Caterpillar 30
Maintenance schedule, common problems & OEM parts breakdown
The Caterpillar Thirty is a gasoline-powered track-type tractor (crawler tracklayer) built for agricultural and light construction work, rated in the roughly 30-drawbar-horsepower class its name reflects. It runs a Caterpillar-built inline 4-cylinder gasoline engine of about 7.6 L (461 cu in) displacement, liquid-cooled, fed by a Zenith carburetor and fired by magneto ignition. Operating weight lands around 9,900 lb (4.5 t / 4,495 kg) for a standard machine, varying with track gauge, belt pulley, and PTO fitments. The nameplate traces to the Best 30 Tracklayer, introduced by the C.L. Best Tractor Co. in 1921; after Best and Holt merged into the Caterpillar Tractor Co. in 1925, production continued as the Caterpillar Thirty through 1938, built at both the San Leandro, California and Peoria, Illinois plants. It was directly succeeded by the gasoline Caterpillar R4 (1938-1944), while Caterpillar's parallel diesel development produced the RD4 in 1936, renamed D4 in 1937, starting the long-running D-series line. Configurations offered across the run included standard and wide-gauge track, belt pulley, front and rear PTO (including dual rear PTO), and a cable-operated blade with no hydraulics, all on an open operator platform with no cab and no electronics.
Across its production run the Thirty evolved from a Best-inherited design into a fully Caterpillar-built machine. Early Cat-badged tractors carried over Best 30 components, including cast iron upper track roller holders, the narrow seat, roller track guards, the manifold, and the air cleaner, before Caterpillar standardized its own parts as Peoria took over volume production; the Peoria-built serial series alone accounts for roughly 14,294 machines built between 1927 and 1932. Reported horsepower and transmission speeds shift somewhat between sources and years, with drawbar output documented anywhere from about 30 to 35 hp and belt output from 36 to 40 hp, reflecting running changes and different test dates rather than one fixed rating - treat these as a range that varies by series and configuration. In today's market the Thirty is a foundational antique-tractor and collector machine: it marks the hinge point between the Holt/Best tracklayer era and Caterpillar's diesel D-series dynasty, and complete running examples or donor parts are scarce enough that restoration depends heavily on reproduction components, salvage engines, and knowledge shared within the vintage Caterpillar owner community.
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 30 specifications
Overview
| Model identity | Caterpillar Thirty (commonly listed as "Caterpillar 30" or "Cat 30" in parts/collector references), gasoline/distillate track-type tractor. |
| Production years | 1925-1938. |
| Production plants | San Leandro, California and Peoria, Illinois. The Peoria-built run is referred to in period literature as the PS series and is described as functionally identical to the California-built machine. |
| Lineage | Successor to the Best Thirty (absorbed into the Caterpillar line after the 1925 Holt/Best merger). Followed in the line-up by the diesel RD4 (introduced 1936), which became the D4 - the RD4 uses a different engine (D4400, 108 x 140 mm / 4.25 x 5.50 in bore/stroke) and is a distinct model, not a Thirty variant. |
Engine
| Engine type | Caterpillar-built inline 4-cylinder, liquid-cooled, tube-type radiator. |
| Fuel | Gasoline on early production; later production rated for distillate (kerosene-type) running fuel with a small gasoline starting tank. Carburetor: Zenith K5A. |
| Displacement | 7.6 L (461 cu in). |
| Bore x stroke | 121 x 165 mm (4.75 x 6.50 in). |
| Rated engine speed | Varies by series/source: roughly 850 rpm on earlier (pre-1930) production; up to 1,400 rpm cited for later/tested production. Treat as a range, not a fixed factory figure. |
| Power output | Belt/PTO (tested): 27.1 kW (36.37 hp). Drawbar (tested): 23.0 kW (30.88 hp). Catalog/advertised ratings of roughly 30 kW (40 hp) belt and 26 kW (35 hp) drawbar also appear in period literature - the drawbar figure near 30 hp is the basis for the "Thirty" model name. Figures vary by source and by which production series is being described. |
