Caterpillar 321B
Maintenance schedule, common problems & OEM parts breakdown
The Caterpillar 321B is a mid-size hydraulic excavator in the roughly 20 to 21 metric ton class, built during Caterpillar's B-series program from the mid-to-late 1990s into the early 2000s. It runs the Cat 3066T, a turbocharged diesel inline six of 6.4 liters displacement, rated at approximately 134 hp (100 kW) gross and 128 to 129 hp (95 to 96 kW) net. Operating weight varies by configuration, typically falling between about 23,000 and 23,600 kg (50,700 to 52,000 lb). Caterpillar sold the 321B in three configurations: a base model with standard swing radius, a CR (compact radius) version, and an LCR (long compact radius) version, the latter two designed so the counterweight stays within the width of the tracks during rotation. The 321B slotted into the B-series lineup between the roughly 20-ton 320B and the 25-ton 325B, filling a size and swing-radius niche rather than directly replacing a single prior model.
Across its production run, the 321B's compact-radius and long-compact-radius variants answered growing demand for excavators that could work close to buildings, traffic, and other equipment without swinging the tail end into obstacles, a configuration idea Caterpillar carried forward into every generation since. Caterpillar eventually replaced the 321B with the 321C and 321C LCR in the early-to-mid 2000s, followed later by the 321D CR/LCR family, each generation refining the swing-frame geometry and engine electronics. In today's used and parts market, the 321B remains relevant because it shares its 3066T engine and much of its hydraulic and undercarriage architecture with siblings like the 320B and 322B, which keeps parts sourcing broad even though the platform itself is long out of production. Buyers value it for straightforward mechanical systems that are cheaper to maintain than later electronically controlled generations, provided the swing bearing, final drives, and undercarriage have been kept up on schedule.
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 321B specifications
Engine
| Engine model | Cat 3066T, turbocharged diesel, 6 cylinders inline |
| Gross power | 100 kW (134 hp) |
| Net power | 96 kW (129 hp) per ISO 9249; 94.8 kW (127 hp) per SAE J1349 - figure varies by rating standard used |
| Displacement | 6.37 L (389 in3) |
| Bore x stroke | 102 mm x 130 mm (4.02 in x 5.12 in) |
| Emissions tier | Not documented for this build era in available factory literature - predates standardized nonroad emissions certification for this power class; verify with dealer if certification matters for your application |
Weights
| Operating weight | 23 100 kg (50,927 lb), long-undercarriage compact-radius configuration with standard 800 mm (32") triple grouser shoes - this is the only factory undercarriage/frame configuration documented for this model; some dealer listings round to approximately 50,980 lb (23.1 t) |
| Ground pressure | Not documented in available factory literature for this model - varies by track shoe width (600 mm, 700 mm, or 800 mm triple grouser); confirm with dealer for exact figure |
| Transport weight | Same as operating weight - no separate reduced transport weight published |
Dimensions
| Transport (shipping) length | 8800 mm (28'10") |
| Transport (shipping) height | 3100 mm (10'2") |
| Transport width | 3180 mm (10'5") with 800 mm (32") shoes, long undercarriage |
| Tail swing radius | 1600 mm (5'3") |
| Minimum front swing radius | 1850 mm (6'1"); 180-degree swing radius 3450 mm (11'4") |
| Track shoe width | 800 mm (32") triple grouser standard; 600 mm (24") and 700 mm (28") triple grouser optional |
| Ground clearance | 475 mm (1'7"), including grouser lug |
| Undercarriage length | Track length 4455 mm (14'7"); length to centers of rollers 3650 mm (12'0"); track gauge 2380 mm (7'10") |
Performance
| Maximum digging depth | 6800 mm (22'4") with reach boom and 2.9 m (9'6") stick, general-purpose bucket |
| Maximum reach at ground level | 9600 mm (31'6") |
| Maximum dump/loading height | 8510 mm (27'11"); minimum loading height 3390 mm (11'1") |
| Maximum cutting height | 11 300 mm (37'1") |
| Swing speed | 10.7 rpm |
| Maximum travel speed | 5 km/h (3.1 mph), two-speed travel system |
| Gradeability | Not documented in available factory literature - verify with dealer |
| Maximum drawbar pull | Approximately 196 kN (44,063 lbf) |
Forces
| Bucket digging force - general excavation bucket | 148 kN (33,300 lbf) per ISO; 131 kN (29,400 lbf) per SAE |
| Stick digging force - general excavation bucket | 103 kN (23,200 lbf) per ISO; 100 kN (22,500 lbf) per SAE |
| Bucket digging force - heavy-duty bucket | 145 kN (32,600 lbf) per ISO; 128 kN (28,800 lbf) per SAE |
| Stick digging force - heavy-duty bucket | 100 kN (22,500 lbf) per ISO; 97 kN (21,800 lbf) per SAE |
| Note on configuration | Only one stick length (2.9 m / 9'6") and one reach boom option documented for this model, so digging force figures do not vary by stick length as they do on some other models - they vary instead by bucket type (general excavation vs heavy-duty) |
Service capacities (summary)
| Fuel tank | 250 L (66 US gal) |
| Hydraulic system, including tank | 260 L (69 US gal) |
| Engine oil | 20.5 L (5.4 US gal) |
| Cooling system | 25 L (6.6 US gal) |
Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.
