governor
3s1465 Carburetor And Breather Group
| 3S***65 | Carburetor And Breather Group | 1 |
7m6038 Carburetor Assembly
| 7M***38 | Carburetor Assembly | 1 |
8m8420 Clutch Group
| 8M***20 | Clutch Group | 1 |
Connecting Rod And Piston (8m2572 N/S)--2 Required
| 8M***72 | Connecting Rod And Piston | 1 |
Crankshaft (8m2574 N/S)
| 8M***74 | Crankshaft | 1 |
Cylinder Block, Cylinder Head And Valve Mechanism--Part 1 Of 2
Cylinder Block, Cylinder Head And Valve Mechanism--Part 2 Of 2
5m1618 Dash Lamp Assembly--24 Volt
| 5M***18 | Dash Lamp Assembly | 1 |
4d9672 Dash Lamp Assembly--24 Volt
| 4D***72 | Dash Lamp Assembly | 1 |
9m1250 Electric Starter Group--24 Volt
| 9M***50 | Electric Starter Group | 1 |
7h7188 Electric Starting Motor Assembly--24 Volt
| 7H***88 | Electric Starting Motor Group | 1 |
4d2130 Fuel System Group
| 4D***30 | Fuel System Group | 1 |
Generator--24 Volt, 20 Ampere
4m9102 Generator Assembly--24 Volt, 20 Ampere
| 4M***02 | Generator Assembly | 1 |
8m2577 Ignition System Group
| 8M***77 | Ignition System Group | 1 |
5d793 Indicator Lamp Assembly--24 Volt
| 5D***93 | Indicator Lamp Assembly | 1 |
1m3379 Head Lamp Assembly--24 Volt
| 1M***79 | Lamp Group | 1 |
1m1882 Flood Lamp Assembly--24 Volt
| 1M***82 | Lamp Group | 1 |
3s2986 Magneto Assembly
| 3S***86 | Magneto Assembly | 1 |
9m1713 Oil Pump Assembly
| 9M***13 | Oil Pump Assembly | 1 |
4d2230 Starting Engine Control Group-Part 2 Of 2
| 4D***30 | Starting Engine Control Group | 1 |
4d2230 Starting Engine Control Group--Part 1 Of 2
| 4D***30 | Starting Engine Control Group | 1 |
Starting Engine Support
7h7290 Disconnect Switch Assembly
| 7H***90 | Switch As-Disconnect | 1 |
3s1764 Transmission And Starter Pinion Group--Part 2 Of 2
| 3S***64 | Transmission And Starter Pinion Group | 1 |
3s1764 Transmission And Starter Pinion Group--Part 1 Of 2
| 3S***64 | Transmission And Starter Pinion Group | 1 |
3s5040 Transmission Oil Level Gauge Group
| 3S***40 | Transmission Oil Level Gauge Group | 1 |
8m2578 Water Lines Group
| 8M***78 | Water Lines Group | 1 |
Wiring Diagram--24 Volt System
Transmission And Chassis
7m8392 Accumulator Body Group
| 7M***92 | Accumulator Body Group | 1 |
Air Horn And Lines--As Viewed From Right Hand Side Of Machine
Air Supply Tank And Lines
4j2743 Brake Chamber Assembly
| 4J***43 | Brake Chamber Assembly | 1 |
Brakes (4j8027 N/S)
| 4J***27 | Brake Group | 1 |
6d6365 Brake Shield Group
| 6D***65 | Brake Shield Group | 1 |
5d3640 Brake Valve Assembly
| 5D***40 | Brake Valve Assembly | 1 |
4d3279 Brake Valve Assembly
| 4D***79 | Brake Valve Assembly | 1 |
Bumper (5d1315 N/S)
| 5D***15 | Bumper | 1 |
8s3515 By-Pass Valve Group
| 8S***15 | By-Pass Valve Group | 1 |
5j5272 Brake Chamber Assembly
| 5J***72 | Chamber As-Brake | 1 |
6j5761 Control Lever Housing Group
| 6J***61 | Control Lever Housing Group | 1 |
4j3100 Control Lever Housing Group
| 4J***00 | Control Lever Housing Group | 1 |
3j7585 Control Lever Housing Group
| 3J***85 | Control Lever Housing Group | 1 |
Control Valves To Scraper Lines (5d7210 N/S)--As Viewed From Right Hand Side Of Machine
| 5D***10 | Control Valves To Scraper Lines | 1 |
Control Valves To Scraper Lines--As Viewed From Right Hand Side Of Machine
4d8591 Cylinder Assembly
| 4D***91 | Cylinder Assembly | 1 |
Differential And Bevel Gear