| Emissions | Not applicable - pre-regulation era engine, no emissions certification exists for this model. |
Weights
| Operating weight | Approximately 4.5 t (9,910 lb). Only one base configuration weight is documented; the Std/XL/LGP weight split used on modern Cat dozers does not apply to this model. |
Dimensions
| Track gauge | 111 cm (43.75 in) standard gauge; 154 cm (60.75 in) wide-gauge option. |
| Track shoe width options | 33 cm (13 in) standard, with 40 cm (16 in), 46 cm (18 in), 61 cm (24 in) and 76 cm (30 in) wide/swamp shoes offered as options. |
| Belt pulley | 30 cm diameter x 21.6 cm width (12 x 8.5 in), operating at 840 rpm. |
| Overall length / height / ground clearance | Not documented in the specification sheets and parts literature reviewed for this model. Verify against original Caterpillar sales literature or a vintage restoration reference for a specific serial-number range. |
Performance
| Transmission | Sliding-gear transmission with dry-disc clutch. Early production: 3 speeds forward, 1 reverse. Later production: 5 speeds forward, 1 reverse. Configuration varies by production series. |
| Travel speeds (3-speed production) | Forward: 2.7 km/h (1.7 mph), 4.2 km/h (2.6 mph), 5.8 km/h (3.6 mph). Reverse speed not documented in sources reviewed. |
| Travel speeds (5-speed production) | Not documented with individual gear speeds in the sources reviewed. |
| Steering | Manual steering clutches with contracting band brakes. |
| Drawbar pull, gradeability, ground pressure | Not published for this model in any source reviewed. Ground pressure and drawbar-pull-by-gear tables are a later Caterpillar dozer publishing convention and do not appear in period Thirty literature or in the modern spec aggregators checked. |
Blades
| Blade options | Not applicable. The Thirty predates Caterpillar's factory blade-type naming system (S/SU/A/VPAT). Dozer blades fitted to this era of machine were typically supplied by outside implement makers rather than sold under a standard Caterpillar blade designation, so no factory blade capacity figures exist to report. |
Service capacities
| Fuel tank | Main tank 141.9 L (37.5 US gal); auxiliary/starting tank 9.8 L (2.6 US gal). |
| Engine oil, cooling system, hydraulic capacities | Not documented in the sources reviewed - this predates factory hydraulic systems, and oil/coolant fill capacities were not found in any specification listing checked. |
Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.
30 fluids & capacities
| System | Capacity | Recommended fluid |
|---|---|---|
| Engine crankcase (with filter, if equipped) | Approx. 18.9 L (20 US qt / 5 US gal). The Thirty shares its oil pan with the larger Caterpillar Sixty, and a 5 US gal fill is reported as sufficient; treat the dipstick/gauge as needing field verification since these are often out of calibration on surviving machines. No independent second source confirms the exact quart figure for the Thirty itself, so verify against the original instruction book for your serial range. | Straight-weight, non-detergent mineral engine oil (period practice, no modern Cat DEO-type spec applies to this pre-war design). SAE 30 general-purpose grade is commonly used by owners of this era of Caterpillar gas/distillate engine; some run heavier SAE 40 in warm climates. No factory seasonal viscosity chart was found specific to the Thirty - confirm against the original instruction book. |
| Cooling system (radiator + block) | Not documented in accessible sources for the Thirty specifically. Do not substitute the Sixty's larger cooling-system figure - that is a bigger engine/tractor and the number does not carry over. | Plain water was standard factory fill for this era, with anti-freeze/rust-inhibitor added for cold-weather storage and operation per period practice. No capacity or additive spec confirmed for this model - verify with the original instruction book or parts book for your serial range. |
| Fuel tank(s) | Main tank approx. 141.9 L (37.5 US gal); auxiliary/starting tank approx. 9.8 L (2.6 US gal). | This generation of Caterpillar gas tractor commonly used a small gasoline tank for cold starting/warm-up, then ran the engine on cheaper distillate (low-grade kerosene-type) fuel from the main tank once warm. Sources disagree on whether the Thirty's engine should be classed as straight gasoline or gasoline-start/distillate-burning - this varies by production series and by how an individual machine was set up. Confirm fuel type for your specific serial range against the original instruction book. |
