Caterpillartrack-type tractorbackhoe

Caterpillar D4

Maintenance schedule, common problems & OEM parts breakdown

The Caterpillar D4 is the original small/mid-class track-type tractor in Cat's crawler line, introduced in 1936 as the diesel-engined RD4 and sold as the D4 through 1959. Early production (roughly 1936 into 1947, serial prefixes 4G, 7J, 2T and 5T) ran the water-cooled inline four-cylinder D4400 diesel, about 5.1 L (312 cu in) and 32 kW (43 hp) at the drawbar, started by a gasoline pony motor. From 1947 to the end of the run (6U narrow-gauge and 7U wide-gauge prefixes), Caterpillar fitted the stronger D315 diesel, about 5.7 L (350 cu in) and up to 47 kW (63 hp) gross, with 24-volt electric start offered alongside the pony motor. Operating weight runs roughly 4,520-4,930 kg (9,970-10,860 lb) depending on gauge and ballast, against a 1936 baseline of about 4,500 kg (10,000 lb). Drive is a 5-speed spur-gear transmission with dry-disc steering clutches and external brake bands, riding on narrow (1.12 m / 44 in) or wide (1.52 m / 60 in) track gauge, with cable-controlled blade angling standard early and factory hydraulic blade control offered later. The D4 traces back to Cat's own gasoline-engined Thirty and was itself replaced by the D4C starting in 1959.

The single biggest change across the run is the 1947 engine cutover: the D4400's non-crossflow head and top-mounted injector return line gave way to the D315's crossflow head and low-pressure injectors, a genuine jump in output rather than a running refinement. Serial identification shifted too, from a letter-number prefix plus a trailing W for wide-gauge (4G/7J/2T/5T) to fully separate narrow (6U) and wide (7U) prefixes. Late production added a factory hydraulic blade option alongside the original cable-and-external-power-unit control, plus a bolt-on loader-backhoe that shares its mounting and parts book with the sibling 922 and 944 machines. In today's used and parts market this D4 sits almost entirely in the collector and vintage-restoration segment rather than active commercial fleets. The main buying risks are confirming which engine and gauge a given serial prefix actually carries, since generic listings routinely blend this tractor's figures with its D4C-and-later successors or with the unrelated modern reissue that reused the bare 'D4' name, and pricing in steering-clutch, final-drive, and undercarriage condition, since those are the costliest systems to put right on a machine this old. Parts sourcing runs through vintage-Cat specialists and salvage rather than a current Cat parts counter, and electrical and fuel-system trim parts (magneto, pony carburetor, gauges) are the hardest items to find.

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 D4 specifications

Identity & Production

Production span	1936 (as diesel RD4, gas-engine predecessor was the Cat Thirty) through 1959
Engine-era generations	D4400-engine production roughly 1936/37-1947 (serial prefixes including 4G and 7J across the pre-war/war years); D315-engine production 1947-1959 in narrow-gauge (6U) and wide-gauge (7U) configurations
Approximate production volume	On the order of 60,000 units built with the D315 engine across the 6U/7U run
Assembly location	Peoria, Illinois, USA
Succession	Replaced by the D4C starting 1959. Later D4B, D4C, D4C Series II, D4C Series III, D4D and D4E generations use different engines and running gear and are separate spec families from this original D4

Engine

Early production engine (through 1947)	Cat D4400, inline 4-cylinder diesel, water-cooled, naturally aspirated. Bore x stroke 108 x 140 mm (4.25 x 5.50 in). Displacement approx. 5.1 L (312 cu in). Rated about 32 kW (43 hp) at the drawbar
Late production engine (1947-1959, 6U/7U series)	Cat D315, inline 4-cylinder diesel, water-cooled, naturally aspirated, 18:1 compression ratio, oil-bath air cleaner. Bore x stroke 114 x 140 mm (4.50 x 5.50 in). Displacement approx. 5.7 L (350 cu in). Rated up to about 47 kW (63 hp) at 1600 rpm flywheel gross
Starting system (D315 era)	Gasoline pony motor start, with 24V electric start offered as an option
Independent test output	Nebraska Tractor Test results reported across the model run: approx. 37 kW (50 hp) drawbar and approx. 44 kW (59 hp) belt/PTO. Figures vary by test year and engine generation
Emissions	Pre-regulation mechanical diesel; no emissions certification tier applies

Weights

Baseline weight (1936 RD4 introduction)	4,500 kg (10,000 lb)
Factory/test operating weight range	Approx. 4,520-4,930 kg (9,970-10,860 lb), varies by track gauge, ballast and production era
Gauge weight tendency	Wide-gauge (7U) configuration runs toward the top of the range; narrow-gauge (6U) configuration runs toward the bottom