Differential Case And Covers--Top View--Rear Half--Part 2 Of 2
Differential Case And Covers--Top View--Front Half--Part 1 Of 2
Differential Lock
Drive Shaft
Drive Shaft Guard
Fenders
Final Drive And Wheels (4d5500 N/S)--37.50 X 39 Tubeless Tires
| 4D***00 | Final Drive And Wheels | 1 |
Foot Control And Lines--As Viewed From Right Hand Side Of Machine
Front Axle And Wheels (5d1305 N/S)-18.00 X 25 Tubeless Tires-Top View-Part 2 Of 3
| 5D***05 | Front Axle And Wheels | 1 |
Front Axle And Wheels (5d1305 N/S)-18.00 X 25 Tubeless Tires-Part 3 Of 3
| 5D***05 | Front Axle And Wheels | 1 |
Front Axle And Wheels (5d1305 N/S)-18.00 X 25 Tubeless Tires-Part 2 Of 3
| 5D***05 | Front Axle And Wheels | 1 |
Front Axle And Wheels (5d1305 N/S)-18.00 X 25 Tubeless Tires-Top View-Part 1 Of 3
| 5D***05 | Front Axle And Wheels | 1 |
Front Axle And Wheels (5d1305 N/S)-18.00 X 25 Tubeless Tires-Part 1 Of 3
| 5D***05 | Front Axle And Wheels | 1 |
Front Axle And Wheels (5d1305 N/S)-18.00 X 25 Tubeless Tires-As Viewed From Front Of Machine-Part 3 Of 3
| 5D***05 | Front Axle And Wheels | 1 |
3d3334 Front Spring Group
| 3D***34 | Front Spring Group | 1 |
Fuel System (5d1335 N/S)--225 U.S. Gallon
| 5D***35 | Fuel System | 1 |
Gauges
2s500 Governor Group
| 2S***00 | Governor Group | 1 |
Hand Control Valves And Lines--As Viewed From Front Of Machine
Hitch
Hitch (5d2110 N/S)
Hood, Dash And Exhaust Extension
5d6998 Air Horn Assembly
| 5D***98 | Horn Group-Air | 1 |
5j5427 Control Lever Housing Group
| 5J***27 | Housing Group-Lever | 1 |
4d6738 Hydraulic Cylinder Group--4" Bore X 10" Stroke
| 4D***38 | Hydraulic Cylinder Group | 1 |
6j1716 Hydraulic Oil Filter Group--Type 1
| 6J***16 | Hydraulic Oil Filter Group | 1 |
6j1716 Hydraulic Oil Filter Group
| 6J***16 | Hydraulic Oil Filter Group | 1 |
4d2960 Hydraulic Oil Filter Group
| 4D***60 | Hydraulic Oil Filter Group | 1 |
6j1716 Hydraulic Oil Filter Group--Type 2
| 6J***16 | Hydraulic Oil Filter Group | 1 |
3d2063 Hydraulic Pump Assembly
| 3D***63 | Hydraulic Pump Assembly | 1 |
4d724 Hydraulic Pump Assembly
| 4D***24 | Hydraulic Pump Assembly | 1 |
Hydraulic Steering Gear, Pump And Lines (4d1830 N/S)--As Viewed From Right Hand Side Of Machine--Part 1 Of 2--Type 1
| 4D***30 | Hydraulic Steering Gear, Pump And Lines | 1 |
Hydraulic Steering Gear, Pump And Lines (4d1830 N/S)--As Viewed From Rear Of Machine--Part 2 Of 2--Type 1
| 4D***30 | Hydraulic Steering Gear, Pump And Lines | 1 |
Hydraulic Steering Gear, Pump And Lines--As Viewed From Rear Of Machine--Part 2 Of 2
Hydraulic Steering Gear, Pump And Lines--As Viewed From Right Hand Side Of Machine--Part 1 Of 2
Hydraulic Steering Gear, Pump And Lines (4d1830 N/S)--End View--Part 2 Of 2--Type 2
| 4D***30 | Hydraulic Steering Gear, Pump And Lines | 1 |
Hydraulic Steering Gear, Pump And Lines (4d1830 N/S)--Side View--Part 1 Of 2--Type 2
| 4D***30 | Hydraulic Steering Gear, Pump And Lines | 1 |
6d1270 Hydraulic Tank Group--Part 1 Of 2
| 6D***70 | Hydraulic Tank Group | 1 |
6d1270 Hydraulic Tank Group--Part 2 Of 2
| 6D***70 | Hydraulic Tank Group | 1 |
Hydraulic Torque Converter
9m2000 Manual Switch Group
| 9M***00 | Manual Switch Group | 1 |
8s3333 Override Selector Valve Group
| 8S***33 | Override Selector Valve Group | 1 |
8s3333 Override Selector Valve