| Transmission / bevel gear / steering clutch compartment | Not documented in accessible sources. | The Thirty predates Caterpillar's later powershift transmissions - it uses a mechanical spur-gear transmission with dry-plate steering clutches, not the pump-and-filter bevel-gear/steering-clutch arrangement seen on later D-series machines. No period lubrication chart entry (capacity or oil grade) for this compartment was found in accessible sources; verify against the original instruction book for your serial range. |
| Final drives (each) | Not documented in accessible sources. | Not documented in accessible sources. Crawler tractors of this era typically used a straight mineral gear oil in the final drive housings rather than a modern GL/TO-rated product; exact grade and fill quantity for the Thirty were not found and should be confirmed against the original instruction book. |
| Hydraulic system / tank | Not applicable - no hydraulic system on this model. | The Caterpillar Thirty predates factory hydraulic implement circuits. Blade and implement control on this generation of machine is cable- and winch-operated (mechanical), not hydraulic, so there is no hydraulic tank or hydraulic fluid spec for this model. |
| Pivot shaft / track frame pivot | Not documented / likely not a filled compartment on this model. | No pivot-shaft oil-fill entry was found for the Thirty. Track-frame and equalizer-bar pivot points on this era of Caterpillar crawler were typically serviced with chassis grease through zerk fittings rather than housed in a filled oil compartment. Confirm against the original instruction book for your serial range. |
| Grease points (track rollers, idlers, steering linkage, etc.) | Not applicable - hand or gun-applied grease, not a fill-to-level compartment. | No model-specific grease specification was found for the Thirty. General period practice for this class of Caterpillar crawler used a general-purpose chassis/cup grease at track roller, idler, and steering linkage fittings. Confirm exact grade and service interval against the original lubrication chart for your serial range. |
Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.
Caterpillar 30 maintenance schedule
| Service interval | Tasks |
|---|---|
| Every 50 h |
|
| Every 250 h |
|
| Every 500 h |
|
| Every 1,000 h |
|
| Every 2,000 h |
|
| Every 3,000 h |
|
Servicing the 30 beyond the schedule
Predictive Maintenance & Fluid Analysis
On the Thirty, 'predictive' service means watching the basics closely. Check crankcase oil for the milky look that signals coolant leaking past the head gasket. Watch for gasoline dilution if the engine is run on distillate before it warms fully. Track magneto point gap and coil insulation - weak spark shows up first as hard starting and popping through the Zenith carburetor. Run a compression check across all four cylinders each season; a low reading on one jug flags valve or ring wear before it costs a full rebuild.
Corrective & Common Repairs
Most Thirty repairs trace to three systems: ignition, steering clutches, and the cable blade control. A weak or arcing magneto is the top no-start complaint - points, condenser, and coil age out first. Steering clutches glaze and slip with heat, making the tractor pull to one side under load; relining or adjusting band tension cures it. The cable-operated blade has no hydraulics to mask wear, so a stretched or frayed control cable and worn sheaves show up as sloppy blade response long before anything else fails.
Overhaul & Rebuild Points
Undercarriage wear dominates overhaul planning: track pins and bushings wear oval under sustained pulling load and need turning or replacement, along with roller and idler bushings carried over from the original Best 30 design. Engine rebuilds typically mean a rebore, new rings, and a valve job once compression drops across the cylinders. Main and rod bearings on these gasoline fours were shimmed or repoured depending on production year. Steering clutch and final drive teardown is the other big-ticket job, since linings and gears wear together under repeated engagement.