Dimensions

Width over tracks	Approx. 1.98 m (6.5 ft), standard-gauge configuration
Height to top of hood	Approx. 1.75 m (5.75 ft)
Length without blade	Approx. 3.12 m (10.25 ft)
Track length on ground	Approx. 1.58 m (5.17 ft)
Ground clearance	Approx. 0.29 m (0.95 ft)
Track gauge, narrow (6U)	1.12 m (44 in)
Track gauge, wide (7U)	1.52 m (60 in)
Standard track shoe width	330 mm (13 in)
Ground contact area (per pair, standard gauge)	Approx. 10,250 cm2 (1,589 sq in)

Performance

Travel speed forward	Up to approx. 8.7 km/h (5.4 mph) in top of a 5-speed range
Travel speed reverse	Up to approx. 3.1 km/h (1.9 mph)
Transmission and steering	5 forward speeds, 1 reverse; dry multiple-disc main clutch with manual steering clutches and brakes
Drawbar output	Approx. 37 kW (50 hp) per independent test data; varies by engine generation and test conditions
Ground pressure	Approx. 0.48 kg/cm2 (6.8 psi), computed from published operating weight and ground contact area for standard-gauge configuration. Wide-gauge and swamp-type running gear lower this figure. Treat as an approximation rather than a single published factory figure
Drawbar pull force and gradeability	Not consistently documented in available literature for this era; figures vary by engine generation and gauge. Verify against period OEM literature for the specific serial-prefix version

Blades

Blade control, early production	Cable-operated angling blade
Blade control, later production	Factory hydraulic blade control became available on later (6U/7U) production alongside cable-operated setups
Blade capacity	Not consistently documented across available sources for this era; published capacity figures vary widely and some appear to reference unrelated later model generations. Verify blade capacity against period literature for the specific blade actually fitted

Service capacities summary

Fuel tank	Approx. 76 L (20 gal); some references cite up to 114 L (30 gal) - varies by source, confirm against the tank itself or the parts book for your serial prefix
Cooling system	Approx. 42 L (11 gal)
Engine oil (crankcase)	Approx. 15-19 L (16-20 US qt), varies by engine generation (D4400 vs D315) and source
Final drive, each side	Not consistently documented for this vintage machine - fill to the level/fill plug on each sealed side compartment and confirm against the parts book for your serial prefix
Hydraulic system	No standardized reservoir capacity published; early production used cable-operated blade control with no hydraulic reservoir, later factory hydraulic blade control added a separate small circuit

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

D4 fluids & capacities

System	Capacity	Recommended fluid
Engine crankcase (with filter)	Approx. 15-19 L (16-20 US qt). Exact figure varies by source and likely by which D4 engine generation is fitted (D4400, 1938-47 production, vs D315, 1947-59 production) - confirm against the OMM/parts book for your machine's own serial prefix.	Era-original guidance calls for a straight SAE 30 diesel engine oil in warm weather and a lighter SAE 10W (or 10W equivalent) in cold weather. Owners running these engines today commonly use a 15W-40 diesel engine oil of at least a CF-4-class quality, or stick with straight SAE 30 non-detergent oil for originality on an unrebuilt, unfiltered-sump application.
Cooling system	Approx. 41-42 L (about 11 US gal / 44 qt), tied to the earlier D4400-engine tractor. Not separately confirmed for the later D315-engine D4 in accessible literature - treat as approximate and verify by generation.	Water with a rust/corrosion inhibitor was the era-original fill. A modern heavy-duty diesel antifreeze/coolant is the practical equivalent today.
Fuel tank	About 76 L (20 US gal).	No. 2-D diesel fuel.
Transmission (spur-gear, non-powershift type, with dry-disc master clutch)	Approx. 19 L (20 US qt) per period spec data; fill to the dipstick or level plug rather than by a fixed volume.	SAE 90 (or 85W-90) gear lubricant per club/owner guidance. Some very early production is noted by owners as originally spec'd with a straight SAE 30 motor oil before gear-lube practice was standardized on this line - confirm against the OMM for your serial range.
Bevel gear / steering clutch compartment	Not confirmed in accessible literature for the base D4 (pre-D suffix production). Fill to the dipstick or level plug.	SAE 90 (or 85W-140) gear oil, matching the final-drive spec. Avoid extreme-pressure gear lubes with heavy zinc/sulfur additive packages - the steering-clutch and bevel-gear assembly on this vintage machine uses bronze parts that aggressive EP additives can attack.
Final drives (each side)	Not confirmed in accessible literature. Each side is its own sealed compartment, filled to its level/fill plug.	SAE 90 gear oil above freezing, SAE 80 (or lighter) below freezing, per era guidance; an 85W-140 gear oil is the practical modern equivalent. Avoid high-zinc/sulfur extreme-pressure lubricants because of the bronze gears and bushings used in these final drives.
Hydraulic system/tank (blade control, where fitted)	Not documented in accessible sources. Not every D4 configuration has a hydraulic blade-control system - cable-controlled-blade units carry none.	Where fitted, owners cite a 10W engine oil (era-original) or an anti-wear hydraulic oil as the practical modern substitute.
Pivot shaft / equalizer-bar and track-frame pivot points	Not separately documented under this heading for the base D4; serviced on the same interval as the track-roller/idler grease points below.	General-purpose chassis grease applied via grease fitting. Verify point-specific intervals against your own OMM.
Track roller / idler grease points	Zerk-fitting points, no bulk reservoir - grease to the fitting until it purges, or top off per roller design.	Early, sealed-roller design (button-type fittings): a soft, semi-fluid grease rather than a standard chassis grease. Later, redesigned rollers are oil-filled instead of greased and are simply topped off with an SAE 30 (or SAE 50 in hot climates) engine oil - identify which roller style is fitted before servicing.
Starting (pony) engine crankcase	Not documented in accessible sources - a small gasoline engine used to crank the main diesel over for starting, with its own separate, unfiltered sump.	SAE 30 engine oil, changed frequently. Because this engine runs without an oil filter, club guidance favors a non-detergent oil on older, unrebuilt units to avoid re-suspending accumulated sludge.

