Replacing belts on your mower is like replacing tires on your car, there are only so many miles in them. Replacing a deck belt can be a challenge, especially if the mower throws the belt and you don’t know the routing, but we’ll get it figured out!
So how difficult is mower deck belt replacement? Getting the correct belt is the first important step. Some manufacturers place a label detailing the belt part number and belt routing. Try under the hood, or under the footrest, if not check out “Belt routing”.
Deck types vary, some are easy to work on, some not so much. It’s important to check over the deck looking for any damaged or loose components, especially if your old belt was damaged and not just worn out. You don’t want to damage the new belt needlessly.
Most mowers won’t require deck removal to fit the belt, others will. So take some time and consider the routing, don’t put extra work on yourself. Take lots of photos, it saves time and head-scratching later.
Fitting the new belt means removing some plastic protective covers. And in most cases, it involves wrapping the belt around the pulleys and making sure the belt guide (guide not on all pulleys) is on the outside of the belt. The last pulley to fit is the crank pulley (engine pulley).
Tensioned or Not
A tension-ed belt is a deck drive belt that is always tight on the pulleys. Push-button blade engages type mowers usually run an always tension-ed belt. Replacing it will require manhandling the belt onto the side of the crank pulley, then turning the crank pulley clockwise by hand until the belt slips on.
If your mower has a lever then you likely have a slack belt which is then tensioned by moving the blade to engage the lever. This type of belt is easier to replace and will take no time at all.
Tensioned – This type of belt is always tight on the pulleys.
Un-Tensioned – This type of belt setup is slack on the pulleys until you engage the blades.
What Deck Type?
Cutting deck setup types vary side discharge; rear discharge; mulching decks; out front decks; cutting and sweeping.
They may have features such as single blade; twin-blade; tri-blade; tensioned belt; manual tensioned belt; electromagnetic blade engage; fan assisted deck; timed overlapping blades and the list goes on.
The one thing they all have in common – the deck drive belt. It’s how engine power is transferred into cutting power.
A timed deck means both your mower blades are set at a fixed angle in relation to each other. The toothed belt maintains the blade position, this allows the blades to overlap.
Some say the overlapping blades give a superior cut, I like the lawn finished with the overlapping twin cut, especially the smaller decks.
This deck type is also referred to as an interference deck. They call it Interference because if the blades go out of time, they’ll smack each other.
Resetting the timing of the blades or replacing the belt is a job that can be done without much difficulty, but it does require removing the deck, tension assembly, and various plastic guards. No special tools are needed.
Rear – Rear discharge great at collecting grass, but doesn’t like long grass so much.
Timed – Timed deck has a toothed belt that can break or slip out of time. Timing the blades allows them to overlap.
Mulching – Some decks will have a flap that closes off the chute when the operator wants to mulch.
Side Discharge – Side discharge is great for tall grass and rough terrain.
Measuring The Belt
A belt will be marked with a type code, length, and part number. Belts are usually measured by their inside length (Li) or outside length (La). If you can find this info on the side of your old belt, great!, but usually it’s worn away.
Some mowers like Husqvarna place a sticker inside the hood with a list of helpful part numbers like belts, filters, plugs, etc.
What Belt Width? – The width and depth of a belt are also very important. A new V belt should fit snugly into a v pulley, the belt should sit just proud of the pulley shoulder. A belt that sits further down into the pulley, is worn out.
What Belt Length? – If your belt was shredded, then you’ll need your make and model number to order the correct belt. An easy way to measure an old belt – use a string to follow the outside of the belt, now measure the string.
This measurement will be marked on belts by the letters La (outside measurement), alternatively, run the string around the inside of the belt, this measurement is the Li measurement.
A faster way to measure an intact belt is to make a circle of the belt and measure inside to inside, then multiply by 3.14, the result is the Li belt size.
Sizing – Sometimes easier said than done!
Markings – Check under the hood of your mower, you may get lucky with a part number sticker, but be cautious with the Husqvarna labeling, they are often wrong belt part numbers.
Check your old belt for markings, if none, get a tape and some string. These belts are measured in mm.
Pulley – The new belt will be the full width of the pulley. Worn belts usually stretch in length and become narrow in the width.
Check Belt Routing
Belt routing, needless to say, is important. On some mowers, it’s possible to put a belt on backways, which makes the blades turn backward. Not much use for yard work.
If you can, make a diagram or take some pictures of the old belt in place. First, you’ll need to remove both plastic protection pulley covers, one on each side. Some mowers have a handy sticker showing the deck belt routing under the footrest.