Seasonal & Environment Servicing
Seasonal care on the Thirty centers on its tube radiator and dual-fuel setup. In freezing climates, drain the cooling system whenever the tractor sits idle - the tube radiator has no antifreeze protection and cracks easily if ice forms. Cold starts run on the small gasoline auxiliary tank until the engine warms enough to switch to distillate from the main tank; skipping that step floods the Zenith carburetor. In dusty conditions, service the oil-bath air cleaner often, since a fouled cleaner lets grit reach the cylinders on this open-platform machine.
30 attachments & work tools
Blades (types S/SU/A, VPAT availability, widths/capacities)
The Thirty (1925-1938 gasoline crawler, the rebadged Best Thirty) predates Caterpillar's own hydraulic blade program and the later S/SU/A/VPAT type designations entirely; those terms apply only to Cat's post-1960s D-series dozers, not this machine. A dozer blade for the Thirty came from outside attachment houses - chiefly LaPlant-Choate, described as Caterpillar's principal 1930s-40s supplier of blades, rippers and scrapers, plus Bucyrus-Erie's cable-operated Bullgrader/Bulldozer line - bolted to the tractor and actuated by cable, not by integral factory hydraulics. No single documented factory blade width or capacity exists for this model; it varied by attachment supplier and job.
Rippers (multi-shank vs single-shank, weights)
No factory single- or multi-shank ripper option is documented for the Thirty; rear hydraulic ripper frames of that kind only became standard Cat catalog items decades after this model was discontinued. Where a Thirty carried a ripping tool it would have been a single-shank aftermarket unit from the same suppliers that built its blades (LaPlant-Choate among them), drawn or bolted at the rear rather than mounted on an integrated hydraulic ripper frame. No weights are documented for this class of attachment on the Thirty.
Winches (class fit, towing winch options)
Cable control of attachments on a Thirty came from third-party gear such as R.G. LeTourneau's PTO-driven Power Control Unit (introduced 1928), used to raise and lower blades or scrapers by wire rope off the tractor's rear power take-off. Caterpillar's own factory Cable Control Unit line did not launch until 1945, after Thirty production ended in 1938, so a Cat-branded CCU on a surviving Thirty would be a later retrofit, not an original fitment. A parts catalog reference to a "Caterpillar 30 cable control attachment" manual exists but its tie to this specific 1920s-1930s Thirty crawler could not be confirmed - verify serial range with a dealer or archive before relying on it.
Drawbars/counterweights
A rear drawbar was standard equipment on the Thirty, used to tow implements such as the Baker-Maney scraper documented behind a 1929 example. Factory special-equipment options included front PTO, rear PTO, double rear PTO, and a belt pulley; no dedicated factory counterweight package is documented, since front-blade ballast on this model was handled by the aftermarket attachment builder, not sold as a Caterpillar line item.
Cab/ROPS guarding (forestry/waste configs)
No ROPS or FOPS structure exists for the Thirty; those crush-protection standards postdate the model by several decades. The tractor was built with an open operator's station, and many field units (especially in the eastern U.S.) carried a simple canopy/sunshade roof for weather protection. A "tail-seat" variant built for orchard work set the operator further back with no canopy at all for low-clearance work under tree limbs - the closest thing this model has to a specialized guarding configuration; no dedicated forestry or waste-handling guarding package is documented.
Hydraulic arrangement notes (front/rear tools)
The Thirty is a fully mechanical, gear-drive machine (3-speed or 5-speed gear transmission depending on configuration) with no factory hydraulic implement circuit front or rear. Attachment control such as blade lift or scraper apron movement relied on cable-and-sheave rigging run off a PTO-driven winch supplied by the attachment maker, not by any Caterpillar-installed hydraulic pump or valve bank.
All 30 assemblies by section
Every catalogued assembly group for the Caterpillar 30. Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.