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

Caterpillar D4 maintenance schedule

Service interval	Tasks
Every 10 h	Check engine crankcase oil level daily before starting and top off with the season-correct grade. Check radiator coolant level daily and look for leaks at the water pump packing nut and hose joints. Drain water and sediment from the fuel sediment bowl and strainer daily, and check fuel level. Check the pony motor's own crankcase oil and gasoline supply daily, and confirm the gasoline hasn't picked up water or gone stale. Clean and check the oil-bath air cleaner weekly, more often in dusty ground, and refill the oil cup to its mark. Grease track roller, idler, and equalizer-bar pivot points weekly at the zerk fittings, and check track sag for obvious over- or under-adjustment.
Every 250 h	Change engine crankcase oil and clean the sump strainer while the engine is warm. Clean the fuel strainer element and injection-pump inlet screen and clear out any gum or varnish buildup. Clean and regap the pony motor's magneto contact points and check ignition timing. Check steering-clutch and brake-band free play at both compartments; adjust the band adjuster to the manual's free-play figure, not the clutch itself. Check track adjustment at the front idler and inspect link pin bosses for early roller contact. Inspect the generator-to-battery cutout relay contacts for corrosion or pitting.
Every 500 h	Drain and refill the transmission (spur-gear) case and check the magnetic drain plug for metal. Check and adjust valve clearance on the diesel, D4400 or D315 as fitted. Check bevel-gear/steering-clutch compartment oil level and top off, and confirm the compartment is dry rather than oil-fouled as a seal check. Service track roller and idler grease points thoroughly; purge early sealed rollers with fresh grease, top off later oil-filled rollers with the correct-season engine oil. Recheck injection pump timing at top dead center and confirm governor linkage and fuel rack move freely.
Every 1,000 h	Drain and refill each final drive compartment and check the magnetic plug for metal, flushing if contamination is evident. Pull the steering-clutch and brake covers, inspect discs and bands for wear, and reline before they reach the manual's minimum thickness. Re-torque the cylinder head and recheck valve clearance afterward. Send the fuel injection pump and injectors out for calibration and nozzle testing. Inspect front idler and track roller bearings, and check equalizer-bar bushings and pins for play. Inspect the pony-motor-to-main-engine drive coupling and its bearings for wear.
Every 2,000 h	Inspect the dry multiple-disc main clutch facing for wear and reline if approaching limits. Pull the bevel gear and pinion for a wear and backlash check alongside the steering-clutch discs. Inspect final-drive gears and bearings beyond the routine oil-change check and plan replacement by measured wear, not hours alone. Inspect the radiator core and water pump impeller and packing, and rebuild the pump if packing has been chased more than once. Inspect the frame, drawbar, and blade-lift or angle linkage pins for wear or age-related cracking.
Every 4,000 h	Plan undercarriage replacement (track, rollers, idlers, sprockets) by measured pin-boss contact and adjuster travel rather than a fixed hour figure; a machine in steady dirt-moving work often reaches this point well before 4,000 hours. Consider a full engine overhaul, D4400 or D315 as fitted, including cylinder liners, pistons, rings and bearings once compression and oil consumption trend down. Overhaul the steering-clutch and bevel-gear assembly as a set rather than piecemeal, since wear on one side typically means wear on the mating side. Reassess the electrical system's originality and condition (magneto, cutout relay, wiring), since replacement parts for these subsystems are scarce and sometimes require custom fabrication.

