Sprocket Wear and Replacement: Teeth, Pitch and How to Swap One
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On a new drive sprocket the tips of the teeth are roughly 10 to 15 mm thick; on a fully worn one, about 1 mm. That is the rule of thumb from Janssen Track, a European undercarriage supplier (Janssen Track, wear on undercarriage sprockets). It is a guide, not the limit for your machine, but it shows how little steel separates a sprocket that still drives from one that is chewing up the chain. This page is a decision path for buyers and owners. It answers four questions in order: is this sprocket finished, do you replace it alone or with the tracks, one-piece or segmented, and which sprocket exactly. It sits inside the Drive Sprockets guide.
(Last modified date: October 27, 2026)
The decision path in one table
Start here, then read the section that matches your row.
| What you found | Decision | Section |
|---|---|---|
| Teeth thin, pointed or hooked, or cracked and broken | The sprocket is finished; plan the replacement | Decision 1 |
| Sprocket worn, chain or rubber track new or about to be changed | Replace the sprocket with the new tracks, not after them | Decision 2 |
| Sprocket and chain both worn | Replace them as a set | Decision 2 |
| Sprocket looks sound but the track jumps | Check tension, pitch and the idler before buying anything | Decision 2 |
| Large machine where you only want to renew the teeth | Consider segmented sprockets | Decision 3 |
| Ready to order | Match the part number, teeth, holes and bolts | Decision 4 |
Decision 1: is this sprocket finished?
A drive sprocket wears in one direction and does not come back. The teeth thin, then hook, and the chain starts running on a profile that no longer matches its pitch. Janssen's rule of thumb adds a second check besides thickness: tooth height. If the teeth have become too low, the chain or rubber track is likely not to engage properly and will slip over the sprocket, wearing the bushings. Cracks are the other end of the scale; repeated impact causes metal fatigue, which shows up first as surface cracks and later as pieces breaking off.
How to inspect and measure it
- Clean the teeth and look at the profile. Thin, pointed or hooked teeth are the warning. Hooking and the point of no return, shark-tooth diagnosis and identifying pointed teeth show what each stage looks like, and wavy tooth profiles covers the uneven case.
- Measure tooth thickness. Use calipers on the tips, and compare with the figure for a new sprocket on your machine. Measuring tooth thickness with calipers and measuring tooth wear are the field checks.
- Check tooth height and look for cracks. Low teeth and any crack at the root or tip mean replacement, not monitoring.
- Look at what the sprocket is driving. What tooth wear says about chain stretch connects the two, and wear limits and when to replace sets the schedule.
- Write the numbers down and use the wear limit in your machine's service manual to make the call. Limits differ by make and model.
When the sprocket is the problem, the knock-on damage is in the rest of the drive path: accelerated bushing wear, top roller failure and damage to the links. A worn idler feeds back into the sprocket as well, as how idler wear damages sprockets and chains explains. Rollers belong in the same inspection; track roller wear and replacement covers that side.
What wears a sprocket out early
Most early failures trace back to something around the sprocket rather than the sprocket itself. A worn or mismatched chain or track puts the load on the wrong part of the tooth. A badly adjusted track, in either direction, does the same. Debris packed into the drive wears the tooth faces from the outside and a misaligned sprocket loads the teeth unevenly. Sprocket alignment and sprocket maintenance cover the habits that slow it down. Fixing the cause matters as much as replacing the part; a new sprocket on a track that is still running too tight or too loose tends to wear the same way.
Plan the replacement, do not wait for it
The cheapest sprocket job is the one you schedule. Record tooth thickness at each service so you can see the trend, order the replacement when the figures approach your manual's limit, and book the work for a quiet week. An emergency swap on a sprocket that has already hooked usually means a worn chain or track as well, and that turns a one-part job into a bigger one.
Decision 2: sprocket alone, or with the tracks?
This is where most of the money is saved or lost. A sprocket and the chain or rubber track that runs on it wear together, so a new part on one side of a worn pair wears out fast. The table sets out the usual call for each pairing.
| Situation | Usual call | Read more |
|---|---|---|
| Worn sprocket, new chain going on | Replace the sprocket first; a new chain on worn teeth wears quickly | Pairing new chains with worn sprockets |
| New rubber tracks going on | Change the sprocket with them | Why change sprockets with new rubber tracks |
| Sprocket and chain both worn | Replace both together | Is replacing sprockets and chains together cheaper? |
| Sprocket looks fine but the track jumps | Check tension, pitch and idler wear first | Why pitch diameter matters when the track jumps |
Pitch is the key figure behind all of these. On a steel chain it is the distance from the centre of one pin to the centre of the next, and a sprocket has to match it. What the pitch of a sprocket is, pitch explained and pitch diameter take it further, and whether pitch is critical answers the obvious follow-up. The effects of mixing a worn and a new part are also covered in how a worn sprocket raises final drive temperature and debris inside the drive. If you are changing rubber tracks as well, the rubber tracks guide covers the track side of the job.
