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521B Track Felller Buncher
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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.

Standard Equipment

Operating Specifications

Max Reach w/ Attachment
26.5 ft

Tractive Effort

70500.0 lb

Track Length

Track Length
191.1 in

Power Train

537.0 in3
Engine Model
Cat® C9 ACERT™
Gross Power
303.0 hp
Gross Power @ 1,800 rpm
303.0 hp
Rated Power @ 2,100 rpm
282.0 hp


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


Operating Weight w/o Attachment
60629.0 lb

Ground Clearance

Ground Clearance
35.0 in


Gross Power
303.0 hp

Operating Weights (without heads, standard counterweight)

Track Feller Buncher - 521B
60629.0 lb

Track Gauge

102.0 in


8.5 in
330 EX (D7)

Service Refill Capacities

Cooling System
10.8 gal
Engine Oil
10.3 gal
Hydraulic System
118.0 gal
Swing Drive
2.6 gal
Fuel Capacity
226.0 gal

Boom System

Track Feller Buncher - Lift over front of tracks: Bare Pin Lift @ 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

Hydraulic System

Dedicated variable displacement pump for saw motor - 55 cc
Electric hydraulic oil fill pump
Dedicated variable displacement pump for travel circuit - 200 cc
Dedicated variable displacement pump for boom and implement functions - 200 cc
Power Train

Power Train

Built for power, reliability, low maintenance, fuel efficiency and low emissions.

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.

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.


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

Hydraulic Packages

Powerful hydraulic system combined with a powerful boom and stick maximize productivity.

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.

Operator Station

Operator Station

Superior comfort and visibility.

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.


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.



Heavy duty forestry undercarriage.


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.

Boom and Stick

Boom and Stick

Maximum reach and lift capacity.

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.



Work tool attachments to meet your needs.


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.



Maximum efficiency and productivity while conserving natural resources.

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.



Easy servicing and extended service intervals.

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.

Customer Support

Customer Support

Count on your Cat dealer.

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.
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