321B fluids & capacities
| System | Capacity | Recommended fluid |
|---|---|---|
| Engine crankcase (with filter) | 20.5 L (5.4 US gal) | Cat DEO diesel engine oil, API CI-4/CI-4 PLUS. SAE 15W-40 for ambient -10 to 50°C (14 to 122°F); SAE 10W-30 for -18 to 40°C (0 to 104°F). Cat DEO Cold Weather SAE 0W-40 for cold-soaked starts down to -40°C (-40°F). |
| Cooling system | 25 L (6.6 US gal) | Cat ELC (Extended Life Coolant), premixed 1:1 with water, freeze protection to -37°C (-34°F). Cat DEAC conventional heavy-duty antifreeze/coolant is an acceptable alternate. Maintain a minimum 30 percent glycol concentration, 50 percent preferred. |
| Fuel tank | 250 L (66 US gal) | No. 2-D distillate diesel fuel meeting Cat distillate fuel specification, minimum cetane number 40. Use low sulfur or ultra-low sulfur grade per local emissions requirements. |
| Hydraulic system (total, including tank) | 260 L (69 US gal) | Cat HYDO Advanced 10, SAE 10W, factory fill, ambient -20 to 40°C (-4 to 104°F). Cat HYDO Advanced 20 (SAE 20W) or Advanced 30 (SAE 30) for warmer climates. Cat Bio HYDO Advanced (biodegradable, ISO 46) where environmental compliance is required. Cat TDTO is an acceptable second-choice fluid. |
| Hydraulic tank | Not published as a standalone figure. Factory literature gives only the combined system-including-tank total of 260 L (69 US gal); no separate tank-only volume is listed. | Same fluid as the hydraulic system, see above. |
| Final drive (each side) | 9.9-10 L (2.6 US gal) per side | Cat TDTO SAE 30, factory fill, ambient -30 to 35°C (-22 to 95°F). SAE 10W for colder climates, SAE 50 for continuous hot-climate duty. Cat TDTO-TMS multigrade is an acceptable alternate across a wider temperature range. |
| Swing drive | 6.1 L (1.6 US gal) | Cat TDTO SAE 30, factory fill, same range as the final drives. SAE 10W or SAE 50 depending on climate; Cat TDTO-TMS multigrade is an acceptable alternate. |
| Pilot / auxiliary hydraulic circuits | Not broken out separately in factory literature; included within the hydraulic system total above. | Same hydraulic oil as the main hydraulic system. |
| Grease (chassis, swing gear, pin joints) | Per grease point, not a sump volume; no factory-published total. | Cat Extreme Application Grease, NLGI 1 or 2, for swing gear and pin joints under normal to heavy excavator duty (moderate to high loads, -30 to 50°C / -22 to 122°F). Cat Prime Application Grease NLGI 2 is acceptable for lighter duty. Grease must resist water washout and cling to the vertical swing-gear face. |
Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.
Caterpillar 321B 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 |
|
| Every 6,000 h |
|
Servicing the 321B beyond the schedule
Predictive Maintenance and Fluid Analysis
Run a standing oil-sampling program on the 3066T engine, the swing drive, both final drives, and the hydraulic tank rather than waiting for symptoms. Watch engine samples for turbocharger bearing metals and injector wear trends given this engine's mechanical fuel system. Track hydraulic sample cleanliness closely on CR/LCR machines, where the tighter swing frame packaging means a failing swing motor or main pump shows up in particle counts before it shows up as noise. Coolant sampling catches liner and thermostat issues early on engines this age.
