The new B Series track feller bunchers feature smoother hydraulic performance, increased engine HP, a more comfortable operator station and better right side operator visibility. The Cat 521B Series are zero tail swing machines designed for medium to high production clear cut and or select cut on rough terrain. These machines perform in a variety of small through larger tree size applications. A robust undercarriage utilizes proven components, customized for demanding forestry conditions. The unique boom design provides industry leading lift capacity throughout full reach range, a benefit in large timber.

Gross Power 303 HP

Operating Specifications
Maximum Reach - With Attachment 26.5 ft

Hydraulic System
Pumps (1) Electric hydraulic oil fill pump
Pumps (2) Dedicated variable displacement pump for travel circuit - 200 cc
Pumps (3) Dedicated variable displacement pump for boom and implement functions - 200 cc
Pumps (4) Dedicated variable displacement pump for saw motor - 55 cc

Service Refill Capacities
Cooling System 10.8 gal (US)
Engine Oil 10.3 gal (US)
Fuel Capacity 226 gal (US)
Hydraulic System 118 gal (US)
Swing Drive 2.6 gal (US)

Operating Weight - Without Attachment 60629 lb

Power Train
Displacement 537 in³
Engine Model Cat® C9 ACERT™
Gross Power 303 HP
Gross Power - 1,800 rpm 303 HP
Rated Power - 2,100 rpm 282 HP

Boom System
Track Feller Buncher - Lift Over Front of Tracks: Bare Pin Lift at Maximum Reach - Without Head 6100 kg @ 6.2 m (13,500 lb @ 20 ft 6 in)
Track Feller Buncher - Lift Over Front of Tracks: Maximum Reach - With Head 26.5 ft

Pitch 8.5 in
Size 330 EX (D7)

Cab Cab meets FOPS/OPS/ROPS/WCB/OR-OSHA requirements

Operating Weights - Without Heads, Standard Counterweight
Track Feller Buncher - 521B 60629 lb

Track Gauge
521B 102 in

Track Length
Track Length 191.1 in

Ground Clearance
Ground Clearance 35 in

Tractive Effort
521B 70500 lb

Power Train

Powerful Performance
The Cat C9 electronically controlled ACERT engine delivers power while meeting all current U.S. EPA emission standards. All models contain a fuel/water separator, engine air intake heater and a dual-stage combination air cleaner with precleaner.

High Capacity Cooling System
The side-by-side radiator/oil cooler gives excellent cooling capacity for high production bunching and harvesting. On-demand Flexxaire fan activates only when required and allows the cooling fan to purge debris from the radiator area, keeping a cleaner cooling system. The IQAN control system automatically adjusts the Flexxaire fan blade angle, reducing horsepower demand and lowering fuel consumption. The purge cycle frequency and duration can be set by operator in the cab to suit varying application conditions.

High Capacity Cooling System

Engine Oil
Cat engine oil is formulated to optimize engine life and performance and is recommended for use in Cat diesel engines. Extended oil change intervals reduce maintenance and machine downtime.

The clamshell style engine compartment provides easy access to all filters, oil dipstick, coolant reservoir, hydraulic fill pump, valves and hoses reducing maintenance time.


Low Exhaust Emissions
The Cat C9 ACERT engine is a low emission engine designed to meet U.S. EPA emission regulations.

Low Exhaust Emissions

An optional starting aid for colder climates is available for 521B machine models. This option provides two (2) additional batteries and HD battery cables.

Hydraulic Packages

Electric Hydraulic Oil Fill Pump
An easy accessible service wand allows hydraulic oil to be pumped through the hydraulic filtering system and into the tank. This reduces maintenance downtime and helps to maintain oil cleanliness requirements.

Variable Displacement Pump
521B track feller bunchers are equipped with variable displacement pumps for travel and implement functions. A dedicated variable displacement pump is available for Hot Saw and Intermittent Saw attachments.

Valve Design Flexibility
Two hydraulic valve groups are available to allow configuration of machine to operate various felling heads. Valve designs allow maximizing of machine multi-functioning performance. 1) High flow felling heads requiring 170-190 L/min (45-50 gal/min) to the grab and accumulator arms. Typically have one cylinder operating each arm, two cylinders per arm set. 2) Low flow felling heads requiring 110-120 L/min (29-32 gal/min) to operate grab and accumulator arms. Typically one hydraulic cylinder and a link arm activate the arm set. Note: Consult the work tool manufacturer for flow requirements before selecting one of the above Hydraulic Arrangements.

Cat Hydraulic Oil
Provides maximum protection against mechanical and corrosive wear in all hydraulic systems. Its high zinc content reduces wear and extends pump life. Biodegradable hydraulic oil (HEES) is also available from your Cat dealer and provides an environmentally-sound alternative to mineral based oils.