Decision 3: one-piece or segmented?
A one-piece sprocket is replaced whole. A segmented sprocket has bolt-on tooth segments, so you can renew the teeth without replacing the hub. Segments are usually replaced as a set, because the others have worn to the same point. One-piece versus segmented and the dozer version weigh the two, replacing segments shows the job, and how a split sprocket works explains the design. Check which type your machine uses before you search, because the two are not interchangeable on the same hub.
Decision 4: which sprocket exactly?
Match four things before you order: the machine details, the existing part number, the tooth count and the mounting. Our contact page is direct about it: a model name alone does not confirm fitment, and search results are not a fitment confirmation, so confirm fitment before ordering.
Why the type of machine matters
Sprocket design varies with the machine's type, size and brand. According to Fortis HD, a specialist in compact-equipment undercarriage, the position of the sprocket changes how much of the track wraps around it, which is why compact track loader sprockets, which sit higher, have beefier teeth than mini excavator ones, and some compact track loaders use a positive-drive design in which pins and bushings replace the teeth (Fortis HD, sprockets: purpose, types and wear signs). In practice that means a sprocket from a different type of machine is not a substitute even when the bolt circle looks close. Always match by machine and part number, not by appearance.
| What to record | How to get it |
|---|---|
| Machine make, full model and serial number | Copy them from the machine plate. |
| Existing sprocket part number | Read it from the old sprocket, if it is legible. |
| Tooth count | Count the teeth on the old sprocket. |
| Mounting holes and bolts | Count the bolt holes and check the bolt thread size. |
| Chain or track type and pitch | Steel chain or rubber track; measure pin centre to pin centre on a steel chain, or ask for help with a rubber track. |
Reading the codes on a sprocket listing
Our sprocket listings carry codes such as 21T/9H or 17T/8H. Coding conventions vary by supplier, so treat the figures on a listing as a shorthand to compare against your old sprocket, and confirm the match with the part number or your supplier before you order. The same codes are used to group sprockets: 21T/9H and 17T/8H are examples of collections built around them. If your old sprocket has a different tooth or hole count from the listing, it is not the one you need.
| Listing | Reference part number | Listed machines (first entries) | Listing code |
|---|---|---|---|
| Sprocket 1404022 | 140-4022 | Caterpillar 302.5, 302.5C | 21T/9H, M12 |
| Sprocket 3860584 | 386-0584 | Caterpillar 302.7D, 302.4D | 21T/9H |
| Sprocket 7196807 | 7196807 | Bobcat T740, T750, T630, T770 | 17T/8H, M20 |
| Sprocket T254141 | T254141 | John Deere 319D, 323D, 323E, 329D | 17T/10BH |
Each of these listings, checked on October 27, 2026, states a 12 Month Pro Rated Limited Warranty. Browse the full sprocket collection by machine or part number. If your old number is gone or you are unsure, send the make, full model, serial number and photos through the fitment request on the contact page.
Doing the swap
The service manual decides torque values and tension release. The outline is the same on most machines: make the machine safe, release track tension as the manual directs, support the track, remove the sprocket bolts, clean the mating face, fit the new sprocket and tighten the bolts evenly to the specified torque, then re-tension and check alignment. Whether the track has to come off depends on the machine; can you replace a drive sprocket without removing the track explains when it does not. For the walk-through, installation best practice and the complete installation guide are the references, with alignment and maintenance for afterwards. Machine-specific versions: the Cat D7 and the Kubota KX series.
While the track is slack, look at the rest of the undercarriage too. The excavator undercarriage parts guide shows how the components work together.
FAQ
How do I know a sprocket is worn out?
Thin, pointed or hooked teeth, low teeth, or any crack. Measure tooth thickness against your manual's limit.
Can I replace just one tooth?
No, not on a one-piece sprocket. On a segmented sprocket you replace the segments, usually as a set.
Does a worn sprocket stretch the chain?
Not literally, but it wears the links and bushings so the chain behaves as if it had stretched.
Should I change the sprocket with new tracks?
Yes, as a rule. A new track on worn teeth wears out quickly.
What do 21T/9H and M12 mean on a listing?
Coding conventions vary by supplier. Compare the figures on the listing with your old sprocket, and confirm with the part number or your supplier before you order.
Can I use a sprocket from a different type of machine?
No. Tooth size and design vary with machine type, size and brand, so match by machine and part number rather than by how the sprocket looks.
Is my machine model enough to pick a sprocket?
No. A model name alone does not confirm fitment. Add the serial number, the part number and the tooth and hole counts.
Ready to order? Start with the AFTparts sprocket collection, or send your machine details to the fitment request form and we will review them before you buy.