Servicing the D4 beyond the schedule

Predictive Maintenance & Fluid Analysis

This machine predates any scan tool, so prediction means watching mechanical tells. Check crankcase oil at every fill: metal in the pan or on a magnetic drain plug flags bearing or ring wear on the D4400 or D315 four-cylinder early. Do the same at the bevel-gear/steering-clutch case and each final drive. A dry steering-clutch cover should stay dry; oil seepage there means a failed bevel-gear seal is migrating oil into the clutch pack. Track adjuster travel predicts undercarriage replacement before pins contact the rollers.

Corrective & Common Repairs

Most no-start complaints trace to the gasoline pony motor, not the diesel: check for flooding at the petcocks, weak magneto spark, worn points, or a gummed carburetor float bowl from stale fuel. A sticky throttle or governor is usually varnish in the fuel rack, not a bad injection pump; clean the fuel strainer and sediment-bowl screen first. Dragging or grabbing steering clutches are almost always a brake-band adjustment issue, not the clutch itself, unless oil has entered the compartment. A corroded generator cutout relay is a frequent cause of erratic charging.

Overhaul & Rebuild Points

Major rebuild items on this tractor are the dry multiple-disc main clutch, the steering-clutch discs and bevel-gear/pinion set, and the undercarriage. Judge undercarriage replacement by track-adjuster travel and pin-boss contact against the rollers, not by hours alone. Final-drive bearings and gears wear slowly but check them at every oil change. Confirm which engine is fitted, D4400 or D315, before ordering head or valve-train parts; the two use different, non-interchangeable cylinder heads. Send the injection pump out for calibration and nozzle testing rather than guessing at fuel-delivery problems in the field.

Seasonal & Environment Servicing

Below freezing, run 10W engine oil and SAE 80 gear oil in the transmission, bevel-gear case, and final drives; above freezing, use SAE 30 and SAE 90. Protect the cooling system with an inhibited fill or modern heavy-duty coolant; these blocks were never built for glycol-only or hard water. Keep pony-motor gasoline fresh and separate from diesel, stale gas causes most hard-starting complaints. In dusty conditions, clean the oil-bath air cleaner more often than the routine weekly check. On idle machines, watch for corrosion and dried lubricant, not wear.

D4 common service parts

Part number	Part	Notes
6K-3454	Oil Filter Assembly	Lubrication service	Check fitment →

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

D4 attachments & work tools

Blades

The earliest D4 (RD4/wartime M1-era) tractors did not carry a Caterpillar-branded blade at all — bulldozer and angledozer blade packages were built by implement houses such as LaPlante-Choate and LeTourneau and bolted to the Cat chassis, raised and angled by an external cable-operated control unit rather than the tractor's own hydraulics. Postwar production moved toward factory-integrated blade packages; for this weight class the shorter, less-curved straight/combination-style blade was the practical fitment, while the taller, heavily-curved universal blade associated with bigger dozers was rarely specified here. There is no Variable Pitch Angle Tilt (VPAT) blade for this vintage chassis — that feature belongs only to the unrelated small-dozer line that reused the D4 name decades later, and should not be attributed to this machine.

Rippers

No catalogued factory ripper package is documented for this vintage chassis; ripper attachments became a mainstream factory option mainly on the larger dozer classes from the 1950s-60s onward, not on a tractor this size. Any single-shank field/shop-fabricated ripper seen on period small crawler tractors of this class is an improvised fitment, not a listed Caterpillar attachment. Multi-shank parallelogram rippers currently marketed for a modern D4 belong to the unrelated Next-Gen small-dozer line and do not apply to this machine — don't cross-apply their weights or shank counts here.

Winches

On early production, blade lift/angle and towing ran off an external power-control unit — a cable-and-sheave mechanism, sometimes combined with an integral winch drum — supplied by implement makers like LeTourneau; other-brand winch units were also fitted to this chassis by outside suppliers. Later in the production run the tractor could be paired with a Caterpillar-branded cable control unit that spanned this size class up through the larger dozers, used both for blade control and for a rear-mounted logging/towing winch drum rather than a separate hydraulic winch circuit.

Drawbars/Counterweights

A rear drawbar was standard fitment for towing scrapers, wagons, or other towed implements typical of a small crawler tractor in this class. A rear counterweight/ballast option is documented as available across this general dozer size range to offset the added front weight of a blade or angledozer attachment and hold steering/traction balance; an exact ballast figure isn't documented for this vintage chassis specifically and varies by configuration.

