The Troy-Bilt Bronco 13WX78KS011 is one of those riding lawn mowers that earned its spot in thousands of garages across North America. Built under the MTD umbrella, this 42-inch lawn tractor paired a reliable Briggs & Stratton engine with a solid cutting deck, giving homeowners a dependable machine for weekend yard duty. It ran through several production years, and to this day, plenty of these Broncos are still going strong.
What keeps them going is parts availability. Because the Bronco shared components with other MTD-family mowers, replacement parts are still widely stocked by dealers and online retailers. Whether you need a fresh set of blades, a new belt, or an entire spindle assembly, you can usually track down what you need without much hassle. That kind of long-term support is a big reason owners hold on to these tractors well past the decade mark.
But finding the right part starts with knowing what you’re looking at. A parts diagram breaks the mower deck down piece by piece, showing you exactly where each component sits and how everything connects. If you’ve ever stared under your mower deck and wondered what a specific bracket or pulley does, the breakdown below will clear things up fast.
Parts Diagram & Details
The Troy-Bilt 13WX78KS011 mower deck parts diagram is an exploded-view illustration that separates every individual component of the 42-inch cutting deck into numbered, labeled positions. At its center sits the main deck shell, a stamped-steel housing that cradles the cutting blades, spindles, and belt system. Surrounding it, you’ll see pulleys, brackets, fasteners, springs, and control rods all pulled apart and arranged so you can trace exactly how each piece fits with its neighbor. A separate inset in the lower-left corner isolates the drive belt and related hardware for a clearer view.
Every numbered callout in the diagram corresponds to a specific OEM part number, making it easy to cross-reference with parts catalogs from Troy-Bilt, MTD, or third-party suppliers. The diagram covers over 50 individual items, from large assemblies like the spindle and chute down to small but critical pieces like lock nuts and flat washers.
Understanding these parts goes beyond simple identification. Each one plays a specific role in how your mower cuts, drives, and handles terrain. Here’s a closer look at the key components and what they do.
1. Spindle Assembly with Pulley
The spindle assembly is arguably the hardest-working component on your entire mower deck. Labeled as part number 918-04822A, this unit combines the spindle shaft, housing, bearings, and a 6.3-inch diameter pulley into one integrated piece. It’s the mechanical link between the drive belt and the cutting blade, transferring rotational energy from the engine down to the steel that actually cuts your grass.
Your Bronco’s 42-inch deck uses two of these spindle assemblies, one for each blade. Over time, the internal ball bearings wear out, and you’ll hear it before you see it. A grinding noise or wobble at the blade tips is a dead giveaway. Replacing the spindle assembly early prevents a seized bearing from warping the shaft or damaging the deck housing.
Worth noting: the pulley is permanently attached to the spindle on this assembly. If the pulley cracks or the spindle shaft bends, you’re replacing the whole unit rather than swapping individual pieces.
2. Deck Assembly (42-Inch with Wheels)
Part number 983-04572-0637 is the deck shell itself, the large stamped-steel body that houses everything else on this diagram. It comes as a pre-assembled unit with two gauge wheels already mounted. The deck’s offset design positions the discharge opening on the right side, allowing clippings to eject cleanly during mowing passes.
This is the foundation that every other part bolts onto. The deck shell has pre-drilled mounting holes for the spindles, brackets, deflector chute, and hanger arms. It’s built from 13-gauge steel on these Bronco models, which holds up well to normal residential use but can develop rust spots if you don’t clean the underside regularly.
3. Cutting Blades
Every clean stripe on your lawn starts here. Part 942-04308 is a 21.23-inch 2-in-1 blade designed for both side discharge and mulching configurations. Two of these blades mount underneath the deck, each attached to a spindle shaft with a hex bolt and flange nut.
Blade condition directly affects cut quality, fuel efficiency, and how hard your engine has to work. Dull or bent blades tear grass instead of slicing it, leaving brown tips and an uneven finish. Most manufacturers recommend sharpening after every 25 hours of use and replacing the blades at least once per season if you mow frequently. When you flip the blades, pay attention to the orientation. The cutting edge has a slight lift on one side that directs airflow upward for proper discharge.
4. Drive Belt (V-Type)
The belt that keeps everything spinning is part 954-04060B, a V-type, A-section belt measuring 96.5 inches in length. It routes from the engine pulley down to the deck, wrapping around the idler and spindle pulleys in a specific path that maintains proper tension and blade speed.
Belt stretch and cracking are the most common issues you’ll run into. Heat, debris, and age all take their toll. A slipping belt shows up as slower blade speed and poor cut quality, while a frayed belt can snap without warning mid-mow. Checking belt condition should be part of your spring startup routine every year. The inset diagram in the lower-left corner of the parts illustration gives you a clear view of how this belt threads through the pulley system.
5. Idler Pulley
Part 756-04129B is a 4.25-inch diameter idler pulley that plays a key role in belt tension management. Unlike the spindle pulleys that drive the blades, the idler pulley’s job is to keep the belt taut and properly routed as it travels between the engine and the cutting spindles.
This pulley sits on a pivot bracket and works in tandem with a spring-loaded tensioning arm. When you engage the blade control lever from the operator’s seat, the idler pulley shifts position to tighten the belt and start the blades spinning. It’s a wear item with an internal bearing that can dry out and seize up over time, so keep an ear out for squealing or chirping sounds when the blades are running.