No. 30 Cable Control
1j8645 Adapter Group--Part 1 Of 2--Type 2
| 1J***45 | Adapter Group | 1 |
2j3610 Adapter Group--Part 2 Of 2--Type 3
| 2J***10 | Adapter Group | 1 |
1j8646 Adapter Group--Part 2 Of 2--Type 3
| 1J***46 | Adapter Group | 1 |
1j8651 Adapter Group--Part 1 Of 2--Type 1
| 1J***51 | Adapter Group | 1 |
1j8648 Adapter Group--Part 1 Of 2--Type 2
| 1J***48 | Adapter Group | 1 |
1j8645 Adapter Group--Part 2 Of 2--Type 1
| 1J***45 | Adapter Group | 1 |
1j8645 Adapter Group--Part 2 Of 2--Type3
| 1J***45 | Adapter Group | 1 |
1j8646 Adapter Group--Part 1 Of 2--Type 1
| 1J***46 | Adapter Group | 1 |
1j8648 Adapter Group--Part 2 Of 2--Type 1
| 1J***48 | Adapter Group | 1 |
1j8649 Adapter Group--Part 1 Of 2--Type 1
| 1J***49 | Adapter Group | 1 |
2j3610 Adapter Group--Part 2 Of 2--Type 1
| 2J***10 | Adapter Group | 1 |
2j3610 Adapter Group--Part 2 Of 2--Type 2
| 2J***10 | Adapter Group | 1 |
1j8647 Adapter Group--Part 1 Of 2--Type 2
| 1J***47 | Adapter Group | 1 |
1j8650 Adapter Group--Part 1 Of 2--Type 1
| 1J***50 | Adapter Group | 1 |
1j8648 Adapter Group--Part 1 Of 2--Type 3
| 1J***48 | Adapter Group | 1 |
2j3610 Adapter Group--Part 1 Of 2--Type 1
| 2J***10 | Adapter Group | 1 |
1j8650 Adapter Group--Part 1 Of 2--Type 2
| 1J***50 | Adapter Group | 1 |
2j3610 Adapter Group--Part 1 Of 2--Type 2
| 2J***10 | Adapter Group | 1 |
1j8646 Adapter Group--Part 1 Of 2--Type 3
| 1J***46 | Adapter Group | 1 |
1j8649 Adapter Group--Part 2 Of 2--Type 1
| 1J***49 | Adapter Group | 1 |
1j8647 Adapter Group--Part 2 Of 2--Type 1
| 1J***47 | Adapter Group | 1 |
1j8650 Adapter Group--Part 2 Of 2--Type 2
| 1J***50 | Adapter Group | 1 |
1j8645 Adapter Group--Part 1 Of 2--Type 1
| 1J***45 | Adapter Group | 1 |
2j7776 Adapter Group--Part 1 Of 2
| 2J***76 | Adapter Group | 1 |
1j8649 Adapter Group--Part 1 Of 2--Type 3
| 1J***49 | Adapter Group | 1 |
1j8646 Adapter Group--Part 1 Of 2--Type 2
| 1J***46 | Adapter Group | 1 |
1j8646 Adapter Group--Part 2 Of 2--Type 2
| 1J***46 | Adapter Group | 1 |
1j8649 Adapter Group--Part 1 Of 2--Type 2
| 1J***49 | Adapter Group | 1 |
1j8649 Adapter Group--Part 2 Of 2--Type 3
| 1J***49 | Adapter Group | 1 |
1j8651 Adapter Group--Part 2 Of 2--Type 1
| 1J***51 | Adapter Group | 1 |
1j8651 Adapter Group--Part 1 Of 2--Type 2
| 1J***51 | Adapter Group | 1 |
1j8647 Adapter Group--Part 2 Of 2--Type 3
| 1J***47 | Adapter Group | 1 |
1j8648 Adapter Group--Part 1 Of 2--Type 1
| 1J***48 | Adapter Group | 1 |
1j8647 Adapter Group--Part 1 Of 2--Type 3
| 1J***47 | Adapter Group | 1 |
1j8650 Adapter Group--Part 2 Of 2--Type 3
| 1J***50 | Adapter Group | 1 |
1j8647 Adapter Group--Part 1 Of 2--Type 1
| 1J***47 | Adapter Group | 1 |
1j8647 Adapter Group--Part 2 Of 2--Type 2
| 1J***47 | Adapter Group | 1 |
1j8650 Adapter Group--Part 2 Of 2--Type 1
| 1J***50 | Adapter Group | 1 |
1j8649 Adapter Group--Part 2 Of 2--Type 2