Cab/ROPS Guarding (Forestry/Waste)

This vintage tractor predates rollover-protective-structure standards — it shipped with an open operator station and no certified ROPS or FOPS was ever factory-fitted. Owners and rebuilders commonly fabricate a frame- or fender-mounted overhead canopy for weather and falling-branch protection in brush-clearing or forestry-adjacent work, but a true load-bearing rollover structure must anchor to the main frame to support the tractor's full weight in a rollover; a shop-built canopy of this era is debris/weather protection only and should not be treated as ROPS-equivalent.

Hydraulic Arrangement (Front/Rear Tools)

Front blade control on the earliest units is mechanical/cable-based, run from an external power-control unit driving lift and angle cables rather than an onboard hydraulic pump; this matches the broader industry pattern where hydraulic blade control did not become the Caterpillar-wide standard until the very end of the 1950s. A rear-mounted winch on this class shared the same cable-and-drum control principle as the blade rather than running its own hydraulic circuit; treat any fully hydraulic raise/tilt/angle blade control seen on a modern-day D4 as a late-production fitment or a non-factory conversion unless a source ties it explicitly to this vintage chassis.

Backhoe (Bolt-On Utility Attachment)

A dedicated loader-backhoe attachment package was catalogued for late-production D4 tractors, sharing one mounting-and-control parts book with the sibling 922 and 944 Traxcavator/wheel loader machines rather than being unique to the D4. The package is a hydraulically controlled boom-and-dipper unit with crowd-and-lift cylinders, a swing cylinder, and stabilizer cylinders, offered with a small choice of bucket widths; it bolts to a specific late D4 serial break, not the full production run, so fitment should be confirmed against that serial break before ordering.

All D4 assemblies by section

Every catalogued assembly group for the Caterpillar D4. Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.

D4, 922 And 944 Backhoe

7k458 Actuating Cylinder Group--3.50" Bore X 32.60" Stroke

7K***58

Actuating Cylinder Group

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6k3567 Actuating Cylinder Group--3.40" Bore X 32.60" Stroke

6K***67

Actuating Cylinder Group

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7k105 Backhoe Arrangement

7K***05	Backhoe Arrangement	1
7K***06	Backhoe Mounting And Control Arrangement	1

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6k5500 Backhoe Arrangement

6K***58	Backhoe Mounting And Control Arrangement	1
6K***00	Backhoe Arrangement	1

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6k9720 Backhoe Arrangement

6K***16	Backhoe Mounting And Control Arrangement	1
6K***20	Backhoe Arrangement	1

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7k101 Backhoe Arrangement

7K***01	Backhoe Arrangement	1
7K***02	Backhoe Mounting And Control Arrangement	1

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6k2400 Backhoe Arrangement

6K***00	Backhoe Arrangement	1
6K***70	Backhoe Mounting And Control Arrangement	1

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6k2700 Backhoe Arrangement

6K***00	Backhoe Arrangement	1
6K***69	Backhoe Mounting And Control Arrangement	1

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6k9730 Backhoe Arrangement

6K***18	Backhoe Mounting And Control Arrangement	1
6K***30	Backhoe Arrangement	1

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6k9718 Backhoe Mounting And Control Arrangement

6K***85	Mounting Group	1
6K***18	Backhoe Mounting And Control Arrangement	1
6K***19	Engine Control Linkage Group	1
6K***21	Hydraulic Control Group	1

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6k5070 Backhoe Mounting And Control Arrangement

5S***68	Film- 922 Backhoe	1
6K***79	Mounting Group	1
6K***50	Engine Control Linkage Group	1
6K***90	Hydraulic Control Group	1

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6k5058 Backhoe Mounting And Control Arrangement

6K***50	Engine Control Linkage Group	1
6K***85	Mounting Group	1
6K***58	Backhoe Mounting And Control Arrangement	1
6K***52	Hydraulic Control Group	1

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6k5069 Backhoe Mounting And Control Arrangement

6K***81	Mounting Group	1
6K***47	Engine Control Linkage Group	1
6K***69	Backhoe Mounting And Control Arrangement	1
7S***42	Film D4 Backhoe	1

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6k9716 Backhoe Mounting And Control Arrangement

5S***68	Film- 922 Backhoe	1
6K***79	Mounting Group	1
6K***50	Engine Control Linkage Group	1
6K***16	Backhoe Mounting And Control Arrangement	1

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7k106 Backhoe Mounting And Control Arrangement

6K***85	Mounting Group	1
6K***52	Hydraulic Control Group	1
7K***03	Engine Control Linkage Group	1
7K***06	Backhoe Mounting And Control Arrangement	1

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7k102 Backhoe Mounting And Control Arrangement