| 8S***33 | Override Selector Valve Group | 1 |
5m5757 Override Selector Valve Group
| 5M***57 | Override Selector Valve Group | 1 |
Planetary Transmission--Part 2 Of 2
Planetary Transmission--Part 1 Of 2
Platform
8m8249 Pressure Control Valve Group
| 8M***49 | Pressure Control Valve Group | 1 |
8s698 Pressure Control Valve Group
| 8S***98 | Pressure Control Valve Group | 1 |
2s4996 Pressure Control Valve Group
| 2S***96 | Pressure Control Valve Group | 1 |
8m7353 Range Transmission Arrangement
| 8M***53 | Range Transmission Arrangement | 1 |
4f6941 Quick Release Valve Assembly
| 4F***41 | Release Valve Assembly | 1 |
1s3249 Relief Valve Group
| 1S***49 | Relief Valve Group | 1 |
5j923 Relief Valve Group
| 5J***23 | Relief Valve Group | 1 |
4j2555 Relief Valve Group
| 4J***55 | Relief Valve Group | 1 |
5d6636 Retarder Assembly
| 5D***36 | Retarder Assembly | 1 |
Retarder Lines (5d3120 N/S)--As Viewed From Right Hand Side Of Machine--Part 1 Of 2
| 5D***20 | Retarder Lines | 1 |
Retarder Lines (5d3120 N/S)--As Viewed From Right Hand Side Of Machine--Part 2 Of 2
| 5D***20 | Retarder Lines | 1 |
Retarder Lines--As Viewed From Right Hand Side Of Machine--Part 2 Of 2
Retarder Lines--As Viewed From Right Hand Side Of Machine--Part 1 Of 2
Retarder Oil Cooler (7m5697 N/S)--As Viewed From Rear Of Machine--Part 2 Of 2
| 7M***97 | Retarder Oil Cooler | 1 |
Retarder Oil Cooler (7m5697 N/S)--As Viewed From Rear Of Machine--Part 1 Of 2
| 7M***97 | Retarder Oil Cooler | 1 |
Retarder Oil Cooler (7s9418 N/S)--As Viewed From Rear Of Machine--Part 2 Of 2
| 7S***18 | Retarder Oil Cooler | 1 |
Retarder Oil Cooler (7s9418 N/S)--As Viewed From Rear Of Machine--Part 1 Of 2
| 7S***18 | Retarder Oil Cooler | 1 |
8s3682 Retarder Oil Cooler And Lines Group
| 8S***82 | Retarder Oil Cooler And Lines Group | 1 |
3d2744 Rotochamber
| 3D***44 | Rotochamber | 1 |
Rotochamber And Lines--As Viewed From Right Hand Side Of Machine
2m3409 Safety Reset And Converter Hold Valve Group
| 2M***09 | Safety Reset And Converter Hold Valve Group | 1 |
8s3330 Safety Reset And Converter Hold Valve Group
| 8S***30 | Safety Reset And Converter Hold Valve Group | 1 |
Scavenge Pump
1s3201 Scavenge Pump Group
| 1S***01 | Scavenge Pump Group | 1 |
6j666 Scraper Control Valve Group--Part 1 Of 2
| 6J***66 | Scraper Control Valve Group | 1 |
4j5825 Scraper Control Valve Group--Part 1 Of 2
| 4J***25 | Scraper Control Valve Group | 1 |
Effective With Serial No. 64h192 Of 2
| 6J***66 | Scraper Control Valve Group | 1 |
4j5825 Scraper Control Valve Group--Part 2 Of 2
| 4J***25 | Scraper Control Valve Group | 1 |
Scraper Pump, Lines And Tank (5d7390 N/S)--Part 4 Of 4
| 5D***90 | Scraper Pump, Lines And Tank | 1 |
Scraper Pump, Lines And Tank (5d7390 N/S)--As Viewed From Right Hand Side Of Machine--Part 3 Of 4
| 5D***90 | Scraper Pump, Lines And Tank | 1 |
Scraper Pump, Lines And Tank (5d7390 N/S)--As Viewed From Front Of Machine--Part 2 Of 4
| 5D***90 | Scraper Pump, Lines And Tank | 1 |
Scraper Pump, Lines And Tank (5d7390 N/S)--As Viewed From Right Hand Side Of Machine--Part 1 Of 4
| 5D***90 | Scraper Pump, Lines And Tank | 1 |
Scraper Pump, Lines And Tank--Part 4 Of 4
Scraper Pump, Lines And Tank--As Viewed From Right Hand Side Of Machine--Part 1 Of 4
Scraper Pump, Lines And Tank--As Viewed From Right Hand Side Of Machine--Part 3 Of 4