That’s great advice, but what if your belt has snapped or derailed? Then you’ll need to check out the links below. Bear in mind, even if you don’t see your maker in the list, check the link out anyway, because lots of makes share the same decks.
Look at the pulley configuration to see if yours looks similar.
Belt Routing Links
The following link to Google belt routing pictures:
- Poulan Pro
- John Deere
- Cub Cadet
- Troy Bilt
- Yard Machines
Sticker – Check under the footrest of your mower, some models have a belt routing sticker. This sticker is on a Husqvarna tractor.
Check out the Amazon link below for deck bels.Amazon Lawnmower Deck Belts
What Belt Type?
Belts are belts, right! Well, No. The correct belt is crucial. An ill-fitting or wrong type belt will cause endless trouble. Throwing the belt, vibration, poor cutting, and collecting, and because the belt doesn’t fit correctly, it won’t last very long. I had one customer who fitted a belt that was so tight, it broke the end off the crankshaft, Ouch!!
There are many different types of belts, however, when it comes to lawn mowers, they are usually fitted with a standard V-type belt. Other belts used are AA belts, timed belts, poly V belts.
Sure you can fit a basic quality belt with a polyester cord but it’s going to wear out quickly, for durability you’ll want Kevlar, they cost more but last a lot longer. Some models will only work well with OEM belts, like John Deere and MTD.
I recommend fitting only OEM (Original Equipment Manufacturer). Fitting a belt can be a challenge. Reassembling and discovering what doesn’t work right, can be demoralizing.
V belts are so-called because of their cross-section shape (wider at the top than the bottom). They are used almost exclusively to drive power from lawn tractor engines to their transmissions. They are also used to drive deck blades. The V belt drives power from one side of the belt only.
They come in different heights/widths and are marked type A, B, C, D. The most common V belts used on mowers are the A and B types, and obviously, they come in a long list of lengths.
Each belt is marked by type and belt length, it may also have a part number, unfortunately, the markings usually wear off, making the id process difficult.
AA Hex Belts
The AA belt is a double-sided hexagonal belt, it is mostly used on tractor mower decks to drive the blades. The belts are unique as they have the ability to drive from either side of the belt.
AA Hex Type – This is a double-faced belt, it gives greater flexibility to deck design, as it allows both sides of the belt to drive. It’s used on higher-end tractor decks.
Toothed (Timed) Belts
A mower toothed or timed belt does two jobs, it transfers power and at the same time keeps the mower blades from hitting each other. The toothed belt is a very exact belt in that the teeth of the belt must match that of the mower cogs.
Timed Belt – This type of belt has become more popular in recent years. It’s fitted to mower decks with overlapping blades.
A Type -This is the most common type of lawn tractor belt, it’s used by many lawn tractor drive systems and most decks too.
The B-type belt is a heavy-duty A belt, it’s an older well-fed brother.
Check Belt Wear
Belts have a difficult job and can be the cause of various issues. Regular inspection will tell you if your belt is at the end of its life. Things to look for are flat-spotting, glazing, cracking, fraying, and contamination.
As you know a V belt should sit just proud of the pulley shoulder, if it’s a lot lower than the shoulder of the pulley, it’s worn out.
How Long Do They Last?
The life of a belt is hard to gauge, it really depends on how much grass you’re cutting and how heavy the workload is, but usually, we’re talking years. Typically a belt should be changed after 3-4 years, but we know this doesn’t happen.
A worn or damaged pulley can shorten the life of your belt. An engine or transmission oil leak can destroy the belt, you can try cleaning it, but it causes slip. A derailing belt can get twisted and damaged, a mower that throws belts regularly, probably has a worn or damaged pulley.
But the real killer of belts – tall heavy grass, jams the blades which cause flat spots on the belt. The flat spot will then cause a lot of vibration which in turn can throw the belt.
Belt damage is usually caused on the first cut of the season when the grass is heavy. So if your grass is tall and heavy, just take a little off on the first pass and make a second pass with the deck a notch lower. Yes, it’s twice the mowing, but it’s better for your mower and your lawn.
Flat Spot – Flat spotting is usually caused when the blade jams but the engine pulley keeps running. This has a grinding effect on the belt.
The flat spot will cause excessive vibration in the mowing deck. The fix – Replace the belt.
Blistering – This can happen when a belt gets old, the material is starting to break down. Your mower won’t cut or collect very well. Better to take care of it now, before it breaks.