5S***68	Film- 922 Backhoe	1
6K***79	Mounting Group	1
7K***02	Backhoe Mounting And Control Arrangement	1
7K***03	Engine Control Linkage Group	1

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Boom

6k3665 Braking Signal Assembly

6K***65

Braking Signal Assembly

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6k3652 Bucket Assembly--24" Wide

6K***52

Bucket Assembly

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6k8522 Bucket Assembly--12" Wide

6K***22

Bucket Assembly

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6k8523 Bucket Assembly--18" Wide

6K***23

Bucket Assembly

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6k8524 Bucket Assembly--30" Wide

6K***24

Bucket Assembly

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6k3623 Control Valve Assembly

6K***23

Control Valve Assembly

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6k3551 Crowd And Lift Cylinder Group--3.90" Bore X 36.50" Stroke--2 Required

6K***51

Crowd And Lift Cylinder Group

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7k437 Crowd And Lift Cylinder Group--4" Bore X 36.50" Stroke--2 Required

7K***37

Crowd And Lift Cylinder Group

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D4, 922 And 944 Backhoe

Dipper

6k3147 Engine Control Linkage Group--Type 2

6K***47

Engine Control Linkage Group

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7k103 Engine Control Linkage Group

7K***03

Engine Control Linkage Group

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6k9719 Engine Control Linkage Group

6K***19

Engine Control Linkage Group

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6k2950 Engine Control Linkage Group

6K***50

Engine Control Linkage Group

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6k3147 Engine Control Linkage Group--Type 1

6K***47

Engine Control Linkage Group

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6k3624 Flow Control Valve Assembly

6K***24

Flow Control Valve Assembly

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6k5452 Hydraulic Control Group--Top View--Part 2 Of 2

6K***52

Hydraulic Control Group

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6k3390 Hydraulic Control Group--Top View--Part 1 Of 2

6K***90

Hydraulic Control Group

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7k104 Hydraulic Control Group--Top View--Part 2 Of 2

7K***04

Hydraulic Control Group

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6k9717 Hydraulic Control Group--Top View--Part 1 Of 2

6K***17

Hydraulic Control Group

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6k3390 Hydraulic Control Group--Top View--Part 2 Of 2

6K***90

Hydraulic Control Group

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6k9721 Hydraulic Control Group--Top View--Part 1 Of 2

6K***21

Hydraulic Control Group

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7k104 Hydraulic Control Group--Top View--Part 1 Of 2

7K***04

Hydraulic Control Group

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6k5452 Hydraulic Control Group--Top View--Part 1 Of 2

6K***52

Hydraulic Control Group

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6k9721 Hydraulic Control Group--Top View--Part 2 Of 2

6K***21

Hydraulic Control Group

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6k9717 Hydraulic Control Group--Top View--Part 2 Of 2

6K***17

Hydraulic Control Group

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6k3625 Hydraulic Valves And Lines Group--As Viewed From Rear Right Hand Side Of Machine--Part 2 Of 2

6K***25

Hydraulic Valves And Lines Group

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6k3625 Hydraulic Valves And Lines Group--As Viewed From Rear Right Hand Side Of Machine--Part 1 Of 2

6K***25

Hydraulic Valves And Lines Group

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Main And Swing Frames

6k2679 Mounting Group

6K***79

Mounting Group

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6k4985 Mounting Group

6K***85

Mounting Group

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6k2681 Mounting Group--Type 2

6K***81

Mounting Group

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6k2681 Mounting Group--Type 1

6K***81

Mounting Group

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6k3454 Oil Filter Assembly

6K***54

Oil Filter Assembly

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6k3456 Relief Valve Assembly

6K***56

Relief Valve Assembly

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6k3651 Seat And Console Group

6K***51

Seat And Console Group

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6k3648 Seat Assembly

6K***48

Seat Assembly

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Stabilizer

7k850 Stabilizer Cylinder Group--4" Bore X 18.20" Stroke

7K***50

Stabilizer Cylinder Group

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6k3561 Stabilizer Cylinder Group--3.90" Bore X 18.20" Stroke

6K***61

Stabilizer Cylinder Group

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6k3583 Swing Cylinder Group--3.90" Bore X 12.90" Stroke

6K***83

Swing Cylinder Group

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D4 serial number reference

The PIN plate on early D4 tractors (4G/7J/2T/5T) sits on the left side rear of the engine block and again on the rear of the main case, left side; on the later 6U/7U tractors it moves to the rear of the tractor, upper-left of the transmission case. The plate reads as a letter-number prefix (or number-letter for 4G/7J/2T/5T) followed directly by the sequential unit number. On the 4G through 5T runs a trailing W after the number marks a wide-gauge machine, while from 6U/7U onward standard and wide-gauge tractors use entirely separate prefixes instead of a suffix.