| 1J***49 | Adapter Group | 1 |
1j8646 Adapter Group--Part 2 Of 2--Type 1
| 1J***46 | Adapter Group | 1 |
2j3610 Adapter Group--Part 1 Of 2--Type 3
| 2J***10 | Adapter Group | 1 |
1j8651 Adapter Group--Part 1 Of 2--Type 3
| 1J***51 | Adapter Group | 1 |
1j8650 Adapter Group--Part 1 Of 2--Type 3
| 1J***50 | Adapter Group | 1 |
1j8645 Adapter Group--Part 2 Of 2--Type 2
| 1J***45 | Adapter Group | 1 |
1j8645 Adapter Group--Part 1 Of 2--Type 3
| 1J***45 | Adapter Group | 1 |
1j8648 Adapter Group--Part 2 Of 2--Type 2
| 1J***48 | Adapter Group | 1 |
1j8651 Adapter Group--Part 2 Of 2--Type 3
| 1J***51 | Adapter Group | 1 |
2j7776 Adapter Group--Part 2 Of 2
| 2J***76 | Adapter Group | 1 |
1j8648 Adapter Group--Part 2 Of 2--Type 3
| 1J***48 | Adapter Group | 1 |
1j8651 Adapter Group--Part 2 Of 2--Type 2
| 1J***51 | Adapter Group | 1 |
1j6041 Bearing Group
| 1J***41 | Bearing Group | 1 |
1j6680 Bearing Group
| 1J***80 | Bearing Group | 1 |
3j8333 Brake Band Assembly
| 3J***33 | Brake Band Assembly | 1 |
4j5404 Brake Band Assembly
| 4J***04 | Brake Band Assembly | 1 |
5j8897 Brake Band Assembly
| 5J***97 | Brake Band Assembly | 1 |
1j6311 Brake Band Assembly
| 1J***11 | Brake Band Assembly | 1 |
6j2532 Brake Band Assembly
| 6J***32 | Brake Band Assembly | 1 |
3j8333 Brake Band Assembly--Second Type
| 3J***33 | Brake Band Assembly | 1 |
Cable Control--Part 1 Of 3
Cable Control--Part 3 Of 3
Cable Control--Part 2 Of 3
2j5524 Cable Control Improvement Group
| 2J***24 | Cable Control Improvement Group | 1 |
1j9698 Cable Cutter
| 1J***98 | Cable Cutter | 1 |
2j3606 Hand Control Group
| 2J***06 | Control Group-Hand; Hand Control Group | 1 |
3j7659 Field Installation Group--Type 3
| 3J***59 | Field Installation Group | 1 |
3j7659 Field Installation Group--720 Degrees Brake Band Assembly--Type 1
| 3J***59 | Field Installation Group | 1 |
5j8958 Field Installation Group--324 Degrees Brake Band Assembly
| 5J***58 | Field Installation Group | 1 |
3j7659 Field Installation Group--Type 2
| 3J***59 | Field Installation Group | 1 |
1j8584 Universal Joint
| 1J***84 | Universal Joint | 1 |
30 serial number reference
The Cat 30 (Thirty) serial tag is a large brass plate, roughly 4-5 inches (100-125 mm) square, screwed to the tail end of a fender - commonly cited as the back of the left fender - with the same number also stamped into the steel underneath in case the tag is lost or the fender is bobbed/removed. Some units also carry a secondary stamping on the flywheel housing near the number-4 cylinder, useful if the fender tag and its backup stamp are both gone. The first letter(s) of the serial are the plant/series prefix (S, PS, or 6G for this model), and the digits that follow are the sequential unit number within that prefix; the prefix alone tells you which production run and roughly which years, not the digits.