Glazing – This belt has a shiny hard surface, not much good for traction. A belt like this will cause horrible vibration in the mowing deck.
Frayed – Wear and tear, this belt are at the end of its useful life.
Cracked – Natural wear and tear
Check Pulley Wear
A pulley’s job is to route the belt around the chassis of the mower or mowing deck and transfer power from the engine pulley to the driven pulleys. As a rough guide, pulleys usually wear out at the same time as a belt, so best to check them while you have the belt removed.
Tension and idler pulleys should move freely, be quiet when spun, and should feel smooth when turned. If they’re worn, now’s the time to take care of it, when a pulley bearing breaks it will likely damage the belt.
Spin To Test
You don’t need to remove them to check. Spin them while the belt is off, they should be smooth and quiet. Changing them now is easy.
Most pulleys come with the new bearings pressed in place, the exceptions are driven pulleys (Mandrel, engine, or transmission pulleys).
Pulleys come in all sizes, some metal, some plastic. Tensioners and idlers will have a bearing fitted and when it wears out, the whole pulley is replaced. Pulleys are usually broken into two main types, flat or V.
A flat pulley is not a driven pulley, it runs on the back of the belt which isn’t powered (unless it’s on a AA belt).
A V pulley can be driving, driven, tensioner or idler. A V pulley is described as a driven pulley if it’s connected directly to the output, such as a transmission or a blade Mandrel.
The driving pulley is the engine pulley, it’s the pulley supplying the power. Both the driven and driving pulleys are fixed to shafts using a key and key-way.
A tensioner pulley is part of a moving arm, which when operated applies tension to a belt. A tensioner pulley can be a flat or V pulley.
A stationary pulley is usually known as an idler, and its job is to route the belt around the chassis of the mower, or mower deck, they can be flat or V type.
Pulleys – Metal or plastic, V type or flat, driven or idle. So many choices.
V Pulley – This is a V pulley, the driving side of the belt is making contact with the pulley.
Flat Pulley – A flat pulley on a V belt set up is never a driven pulley. Its job is to change the direction of the belt and guide it to the next pulley.
Fitting A Belt
Fitting a cutting blade deck belt that is just worn is the easiest, as you can see the routing of the old belt and make a diagram or take pictures. Removing the old one also gives you an idea of how challenging fitting the new one will be.
As you know there are a few variations of deck belt setups, most mowers will have one belt to drive the blades that are either a tension-ed or a un tension-ed belt. The belt can be fitted to both of these types of setups without removing the deck from the mower.
The timed belt setup is a little more involved but not complicated. It has two belts as do so some of the larger John Deere mowers. They can be a challenge as they have many pulleys and you’ll need to remove the deck.
Likely you’ll have already removed the 2 plastic pulley protection covers, one on each side of the deck. Usually 2 or 3 screws on each side. They’re not there to protect the pulley, they’re there to protect us from catching body parts in the pulleys.
The latest generation mowers are far more challenging to access as the nice people in the health and safety dept. have been working nights and weekends to find new ways to challenge us.
Here’s a quick run-through of what we’ll be doing, but it’s all covered in steps below with pictures. With covers removed, start by removing the belt from the engine pulley. Often the engine pulley will have a belt guide, its job is to prevent the belt from derailing. Depending on the type of belt guide (if fitted), you may need to remove them first.
As you know some belts will be tensioned all the time. By tension-ed I mean the belt is tight around the engine pulley all the time.
The belt tensioner will allow for movement (it’s spring-loaded) so the easiest way to do this is by pulling the belt over the side of the engine pulley, and then with both hands turn the engine pulley until the belt falls off (Removing the spark plug makes turning the pulley easy).
With the belt off the engine pulley, it’s easy to guide it off the other pulleys. Check your old belt against your new belt, just to be sure.
The un-tensioned belt is simple to fit, and by un-tensioned I mean the belt is loose around the engine pulley until you engage the blades. The belt can usually be maneuvered around the guides without much trouble.
As with the tension-ed belt, remove the belt from the engine pulley first.
Reference your diagram or pictures of routing. Refitting the belt is identical except in reverse order, fitting the engine pulley last, by pulling the belt onto the side of the pulley and turning the engine over.
Most mowers have a simple deck belt setup, others like the one covered in this guide are more complex. The demo mower used here has two deck belts driving the mowing deck.
The first one is the main input belt and is powered directly by the engine. This belt is easy to replace and you don’t need to remove the deck, just some plastic covers.