6. Deck Pulley
Separate from the spindle pulleys, part 956-1227 is a standalone 6.3-inch deck pulley that helps direct the belt path across the underside of the deck. Its larger diameter gives the belt a broader contact surface, which improves grip and reduces slipping during heavy cutting loads.
A cracked or wobbly deck pulley throws off belt alignment and can chew through a new belt in a matter of hours. Inspect it for hairline fractures around the hub and make sure it spins freely by hand when the belt is removed.
7. Deck Hanger Bracket (Left-Hand)
Part 683-0254B-0637 is the left-hand deck hanger bracket, one of the mounting points that suspends the mower deck from the tractor frame. These brackets allow the deck to pivot freely as it follows ground contours, which is essential for an even cut across uneven terrain.
A bent or cracked hanger bracket causes the deck to sit unevenly, and you’ll see it in your lawn as a scalped strip on one side and uncut grass on the other. If you’ve hit a stump or a large rock, the hanger bracket is one of the first things to inspect.
8. Deck Brake Assembly
Two brake assemblies appear on this diagram. Parts 983-04511 and 983-04525 serve as deck brakes that stop the blades from spinning after you disengage the cutting system. Federal safety standards require the blades to stop within a few seconds of disengagement, and these brake pads make that happen.
If your blades keep spinning long after you’ve released the engagement lever, the brake pads are likely glazed or worn thin. Replacing them is a straightforward job that doesn’t require pulling the entire deck.
9. Deck Belt Cover
Part 783-06424A-0637 is the 42-inch deck belt cover, a stamped metal shield that protects the belt and pulleys from debris thrown up during mowing. It also keeps your hands and feet away from the spinning belt during routine maintenance.
This cover takes a beating from rocks, sticks, and dirt. Dents are cosmetic, but if a dent is deep enough to contact the belt, it needs straightening or replacement.
10. Discharge Chute Assembly
Part 631-04288 is the chute assembly that mounts to the right side of the deck. It directs grass clippings away from the mowing path and controls the angle of discharge. A hinged deflector at the end of the chute pushes clippings downward so they don’t spray bystanders or vehicles.
The chute bracket (part 783-06074A) holds this assembly in place and allows it to pivot up for deck access. If you’re using a bagging attachment, the chute is where the bagger duct connects.
11. Deck Gauge Wheels
The small but mighty part 734-04155 is a 5.0-inch diameter ball-style deck wheel. These wheels ride along the ground surface and prevent the deck from digging into dips or scalping high spots. They’re adjustable for height, giving you control over your preferred cutting clearance.
Flat spots develop on these wheels over time, especially if the mower sits in one place for extended periods. A flat-spotted wheel causes a rhythmic thumping during mowing and won’t roll smoothly. They’re inexpensive and easy to swap, so replace them at the first sign of wear.
12. Front Lift Rod
Part 747-05141A is a 12.7-inch front deck lift rod that connects the deck to the tractor’s height adjustment mechanism. When you move the deck height lever on the fender, this rod raises or lowers the front edge of the deck to match your selected cutting height.
A bent lift rod throws off your deck leveling. If your front-to-back deck pitch is off despite correct adjustment settings, pull this rod and roll it on a flat surface to check for straightness.
13. Brake Control Rod
Part 747-05105B is the 42-inch brake control rod, which links the operator’s blade engagement lever to the deck brake assembly. Pulling the lever on the dash sends mechanical force through this rod to either release or engage the blade brake.
Because it’s a direct mechanical linkage with no electronic assist, this rod needs to move freely in its pivot points. Corrosion or a bent rod creates a spongy feel at the lever and can prevent the brakes from fully engaging.
14. Stationary Idler Bracket
Part 783-06643-0637 is the support bracket for the stationary idler pulley. While the pivot idler bracket (part 783-05946) allows one pulley to swing for belt tensioning, this bracket holds a fixed-position idler that maintains the belt’s routing path.
Even a small misalignment here alters the belt’s track, leading to uneven wear and premature failure. When replacing a belt, always check that this bracket is tight and square to the deck surface.
15. Water Nozzle Adapter
Part 921-04041 is a feature many Bronco owners overlook entirely. This adapter threads into a port on the deck shell and connects to a standard garden hose. After mowing, you hook up the hose, turn on the water, and run the blades briefly to wash grass buildup off the underside of the deck.
The companion piece, part 737-04003D (the deck water nozzle), sits inside the deck and sprays water across the cutting chamber. Regular deck washouts prevent grass from caking into a thick mat that traps moisture and accelerates rust. It takes about two minutes and saves hours of scraping later.
16. Extension and Torsion Springs
Two types of springs keep the deck’s moving parts under proper tension. The extension spring (part 932-0384) stretches to provide return force on the belt tensioning system, while the torsion spring (part 732-04840) applies rotational pressure to specific pivot points on the idler bracket.
Springs weaken gradually, and a tired spring won’t maintain enough tension to keep the belt from slipping under load. If you’re replacing a belt and the new one still slips, the tensioning spring is the next suspect on your list. Always replace springs in pairs where applicable to maintain balanced tension across the deck.