Corrective and Common Repairs
On high-hour 321Bs, expect swing bearing bolt loosening and gear backlash growth on CR/LCR frames, travel motor and final drive seal seepage, and boom/stick/bucket cylinder rod wear from decades of cycling. The 3066T commonly needs injector and turbocharger attention as hours climb. Pilot control valves can drift out of calibration, and wiring harnesses harden and crack with age, causing intermittent electrical faults in the cab and at the ECM-era fuse panels behind the seat and cab rear.
Overhaul and Rebuild Points
Major rebuild candidates on the 321B center on the engine top end (liners, injectors, turbocharger), the main hydraulic pump and swing motor, and the undercarriage (track chain, sprockets, rollers, idlers), all of which wear out together on a machine of this vintage. Final drive and travel motor internals are a common second-life rebuild. On CR and LCR configurations, inspect the compact swing frame and counterweight mounting for fatigue cracking before committing to a full undercarriage or powertrain rebuild, since frame condition affects the payback.
Seasonal and Environment Servicing
Check coolant freeze protection and battery condition before cold weather, and confirm glow plug and any ether-start aids function correctly to help the 3066T start reliably in low temperatures. Switch to a colder-rated hydraulic oil viscosity if the machine works through wide seasonal temperature swings. In hot, dusty applications, shorten air filter service intervals rather than holding to the standard schedule. In wet or coastal environments, wash accumulated mud and salt from the undercarriage and swing frame regularly to slow corrosion at pins, bushings, and bearing seals.
321B fault codes & troubleshooting
| Code | Meaning | Likely cause | What to do |
|---|---|---|---|
| 1302 | Engine oil pressure below the acceptable threshold for current engine speed. | Low oil level, worn oil pump, clogged suction screen, or a failed oil pressure sensor. | Shut the engine down immediately, check oil level and condition, then verify the sensor before restarting. |
| 1303 | Engine coolant temperature above normal operating range. | Low coolant, blocked radiator core, slipping or failed fan drive, stuck thermostat, or a faulty coolant temperature sensor. | Idle down and let the engine cool, check coolant level and radiator for debris, inspect the fan drive before returning to work. |
| 1304 | Hydraulic oil temperature above normal operating range. | Low hydraulic oil level, plugged oil cooler fins, weak cooler fan, or sustained high-load work in hot ambient conditions. | Reduce load and idle to cool down, check hydraulic oil level, inspect the cooler for blockage. |
| 2201 | Governor actuator feedback sensor circuit open, or shorted to battery voltage (also documented as 2202 depending on monitor software level). | Damaged wiring or connector at the governor actuator, corroded pins, or a failed feedback sensor. | Inspect and clean the governor actuator connector, check circuit continuity and reference voltage against the wiring diagram, recalibrate the governor after any repair. |
| 2301 | Governor actuator feedback signal is unstable or biased (also documented as 2302, the number most often cited by owners of this engine/governor family). | Worn or dirty feedback sensor, loose actuator linkage, or actuator motor wear producing an erratic position signal. | Check the linkage for binding, verify feedback sensor reference voltage, then run the governor calibration procedure. |
| 2303 | Governor actuator fails to move when commanded (also documented as 2304). | Seized or jammed actuator, broken linkage, blown actuator fuse, or a failed drive motor. | Check the actuator for free movement by hand, verify power and ground at the actuator, replace it if it will not stroke. |
| 4301 | Engine speed (throttle) dial is not resting in one of its designated detent positions. | Worn speed-dial potentiometer, loose linkage, or a knob assembly out of adjustment. | Cycle the speed dial through its full range, inspect potentiometer wiring, adjust or replace the dial assembly. |
| A203 | Communication fault between the gauge cluster/monitor and the engine or pump controller (also documented as A201/A20D for related communication faults). | Loose or corroded data-link connector, chafed harness, or a controller that has dropped off the data link. | Check data-link wiring and connectors for damage or corrosion, confirm controller power and ground, reseat connectors and retest. |
| 4101 | Controller supply voltage outside normal operating range, too high or too low (also documented as 420C). | Weak battery, failing alternator or voltage regulator, corroded ground strap, or a chafed supply wire to the controller. | Load-test the battery and charging system, clean and tighten ground connections, verify supply voltage at the controller connector. |
Codes and remedies are general guidance for this model family — always confirm with diagnostic tooling and your dealer before major repairs.