Prefix	Identifies	Notes
4G	Standard/Wide gauge, USA	Earliest D4 production run, originally sold as the RD4 diesel crawler and renamed D4 shortly after launch. Built with the four-cylinder D4400 diesel engine and clutch-and-brake steering typical of the era. A trailing W on the plate (e.g. a unit stamped ...W) marks the wide-gauge track version; units without the W are the base gauge. USA-built.
7J	Standard/Wide gauge, USA	Follow-on run after 4G, same D4400 four-cylinder diesel engine and clutch-and-brake steering. Wide-gauge units carry the same W suffix convention as the 4G run. USA-built.
2T	Standard/Wide gauge, USA	Bridge run between the 7J and 5T series, same D4400 engine family. Standard and wide-gauge (W suffix) versions both exist in this range. USA-built.
5T	Standard/Wide gauge, USA	Last of the pre-U-series D4 runs, still on the D4400 four-cylinder diesel. Standard and wide-gauge (W suffix) versions both exist. USA-built.
6U	Standard/narrow-gauge (1,100 mm / 44 in track), USA	From this point standard and wide-gauge tractors got fully separate serial series instead of a W suffix. Narrow-gauge D4, carried the D315 diesel engine. Sequence ran from the start of this configuration change through the end of the classic D4 line before the D4C replaced it; approximate unit-number range runs from the low single digits up into the low five figures (exact top end varies slightly by source).
7U	Wide-gauge (1,500 mm / 60 in track), USA	Wide-gauge counterpart to 6U, same D315 diesel engine, sold alongside 6U for the same production run. Approximate unit-number range runs from the low single digits up into the low-to-mid five figures (exact top end varies slightly by source).

Frequently asked questions

What engine did the Caterpillar D4 use?

Two engines across the model's life. Production through about 1947 used the Cat D4400, a water-cooled inline four-cylinder diesel of roughly 5.1 L (312 cu in) rated near 32 kW (43 hp) at the drawbar. From 1947 to the end of the run in 1959, Caterpillar switched to the D315, a larger inline four-cylinder diesel of roughly 5.7 L (350 cu in) with a crossflow head, rated up to about 47 kW (63 hp) gross at 1600 rpm. Both start on a gasoline pony motor; 24-volt electric start was optional on D315-engine machines.

What did the Caterpillar D4 weigh?

Operating weight runs roughly 4,520-4,930 kg (9,970-10,860 lb), depending on track gauge, ballast, and production era. The tractor was introduced in 1936 at a baseline of about 4,500 kg (10,000 lb); wide-gauge (7U) configurations sit toward the top of the range and narrow-gauge (6U) toward the bottom.

What replaced the Caterpillar D4?

Caterpillar replaced this tractor with the D4C starting in 1959, and later D4B, D4C Series II, D4C Series III, D4D, and D4E generations carried the model number forward with different engines and undercarriages. None of those should be confused with the unrelated small-dozer line Caterpillar reintroduced under the bare 'D4' name in the late 2010s, a much heavier and more powerful machine built around a modern electronically controlled engine.

What D4 owners discuss

The pony motor cranks over but won't start, or runs rough - what's the usual cause?

On a cold engine, check for flooding first: open the cylinder petcocks and bar the pony over, if raw gas blows out or the plugs come out wet, the carburetor is over-fueling. A shrunk or hardened gasket at the top of the pony can uncover part of an intake port and lean out one cylinder, which shows up as a rough, popping run rather than a no-start. Magneto condition matters as much as fuel: weak spark, worn points, or a magneto driveshaft strained by worn pony main bearings will all cause intermittent or no-start conditions, worse in cold weather even when the plugs are visibly firing. Old fuel that has gone stale or gummed the carburetor bowl is a frequent culprit after long storage - a carb strip and clean of the float chamber, including any lead sealing plugs, usually sorts it. Treat pony fuel as gasoline, never let it sit mixed with the diesel side, and never run the pony on fuel-diluted crankcase oil - that has taken out rod bearings on more than one machine.

What's the practical difference between the early D4400 engine and the later D315 engine in a D4?

The D4400 ran from the mid-1930s into the late 1940s: a non-crossflow head, tall injectors linked at the top by a common return line, and both intake and exhaust manifolds on the same side of the block. Owners rate it around 41 kW (55 hp) flywheel at its top rating and roughly 26 kW (35 hp) at a continuous rating - exact rpm split varies a bit by reference, so treat it as approximate. The D315 replaced it in the late 1940s and ran to the end of D4 production in the late 1950s. It uses a crossflow head, intake and exhaust on opposite sides, and low-pressure injectors without the external return-line manifold. Community consensus is that it breathes noticeably better and is meaningfully stronger, on the order of 61 kW (82 hp) top rating and around 40 kW (53 hp) continuous. Which engine is under the hood on a given serial number is a hard cutover, not a running change - confirm it from the tag and the parts book for that range rather than assuming by decade.