Scraper Pump, Lines And Tank--As Viewed From Front Of Machine--Part 2 Of 4
Seat (5d810 N/S)--As Viewed From Right Hand Side Of Machine--Part 1 Of 2
Seat (5d810 N/S)--As Viewed From Front Of Machine--Part 2 Of 2
Seat--As Viewed From Front Of Machine--Part 2 Of 2
Seat--As Viewed From Right Hand Side Of Machine--Part 1 Of 2
5d8901 Seat Suspension Assembly
| 5D***01 | Seat Suspension Assembly | 1 |
7d2269 Seat Suspension Assembly
| 7D***69 | Seat Suspension Assembly | 1 |
2s7856 Selector Body Group
| 2S***56 | Selector Body Group | 1 |
8m7325 Selector Valve Group
| 8M***25 | Selector Valve Group | 1 |
3s6890 Selector Valve Group
| 3S***90 | Selector Valve Group | 1 |
2s7855 Selector Valve Group
| 2S***55 | Selector Valve Group | 1 |
7d6345 Brake Shield Group
| 7D***45 | Shield Group-Brake | 1 |
5d9525 Slack Adjuster Assembly--R.H.
| 5D***26 | Slack Adjuster Assembly--R.H.,Slack Adjuster Assembly--L.H. | 1 |
4d6590 Steering Gear Assembly
| 4D***90 | Steering Gear Assembly | 1 |
Three Speed Planetary Torque Divider And Converter (8m1083 N/S)
| 8M***83 | Three Speed Planetary Torque Divider And Converter | 1 |
8m1072 Three Speed Planetary Transmission Group
| 8M***72 | Three Speed Planetary Transmission Group | 1 |
Three Speed Torque Divider Hydraulic Control (8m8225 N/S)
| 8M***25 | Three Speed Torque Divider Hydraulic Control | 1 |
Three Speed Torque Divider Hydraulic Control (8m2494 N/S)
| 8M***94 | Three Speed Torque Divider Hydraulic Control | 1 |
3s118 Throttle Pressure And Converter Hold Valve Group
| 3S***18 | Throttle Pressure And Converter Hold Valve Group | 1 |
8m8252 Torque Divider And Retarder Arrangement
| 8M***52 | Torque Divider And Retarder Arrangement | 1 |
5m6679 Torque Divider And Retarder Arrangement
| 5M***79 | Torque Divider And Retarder Arrangement | 1 |
Transmission Control--Part 2 Of 2
Transmission Control--Part 1 Of 2
Transmission Hydraulic Control (8m2500 N/S)--End View - Part 2 Of 2
| 8M***00 | Transmission Hydraulic Control | 1 |
Transmission Hydraulic Control (8m2500 N/S)--Top View - Part 1 Of 2
| 8M***00 | Transmission Hydraulic Control | 1 |
Transmission Hydraulic Control (8m2500 N/S)--End View--Part 2 Of 2
| 8M***00 | Transmission Hydraulic Control | 1 |
Transmission Hydraulic Control (8m2500 N/S)--Top View--Part 1 Of 2
| 8M***00 | Transmission Hydraulic Control | 1 |
8s670 Transmission Pump
| 8S***70 | Transmission Pump | 1 |
Transmission Pump And Lines (5d1365 N/S)--As Viewed From Rear Right Hand Side Of Machine--Part 1 Of 3
| 5D***65 | Transmission Pump And Lines | 1 |
Transmission Pump And Lines (5d1365 N/S)--As Viewed From Rear Right Hand Side Of Machine--Part 2 Of 3
| 5D***65 | Transmission Pump And Lines | 1 |
Transmission Pump And Lines (5d1365 N/S)--As Viewed From Rear Right Hand Side Of Machine--Part 3 Of 3
| 5D***65 | Transmission Pump And Lines | 1 |
Tranmission Pump And Lines (5d1365 N/S)--As Viewed From Rear Right Hand Side Of Machine--Part 3 Of 3
| 5D***65 | Transmission Pump And Lines | 1 |
6d1039 Valve Assembly
| 6D***39 | Valve As; -Exhaust Bypass-Exhaust Diverter | 1 |
6d856 Valve Assembly
| 6D***56 | Valve As; -Differential Lock-Parking Brake | 1 |
4d2287 Valve Assembly
| 4D***87 | Valve As; -Parking, Service | 1 |
5d3056 Double Check Valve Assembly