The second belt is the output belt and it turns both blades in time. This allows the cutting blades to overlap and catch that annoying tuft of grass you sometimes see in the middle of your cutting strip.
To replace the output belt also known as a toothed belt or timed belt, we need to remove the deck from the mower. It’s not difficult to do and the whole job shouldn’t take more than an hour.
This guide covers a timed deck belt replacement procedure. Most timed decks will look something similar. It’s more complex than other deck belt setups but not difficult to work on. In this guide I will: remove the deck; inspect belts; replace the belt; tension the belt; set the timing of the blades.
No special tools are needed on this mission, but an impact gun would make life a lot easier. When you try to open bolts attached to pulleys, they tend to spin which is a real pain. Sure you can wedge it or grab it with grips, but you risk damaging the face of the pulley, and that in turn can damage your new belt. Nooo!
The impact makes small work of pulley bolts, and the better brands have a torque setting built-in which makes reassembly a gift. It’s a super tool to have in the trunk of your car, it makes changing a wheel look NASCAR slick. So treat yourself or drop a few hints before fathers day.
Chute – On this model mower the chute is fitted through the center of the mower. Not all mowers will have a chute like this. If your mower is side discharge, then you don’t have one.
Remove – As said earlier, you may not need to remove your deck to fit a belt. On this model mower, removing to fit the belt just makes life a little easier.
Pins – Locate the deck arms. Most mowers will have one at each of the four corners. The deck will be fixed to the deck arms with Cotter pins. (Some may have nuts and bolts) Remove the two front pins and the two rear pins.
Slide – The deck will be free to move forward, which allows you to remove the deck drive belt from the engine drive pulley. In some cases you may have a cable to remove, this depends on the blade engage type.
Push – With all pins removed and belt off, just push the deck sideways and it will pop off the arm bushing mounts. Apply some grease when refitting. Hey, I make that sound easy!
Inspect – Go ahead and turn the deck over to inspect the blades and blade boss (blade attachment). It’s likely that the blades are damaged, if they are, replace them.
Bent – If your blades are bent or worn, now is the time to take care of them. Replacement blades are easy to fit when the deck is off.
Boss – When your blade hits something hard, the blade boss pins are designed to break, this saves damaging more expensive components.
Replace – Check washers and bolts for damage. Blade bolts and washers are specially designed, so only use the original kit.
Remove – Remove plastic protection covers.
Pulley – Remove the drive belt tension assembly.
Key – Remove and store safely the key and the spacer ring.
Remove – Remove timed belt protective cover.
Belt – Remove the broken belt, and check for damage.
Loosen – Loosen both pulley bolts, the bolts are positioned on the underside of the pulley.
Remove – Remove the two guide bolts. Remember to fit these after fitting the belt, but tighten last.
Loosen – Loosen the two guide bolts.
Loosen – Loosen the lock-nut on the adjusting bolt, and thread it all the way out.
Push – Now push the tensioning assembly in all the way so it hits the adjusting bolt.
Set to 90° – Set the deck blades at 90° to each other.
Mark – Now mark the two main blade pulleys and the deck body as per the picture. Marking them with paint gives us a clear reference point when fitting the new belt.
Align – Keep your paint marks aligned and fit the belt to the tension assembly last.
Check – Check your paint marks again, it’s OK if you’re out by one or two teeth.
Routing – This is a typical timed belt routing.
Adjust – Adjust belt tension first, and tighten down lock-nut. Do not tighten pulley bolts or guide bolts at this stage.
Check – Check belt tension as you adjust. Leave some play in the belt, it should deflect by about 1/2″ at its longest run.
Check 90° – Check that the blades are at 90 degrees. If all is OK, go ahead and fit the guide bolts, but don’t tighten yet. Tighten the two pulley bolts, and now tighten the four guides.
Rebuild covers and fit deck in reverse order, that wasn’t so bad!
Why does my deck belt keep breaking? Common reasons, new belts keep breaking:
- Wrong belt routing
- Misaligned pulley
- Damaged pulley
- Belt guide missing
- Belt guide damaged
- Belt too loose
- Belt too tight
- Belt too narrow
- Wrong belt type
- Blade obstruction
Why does my mower deck shake? Mower decks commonly shake because:
- Engine not at the correct rpm
- Engine not running correctly
- Blades damaged
- Deck belt damaged or worn
- Spindle bearing damaged or broken
As you’re a MacGyver type, you’ll likely find the Riding mower troubleshooting and the Video repair library useful.
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.