ToughGuard™ Hoses
Abrasion resistant hoses along the boom and stick provide strength and flexibility. The ToughGuard cover consists of a thin layer of UHMW polyethylene bonded to the rubber cover, providing exceptional abrasion resistance.

ToughGuard™ Hoses

Operator Station

Operator Comfort
The operator station is designed with joysticks and an air ride seat, reducing operator fatigue and providing easy operation. An HVAC system with updated air ducts provides better cooling and heating. A large side window with side screen allows fresh air into the cab, and an isolated mounted cab and engine reduces noise levels and vibration. Ergonomically located pedals with side-by-side foot rests provide operator comfort while operating machine travel.

Operator Comfort

The cab design optimizes post structures and window placement to provide excellent operator visibility to front, sides and rear. A large polycarbonate skylight provides excellent upward visibility.


The standard 32 mm (1.25 in) thick “Margard” front window maximizes operator protection. All purpose built forestry cabs meet FOPS/ROPS/OPS/WCB, OR-OSHA requirements. The skylight provides an alternate escape hatch.


The IQAN MD3 display allows the operator to continuously monitor the operating system of the machine. The monitor has been downsized slightly to improve the operator's line of sight out the right front corner of the cab. The IQAN control system allows the operator to adjust all control parameters.


Electrical System
The B Series track feller bunchers feature dual in-cab 12 volt power points and one 24 volt power point. Power is provided by two 12 volt maintenance free batteries and 95 amp alternator. An optional cold weather starting attachment may be selected to provide two additional batteries and HD starter cables.


The 521B Series machines utilize larger 330 size components providing extended life and reduced O&O costs.

Travel Motor
The proven final drives and travel motors provide up to 31 978 kg (70,500 lb) of tractive effort as standard on the 521B, giving excellent traction and power to the ground. Three (3) travel speed range selections are standard.

Track Guards
All models feature full length track guards to maximize shoe support in all types of terrain.

The 521B model features a welded track frame to carbody design with a fixed gauge, simplifying and reducing carbody weight. The 521B provides maximum ground clearance and a smooth carbody under-surface to resist snagging on debris or large obstacles. The precision welded track frames reduce flexing and increase the durability of rollers and track links throughout the life of the machine.

Carrier Rollers
B Series models offer a choice of either standard single flange carrier rollers for moderate terrain and warm climates or star carrier rollers for severe terrain and cold climate applications. Nine bottom track rollers are used per side.

Carrier Rollers

Boom and Stick

Boom and Stick
With up to 6124 kg (13,500 lb) of bare pin lift capacity at 6.2 m (20 ft 6 in), the 500 Series machines are dominant over competitors in their size classes. With market leading lift capacity, 8.1 m (26.6 ft) of maximum reach to cut, and tight stick tuck-in capabilities, these machines deliver high performance felling in all applications.


From plantation thinning to select cut of large diameter wood, Cat felling attachments provide reliable productive felling solutions. Designed to allow maximum visibility of the cutting area, all saws utilize high pressure hydraulics for fast saw recovery and quick arm speed. This provides a positive impact on production and better tree control.

All models are available with a Cat 40 degree partial lateral tilt, or new 330 degree full lateral tilt.


The B Series track feller bunchers are designed to maximize efficiency and productivity while conserving natural resources.
  • The C9 ACERT engine meets all current U.S. EPA emission standards.
  • The IQAN control system automatically adjusts the Flexxaire fan blade angle, reducing horsepower demand and lowering fuel consumption.
  • Major structures and components are built to be rebuilt, reducing waste and replacement costs.


Extended Service Intervals
Caterpillar service and maintenance intervals have been extended to reduce machine service time, increase machine availability and reduce operating costs. Hydraulic oil change intervals can be extended from 2,000 hours to 4,000 hours. Engine coolant change intervals are 12,000 hour with Cat Extended Life Coolant/Anti-freeze. Regularly scheduled maintenance extends machine service life and lowers overall operating costs. 500-hour oil change interval reduces overall maintenance costs.

Flexxaire Fan
Standard Flexxaire fan automatically adjusts the fan blade pitch to optimize cooling and reduce fuel consumption. The reversing purge debris cycle frequency and duration can be set by operator in the cab to suit varying application conditions.

Clam-Shell Service Door
The hydraulically actuated clam-shell service door provides easy and quick access, reducing maintenance downtime.