| 5D***56 | Valve As-Double Check; -Brakesaver Air Supply | 1 |
3d9943 Horn Valve
| 3D***43 | Valve As-Horn | 1 |
6d3546 Safety Valve Assembly
| 6D***46 | Valve As-Safety | 1 |
4d1669 Three Way Control Valve Assembly
| 4D***69 | Valve Assembly | 1 |
5d470 Quick Release Valve Group
| 5D***70 | Valve Group-Quick Release; -Parking Brake-Air | 1 |
6j60 Relief Valve Group
| 6J***60 | Valve Group-Relief; -Trailing Unit Main Control | 1 |
Windshield (3d8690 N/S)
| 3D***90 | Windshield | 1 |
666 serial number reference
The PIN plate on the 666 is a riveted metal tag mounted on the tractor-unit frame, typically near the operator's platform or engine compartment on this twin-engine layout. Pre-2000 machines like the 666 carry an 8-character serial number: a 3-character prefix identifying the model/production run, followed by a 5-digit sequence number. Clean the plate before reading it since decades of grease and paint can obscure the stamped characters.
| Prefix | Identifies | Notes |
|---|---|---|
| 77F | Cat 666 wheel tractor-scraper, earliest production run | Covers SN 77F1 through 77F279. Twin-engine layout: D346T turbocharged V8 tractor engine driving the front axle, D343T inline-six scraper engine driving the tandem rear axle. Confirm exact build year with your dealer. |
| 20G | Cat 666 wheel tractor-scraper, primary production run | Covers SN 20G1 and up. Same twin-engine D346T tractor / D343T scraper-engine layout as the 77F run, initially rated near 450 hp tractor / 335 hp scraper. A power upgrade to roughly 550 hp tractor / 400 hp scraper took effect partway through this range, around SN 20G00242, and that later portion of the run is what parts literature associates with the 666B designation; the prefix itself stayed 20G at that point, it did not switch to a new code. Do not confuse with the separate 66K prefix, which sources tie specifically to later 666B-only production (66K00227-up), not to the 666. |
Frequently asked questions
What engine does the Caterpillar 666 use?
The 666 runs two separate diesel engines. The tractor unit uses a turbocharged Caterpillar D346T V8 rated at roughly 450 hp net (up to 560 hp max). The trailing scraper unit uses its own turbocharged Caterpillar D343T inline-six rated at roughly 335 hp net (up to 420 hp max). Combined, the two power units produce close to 980 hp max, delivered through a nine-speed Caterpillar powershift transmission and tandem-power driveline.
What is the Caterpillar 666's operating weight?
Loaded operating weight is not consistently published across surviving sources — figures range from roughly 59,000 kg (130,000 lb) at the lighter end to over 107,000 kg (235,000 lb) loaded, depending on configuration and which source is cited. Verify the actual figure against the machine's data plate or an OMM matched to its serial-number range rather than a single published number.
What replaced the Caterpillar 666?
Caterpillar's direct next generation in the same twin-engine tandem-power family was the 666B, a higher-horsepower version with its own distinct serial prefix. Beyond that, large twin-engine tandem-power scrapers fell out of favor industry-wide through the 1980s as Cat shifted big-scraper development toward push-pull and elevating configurations, so no single later model directly replaced the 666's twin-power layout.