321B attachments & work tools
Buckets
321B LCR runs Cat's B-family bucket linkage, pin-on with 80 mm pins. Factory lineup: General Purpose (GP) 0.70-1.30 m3 (0.88-1.75 yd3) in 775 to 1230 mm (30-48 in) widths; Heavy Duty (HD) 0.45-1.10 m3 (0.62-1.50 yd3) in 625 to 1230 mm (24-48 in) widths; Heavy Duty Rock (HDR) 0.45-1.00 m3 (0.62-1.25 yd3) in 625 to 1082 mm (24-42 in) widths. Ditch-cleaning buckets add 0.90-1.1 m3 (1.12-1.50 yd3) at 1422-1727 mm (56-68 in) wide. Pin grabber and wedge-style couplers sized for the same 80 mm pin/B-family linkage also carry these buckets.
Hydraulic hammers
321B sits in Cat's H120Es/H130Es breaker bracket built for the 20-tonne-class carrier group; the next size up, H140Es, is aimed more at the 323-329 range and runs oversized for routine work on a 321. Breaker operating weight in this bracket runs roughly 1.5-2.4 t depending on the exact hammer (varies by spec sheet), needing about 100-160 L/min feed flow. A single-function, one-way auxiliary circuit is the standard setup for a single-speed hammer on this machine.
Quick couplers
Both wedge-style couplers and pin-grabber couplers built for the 320-325 size bracket fit the 321B, since that bracket shares its 80 mm bucket pin and B-family linkage. Confirm pin center-to-center spacing against the B-family linkage spec before ordering, since coupler families vary slightly by supplier.
Thumbs/grapples
Pin-on hydraulic thumb kits built for the 318-325 class (80 mm main pin) fit the 321B and run off a double-function, bi-directional auxiliary circuit. Demolition, sorting, and rotating grapples sized for 20-tonne-class carriers mount the same way, through the coupler and the same double-function circuit.
Rippers
Single-shank rippers built for the Cat 320's B-family bucket linkage are generally interchangeable with the 321B, since both machines share that linkage family. Verify stick length and pin spacing before fitting, since the LCR's compact-radius stick geometry differs from a standard-radius 320.
Hydraulic kit / auxiliary circuit
Auxiliary hydraulic valve is standard equipment on the 321B LCR. Factory-optional arrangements are Single Function (uni-directional, for single-speed hammers, with an overload relief valve and 3rd pedal), Double Function (bi-directional, for tilt buckets and thumbs, two overload relief valves and 3rd pedal), and Combined Function (three overload relief valves, manual switch between single and double circuit, plus one- or two-pump flow selection for higher-demand tools). Main pump flow runs about 185 L/min (49 US gal/min) at up to roughly 343 bar (4974 psi), covering the hammer, thumb, and grapple classes above.
All 321B assemblies by section
Every catalogued assembly group for the Caterpillar 321B. Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.
Electrical And Starting System
5i8085 Alternator Gp
| 5I***85 | Alternator Group | 1 |
Engine Arrangement
2144457 Engine Arrangement
| 21***57 | Engine Arrangement | 1 |
| 5I***85 | Alternator Group | 1 |
Frame And Body
2144467 Chassis Arrangement
| 21***67 | Chassis Arrangement | 1 |
| 21***68 | Lubricant Group | 1 |
2144468 Lubricant Gp
| 11***23 | Film; (Speed Control) | 1 |
| 12***29 | Film; (Accum, High Press, Read Omm) | 1 |
| 13***44 | Film; (Window Latch) | 1 |
Hydraulic System
4i8267 Cap
Machine Arrangement
2144441 General Arrangement
| 21***41 | General Arrangement | 1 |
| 21***57 | Engine Arrangement | 1 |
| 21***67 | Chassis Arrangement | 1 |
Operator Arrangement
1617795 Operator Arrangement
| 16***95 | Operator Arrangement | 1 |
| 16***89 | Seat Group | 1 |
| 21***97 | Mirror Group | 1 |
Operator Station
2144497 Mirror Gp
| 21***97 | Mirror Group | 1 |
1697989 Seat Gp
| 16***89 | Seat Group | 1 |
Service Equipment And Supplies
1161523 Film
| 11***23 | Film; (Speed Control) | 1 |
2144464 Film
| 21***64 | Film; Film-Shipping | 1 |
1563849 Film
| 15***49 | Film; (Radiator) | 1 |
1r6245 Film
| 1R***45 | Film; (Travel Control) | 1 |
1205129 Film