| Prefix | Identifies | Notes |
|---|---|---|
| S | Best/early Caterpillar Thirty carryover (gas) | Gasoline 4-cylinder Thirty, a direct carryover from Best Manufacturing Co.'s Best 30 Tracklayer (introduced 1921), continuing into the 1925 Best-Holt merger under the Caterpillar name. Sources disagree on which plant built the S-prefix run - some tie it to the former Best plant at San Leandro, others to Stockton - treat the plant call as unresolved and confirm with dealer/collector reference. Production runs roughly mid-to-late 1920s into 1930, tapering off as Peoria (PS) output ramps up. A specific numeric span (cited variously as roughly S3563-S10536) is inconsistent across sources and should be confirmed with a dealer or club reference before relying on it for dating a unit. |
| PS | Caterpillar Thirty, Peoria-built (gas, Standard/Orchard gauge) | Gasoline 4-cylinder Thirty built at the Peoria, Illinois plant, spanning the merger era through the early 1930s (one detailed source bounds this July 1925 to October 1932). Rated in the 30-35 drawbar hp / 36-40 belt-PTO hp range depending on test source. Offered with a standard (narrower) and a wide track gauge; a version with an end-mounted operator seat was also offered to keep the operator clear of low branches in orchard/grove work. Year-by-year serial breaks are documented by collector reference and line up with an overall PS1 to roughly PS14294 span, ending around late 1932. |
| 6G | Redesigned Caterpillar Thirty (gas, late production) - transitions into R4 | Later, redesigned version of the Thirty, still a 4-cylinder gasoline machine (a V8 automotive-engine swap noted on one surviving unit was a field modification, not a factory diesel or factory gas option). Radiator/tank still reads THIRTY through this run. Serial numbers in this series start at 6G1; machines carry the Thirty designation only up to roughly 6G874, after which the same 6G serial sequence continues under the Caterpillar R4 name - a 6G number past that break belongs to the R4, not the Thirty. Documented mid-1930s production (one dated example near 6G101 is a 1936 unit); exact start year varies by source, so treat the early end of this range as approximate. |
Frequently asked questions
What engine does the Caterpillar Thirty use?
The Thirty uses a Caterpillar-built inline 4-cylinder gasoline engine of about 7.6 L (461 cu in) displacement, liquid-cooled, with a Zenith carburetor and magneto ignition. Documented output runs roughly 30-35 drawbar horsepower and 36-40 belt horsepower depending on the test year and tuning - treat the exact figure as varying by series. Many machines started on gasoline from a small auxiliary tank, then switched to distillate from the main tank once warmed.
What does a Caterpillar Thirty weigh?
Operating weight is documented at approximately 9,900 lb (4.5 t / 4,495 kg) for a standard-configuration machine. Actual weight varies with track gauge, belt pulley, and PTO options fitted, so treat this as a working figure rather than an exact constant across every example.
What replaced the Caterpillar Thirty?
The gasoline Thirty was directly continued by the Caterpillar R4, built from 1938 to 1944. In parallel, Caterpillar's diesel development produced the RD4 in 1936, renamed D4 in 1937, which carried the model line forward into Caterpillar's long-running diesel D-series.
What 30 owners discuss
Why do old Caterpillar Thirty owners talk so much about magnetos and hard starting?
How does the cable-controlled blade behave compared to a later hydraulic dozer?
What undercarriage wear pattern should I expect on a Thirty that has done real work?
Steering clutches and brake bands feel heavy or grabby - is that normal for this era of machine?
What cooling and engine issues show up after long storage or heavy use?
Does the engine spec or fuel system change across the 1925-1938 production run?
What should I actually check before buying a used Caterpillar Thirty today?
Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.
Need a specific 30 part?
Search live OEM part data, check fitment, and cross-reference alternatives with Heavy Parts AI.