Clam-Shell Service Door

Customer Support

Caterpillar is known the world over for the quality of customer support from its dealer network - the industry's best. No matter where you are, the expertise of a Cat dealer is always nearby. Your local Cat dealer is your forestry consultant who can recommend the machines, work tools and services to maximize your operation and provide the support to keep you at top productivity.
  • 24-hour parts availability, where and when you need them, to minimize expensive downtime.
  • Remanufactured parts that carry the same warranty as new parts at a reduced cost.
  • Operator training to get the most out of your Cat equipment.
  • Field services to provide on-site help when needed.
  • Timely repair and replacement services.
  • Customer Support Agreements to lower your operating costs.
  • State-of-the-art diagnostic programs, such as S·O·SSM oil analysis, inspection services and trend reporting to help avoid unscheduled repairs.
  • Financing programs for buying, renting or leasing Cat equipment.
  • Cat Access Account, a fast and convenient way to pay for or rent anything offered at any Cat dealer or The Cat Rental Store.
  • Cat Insurance to cover equipment losses from theft, collision, flood, upset or overturn, fire, vandalism and more.
  • Product Link™ to manage your equipment fleet through remote monitoring.
  • Cat Certified Rebuild to get a second life from your equipment.
For more information on Cat products, dealer services and industry solutions, visit www.cat.com.

Standard Equipment

  • 24 volt electrical system
  • Two 12 volt maintenance free batteries with 950 CCA
  • 95 Amp alternator
  • IQAN programmable controls
  • MD3 monitor display screen
  • Hydraulically actuated hinged engine enclosure with warning alarm
  • Engine fan guard
  • Bottom guards
  • Isolation mounted cab and engine
  • Heavy duty radiator guard door
  • OPS/FOPS/ROPS/WCB/OR-OSHA certified cab
  • Dual joystick control
  • Reverse slope windshield
  • Air ride seat
  • Skylight - alternate escape hatch
  • Top escape hatch
  • 76.2 mm (3 in) retractable seat belt
  • DIN mount radio ready with speakers
  • Operator sound suppression to standards
  • Travel alarm
  • Cup holders (2)
  • Two 12 volt and one 24 volt power outlets
  • Front window, 32 mm (1.25 in) thick Margard
  • Positive filtered ventilation, pressurized cab with bi-level air conditioner, heater and defroster with manual control
  • Left side fresh air window with screen
  • Forced air fan
  • Washable floor mat
  • Neutral lever (lockout) for all controls
  • C9 ACERT engine meets U.S. EPA emission standards
  • Variable travel speed (3 ranges)
  • Under hood primary air filter
  • 43° C (109° F) degree ambient temperature cooling package: - Aluminum core ATAAC and side-by-side radiator and oil cooler - Removable debris screen - Swing out A/C condenser
  • Electric fuel priming pump
  • Fuel water separator
  • Track roller guards with access holes
  • Hydraulic track adjusters
  • Double flange track rollers
  • GFT80 Lohmann final drives
  • Cat 330 size undercarriage components
  • Nine (9) track rollers and two (2) carrier rollers per side
  • Welded, non-leveling carbody
  • Dedicated fixed displacement pilot pump
  • Dedicated variable displacement pumps for track and implement functions
  • Variable displacement saw pump for feller buncher hydraulics only
  • 325 EX swing drive and swing motor
  • Electric hydraulic oil fill pump
  • High flow hydraulic arrangement for felling heads with two cylinder arm design 180 L/min (48 gal/min)
  • Low flow hydraulic arrangement for felling heads with single cylinder and link design 120 L/min (32 gal/min)
  • 265 L (70 gal) hydraulic tank
  • 50% concentration extended life coolant
  • Heavy duty hood enclosure
  • Flexxaire fan
  • Vandalism protection
  • Spare hose storage area
  • Grease gun bracket and storage area
  • 856 L (226 gal) fuel tank

Optional Equipment

  • Intermittent
  • Feller Buncher
  • Omission - Linkage
  • Intermittent
  • Feller Buncher - Low Flow
  • Feller Buncher - High Flow
  • HF201B*
  • *Heads available with Full Tilt 330 degree, or Partial Tilt 40 degree
  • 610 mm (24 in) SG pads, clipped corners, with mud/snow reliefs
  • 711 mm (28 in) SG pads, clipped corners, with mud/snow reliefs
  • 711 mm (28 in) DG pads, clipped corners, with mud/snow reliefs
  • 914 mm (36 in) Tri-track, clipped corners, with mud/snow reliefs
  • Star Carrier Rollers
  • Standard Carrier Rollers
  • Lighting Gp-HID, replaces certain standard lights
  • Heavy Counterweight
  • Starting Aid, Cold Weather - includes two additional batteries
  • Starting Aid, Arctic - includes engine and hydraulic oil Arctic heater
  • Skylight, peaked, removable