How do the dry-disc steering clutches and brake bands hold up on a machine this old, and what goes wrong?

Steering is by hand-lever dry-disc clutches, each paired with its own external brake band, both reached through a small bolted cover behind or beside the seat. The clutch itself is not something you adjust in service - what owners are actually turning is the brake-band adjuster, and it's easy to confuse the two when a manual talks about "steering adjustment." A recurring complaint is a band that creeps tight on its own and starts dragging or grabbing if it's set too close to begin with, so adjustments are best done to the free-play figure in the manual for that serial range rather than by feel. When the covers are off, the compartment should be dry - any oil intrusion, especially if it's dark or burnt-looking, points to a failed seal at the bevel gear case letting drivetrain oil migrate into the clutch pack, which will make the clutches slip or grab unevenly. Because a dragging or grabbing steering clutch affects control on a slope, have your dealer or a qualified vintage-Cat mechanic verify brake and clutch adjustment before the machine goes back to work.

What kind of undercarriage wear should I expect on a D4 this old?

After decades of use and likely more than one rebuild, the wear pattern owners flag most is the track link pin bosses starting to contact the rollers - a sign the bushings are worn enough that the track has stretched past the boss clearance. Checking how far the track adjuster is already run out tells you how much stretch has already been taken up and how much life is left before the next undercarriage job. On a machine that has stayed in regular dirt-moving work, a tired undercarriage can need replacing within a few hundred more hours of use, so that cost needs to be weighed into the price. On a machine that has sat idle or been kept as a collector piece, the bigger undercarriage risk shifts from wear to corrosion and dried-out lubricant in the rollers and idlers rather than actual metal loss.

Are there recurring quirks with the injection pump or governor on these engines?

Governor linkage and the fuel rack are known to stick, mostly from old fuel varnish and gum building up in the bushings after long periods of storage or light use - a stiff or sticky throttle response is more often this than a bad pump. The fuel strainer ahead of the injection pump, whether it's a paper element or the older string-type elements on rods, chokes down over time and starves the engine before it ever gets to the injectors, so it's worth checking early in any troubleshooting. Some units also have a small sediment bowl with metal strainer segments in the line between the tank and the lift pump that needs periodic cleaning out. When setting pump timing at top dead center, always finish the check by rotating in the engine's normal direction of rotation - if you go past the mark, back up well beyond it and come forward again, otherwise slack in the gear train gives a false reading.

What electrical issues come up, given this machine predates any electronics?

There's no engine control module, no sensors, and nothing to diagnose with a scan tool - the electrical system exists only to crank the pony motor and run the ignition and gauges, originally on 6 volts. Many owners have since gone to a 12-volt battery for a quicker-cranking pony while keeping the rest of the low-voltage circuit, which is a common and generally trouble-free swap as long as the ignition and any lighting are matched to the voltage in use. The recurring failure point is the mechanical cutout relay between the generator and battery: corroded or pitted contacts cause erratic charging or a battery that won't hold up. Wiring the relay backwards - tying its output to the battery directly instead of through the ammeter - is a common mistake when machines get rewired from memory. If you switch to a solid-state cutout relay, note it is polarity- and ground-sensitive, unlike the original mechanical unit, so get the ground and polarity right before connecting it.

What should I check when looking at a used D4?

Try to see it started cold rather than already warmed up - a cold start tells you far more about the pony motor, magneto, and injection pump condition than a machine that's been idling for an hour before you arrived. Pull the small covers over the steering clutch and brake compartment and check that it's dry inside; any oil, especially if it's dark or burnt-smelling, means a leaking seal at the bevel gear case and points to clutch and brake trouble down the road. Look at the track link pin bosses for contact marks against the rollers and note how much travel is left in the track adjuster, since that tells you how close the undercarriage is to needing replacement. Confirm the serial number and prefix against the tag so you know which engine and configuration is actually fitted - decades of parts swapping and rebuilds mean the machine in front of you does not always match what the paperwork or the seller assumes. Ask whether the magneto, pony carburetor, and gauges are original and functioning, since sourcing new parts for these subsystems is difficult and sometimes means a custom-machined replacement rather than an off-the-shelf part. Given the age of the frame, final drives, and steering components, have your dealer or a qualified vintage-Cat specialist inspect structural and seal condition before you rely on the machine for real work.

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

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