| 12***29 | Film; (Accum, High Press, Read Omm) | 1 |
4i8472 Film
| 4I***72 | Film; (Operator Controls) | 1 |
8t1915 Film
| 8T***15 | Film; (Window Cleaning) | 1 |
1677223 Film
| 16***23 | Film; (Universal, Read Omm) | 1 |
7y0014 Film
| 7Y***14 | Film; (Hydraulic Oil Level) | 1 |
1346144 Film
| 13***44 | Film; (Window Latch) | 1 |
7y0012 Film
| 7Y***12 | Film; (Tank Lifting) | 1 |
1449126 Film
| 14***26 | Film; (Oil) | 1 |
4p6464 Film
4i8088 Film
| 4I***88 | Film; (Engine Start Switch) | 1 |
2144456 Film
| 21***56 | Film; Film-Tread | 1 |
6v4611 Film
| 6V***11 | Film; Film-Warning. Located On Front Of Fuel | 1 |
7y0013 Film
| 7Y***13 | Film; (Hydraulic Tank) | 1 |
1870661 Film
| 18***61 | Film; (Alternate Exit) | 1 |
1887268 Film
| 18***68 | Film; (Machine Swing, Crushing) | 1 |
1677270 Film
| 16***70 | Film; (No Step) | 1 |
8t6842 Film
| 8T***42 | Film; (Pull Ring, Alternate Exit) | 1 |
7y0005 Film
| 7Y***05 | Film; (Engine Speed Control) | 1 |
7y0003 Film
| 7Y***03 | Film; (Clearance) | 1 |
321B serial number reference
The PIN/serial plate on a 321B is a riveted metal tag on the machine's frame, most commonly on the right-hand side of the cab exterior below the window. Read the full PIN off that plate - the first three characters are the serial-number prefix identifying model/plant (e.g. AKG, 9CZ, KGA), and the digits that follow are the sequential unit number within that range.
| Prefix | Identifies | Notes |
|---|---|---|
| AKG | 321B (base/standard tail-swing, standard undercarriage) | Base configuration of the 321B line, built in Japan. Fitted with the Cat 3066T diesel, rated near 95 kW (128 hp) on dealer listings for this range. Units in this range trace to roughly 1999-2000. |
| 9CZ | 321B CR (Compact Radius, reduced tail-swing) | Reduced tail-swing version of the 321B for confined-site work. Cat parts-catalog machine-arrangement documents title this range as the 321B CR configuration specifically. Engine is from the same 3066T family used across the 321B line; exact rating for this prefix range not separately documented - confirm with dealer. |
| KGA | 321B LCR (Long Compact Radius, reduced tail-swing with extended reach/undercarriage) | Reduced-swing machine (1600 mm / 5'3" tail-swing radius) built to match the productivity of a standard 20-tonne class excavator. Runs the Cat 3066T ATAAC (air-to-air aftercooled) diesel, factory-rated near 103 kW (138 hp) gross, a higher output than the base 321B's 3066T. Service and electrical-schematic manuals for this range are titled for the 321B LCR specifically. |
Frequently asked questions
What engine does the Caterpillar 321B use?
The 321B runs the Cat 3066T, a turbocharged diesel inline six of 6.4 liters (389 cu in) displacement, rated at roughly 134 hp (100 kW) gross and 128 to 129 hp (95 to 96 kW) net, across the base, CR, and LCR configurations.
What is the operating weight of the Caterpillar 321B?
Operating weight runs approximately 23,000 to 23,600 kg (50,700 to 52,000 lb), varying by configuration (base, CR, or LCR) and by counterweight and undercarriage options. Check the machine's data plate for the exact configuration weight.
What replaced the Caterpillar 321B?
Caterpillar replaced the 321B with the 321C and 321C LCR in the early-to-mid 2000s, and that generation was later followed by the 321D CR/LCR family.
What 321B owners discuss
What engine does the Cat 321B actually run, and how much power does it make?
Why do some 321B owners see hydraulic temperature spike only while traveling, not while digging?
What undercarriage and swing-drive wear patterns show up on a 321B?
Are there known fuel-system quirks on the 3066T that turn up on the 321B?
What electrical or sensor problems come up most often on the 321B?
What should a buyer check on a used 321B before making an offer?
Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.
Need a specific 321B part?
Search live OEM part data, check fitment, and cross-reference alternatives with Heavy Parts AI.