What 666 owners discuss
What do operators say about the 666's powertrain and stopping power?
The 666 runs a nine-speed barrel-type powershift transmission behind each power unit, backed by a hydraulic retarder that is the main way of controlling a loaded machine on grade. Owners of these tandem-power scrapers treat retarder condition as a go/no-go item on a used machine — a common first question about a 666 for sale is simply whether the retarder still works, and a healthy example gets described as having everything in perfect working order including the retarder. Loaded weight runs past 56,700 kg (125,000 lb), so a soft or slipping retarder is not something to defer. Have your dealer verify retarder and brake function before putting a purchased unit to work.
Are there known problems with the D346T tractor engine?
The V8 tractor engine has a reputation among people who ran fleets of these machines for head and turbocharger wear, often showing up as heavy exhaust smoke. Some longtime 660/666 users say that reputation is part of why Caterpillar eventually moved to a later 90-degree V8 design. It is also why a lot of the 660/666 units still working today, in California in particular, have been repowered with Detroit Diesel Series 60 engines rather than kept on original Cat power — owners who made that swap report the smoke goes away and reliability improves. An original-power machine is not a red flag by itself, but budget for a head and turbo inspection.
Does the D343T scraper engine have any known fuel-system quirks?
The D343 series inline-six used in the scraper unit, the same basic engine family Cat fitted across several scraper models of the era, has a documented tendency for the fuel priming pump to leak into the crankcase, diluting the oil with diesel and showing up as excess discharge from the crankcase breather. If a used 666's scraper engine has oil that smells strongly of fuel or a level that keeps climbing, check the priming pump seals before assuming a worse internal problem.
What wear patterns show up on the axles, tires, and bearings?
Correct tire sizing is 18:25 on the steering axle, 37.5x39 on the drive axle, and 37.5x51 on the scraper axle — mismatched replacement tires from a machine's service history is a recurring point of confusion when sourcing rubber for one of these. Scraper- and trailer-axle wheel bearings are called out repeatedly as a chronic trouble spot on this class of tandem-power scraper, so plan on regular grease intervals and check for play and heat during any pre-purchase inspection.
What about the fenders and cushion hitch — anything to watch there?
Production up to about 1970 used hinged fiberglass front fenders on the tractor unit that could be pulled partway out for engine access; later production changed this, so fender parts interchange depends on which run your serial falls in. The cushion hitch, the load-cushioning gooseneck between tractor and scraper, uses a hydraulic damper backed by a nitrogen accumulator to soak up haul-road shock, and it is expected to lock down solid during loading and dumping. Weak accumulator charge or a sloppy hitch pin shows up as excess shock transmitted to the operator and is worth checking on a used machine.
What's known about electrical issues on the 666?
Documented electrical complaints specific to the 666 are thin in owner discussions. This is a mechanical-injection diesel machine from the 1960s/70s, not one with engine-control electronics, so the electrical system is limited to 24-volt starting and charging circuits plus gauge and warning-light panels for each power unit. The most commonly cited weak point on twin-power scrapers of this type is corrosion or wear in the wiring and connectors that cross the articulated hitch between tractor and scraper, given constant flex and road exposure. Treat that harness and its connectors as the first place to look on a no-start or gauge complaint limited to one engine.
What should I check before buying a used 666?
Confirm the serial prefix — tractor units ran in the 77F series and scraper units in the 20G series — to make sure you are looking at a true 666 and not a later, larger 666B (66K prefix) or a mismatched pairing. Run and load both engines independently: watch the tractor V8 for smoke and check the scraper engine's oil for fuel dilution. Test the retarder and steering under load, not just at idle. Check tire sizes against spec (18:25 steering, 37.5x39 drive, 37.5x51 scraper axle) and feel for play or heat in the axle bearings. Inspect the cushion hitch accumulator and pin wear, and ask directly whether the machine has been repowered — many survivors now run Detroit Diesel Series 60 engines instead of original Cat power, which changes what parts you will need. Original Cat parts for this model are sourced almost entirely from the US, since export numbers were low. Have your dealer verify brakes, retarder, and steering function before the machine goes to work.
Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.
Serial-range & configuration variants
This page covers the Caterpillar 666. Related factory variants of the same model family:
666
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