Excavator Undercarriage Parts: How to Choose the Right One

An excavator undercarriage is five wear parts, and the money is decided by the order you deal with them. Bottom rollers fail first, then the front idler, then the drive sprocket, and the rubber track is usually last - because it is protected until the parts around it stop doing their jobs. Buying the part that is actually due, confirmed by part number rather than by model name, is what keeps the machine working and stops the same repair coming back twice.

Last updated: October 6, 2026

This page is the doorway to what we publish about each of those components. If you already know which part you need, the seven category pages below take you straight to it; if you are working out what is wrong with the machine, start with the wear order and the fitment checks in the next two sections.

The five components, and what each one does

Rubber tracks provide ground contact, traction and shock absorption, and they are the most visible part - which is why they are blamed first and usually fail last. They are sized by width, pitch and link count, and the size code has to match the machine exactly. Start with the rubber tracks guide, or go straight to the rubber track range.

Track rollers - bottom rollers or lower rollers - carry the machine's weight on the lower run of the chain and keep it travelling in a straight line. They are the first component to wear and the one whose failure spreads fastest. Start with the track rollers guide, or go straight to the track roller range.

Carrier rollers - top rollers - run on the upper run of the chain and hold it up so it does not sag onto the frame. They are usually sold alongside the bottom rollers and are often replaced in the same job. Start with the carrier rollers guide, or go to the carrier roller range.

Idlers are the wheels the track runs around at the front of the undercarriage, and on some machines at the rear as well. They set the chain's line and carry the tensioning load, which makes them the component that decides whether the belt runs true. Start with the idlers guide, or go to the idler range.

Drive sprockets engage the links and turn drive torque into movement. A sprocket that has hooked wears the chain, and a stretched chain wears the next sprocket, which is why the two are usually judged together. Start with the drive sprockets guide, or go to the sprocket range.

Two further items complete the picture. Rubber pads and track shoes are what the machine actually runs on when the surface has to be protected or the ground is soft, and repair link kits are what let a damaged section of chain be joined without replacing it. Those are covered in the track shoes and rubber pads guide and the repair link kits guide, and stocked in the track shoe range and the repair link kit range.

The order the parts wear in

On a compact excavator working mixed ground, the sequence is consistent enough to plan around.

The bottom rollers go first. They carry the machine's weight continuously plus the axial loads of turning, and their seals are the first thing grit and water breach. The front idler follows, particularly on machines that pivot a lot, because it carries the tensioning load and takes the lateral forces of turning. The sprocket is next, its teeth peening and hooking as they mesh with the links - and once hooked, they start stretching the belt. The belt is usually last, because nothing wears it until the components around it have stopped doing their jobs.

That order is the reason the cheapest repair is almost always the earliest one. A weeping roller seal caught in a walk-round is one part; the same seal ignored is a roller, a section of chain and a sprocket. The same principle on a specific machine is set out in replacing tracks on a Bobcat, where the steel is checked while the belt is already off.

Confirming fitment

Fitment comes down to a small number of measurements, and all of them are taken from the part you are removing rather than from a catalogue drawing.

For a roller: running-surface diameter and width, bore or shaft diameter with the hardware off, flange type, and the mounting arrangement. For an idler: wheel diameter, running-surface width, flange height, bore and mounting. For a track: width, pitch and link count, where the pitch has to match the drive sprocket. For a sprocket: tooth count, pitch, bore and mounting geometry.

Then the part number, cross-referenced against the machine's model and serial number. Undercarriage specification changes across build series, which is why the serial number is what identifies the part - and why a listing that names a model family rather than a machine has not finished the job. The difference between a bottom roller and a carrier roller, for instance, is a fitment question before it is a description, as how track rollers and carrier rollers differ sets out.

One decision worth making early

Rubber tracks or steel: the question comes up on almost every machine, and the answer is about ground rather than about price. Rubber spreads load, grips on finished surfaces and damps vibration, which is what turf, paving and urban work need; steel and over-the-tire tracks stand up to rock, demolition debris and continuous abrasion, where a rubber belt would be cut to pieces. Where the machine works both, the honest answer is that the belt is a consumable chosen for the majority of the work - and how rubber and steel over-the-tire tracks compare walks through the trade-off.

What to ask a supplier

Four items, and they are the same whichever component you are buying. The part-number cross-reference for your model and serial number, in writing. The specification as supplied - seal and bearing design, material and heat treatment of the running surface on a roller or idler; cord construction, tread depth and the compound's published temperature band on a track. The warranty terms as a period plus an operating-hours cap, whichever comes first, with freight responsibility named. And the stock position with a delivery date, because a machine standing still is the expensive part of any undercarriage job.

Two more questions are worth asking on wear parts, and they cost nothing. What happens if the part arrives wrong, and how quickly can a replacement be shipped - the answer tells you whether the cross-reference you were given was real. And whether the supplier can quote the whole assembly, since undercarriage components wear each other and a new part running beside seized ones wears into the same pattern.

Maintenance that decides how long they last

Three habits do most of the work. Keep the tension right: a belt run slack climbs the sprocket or comes off, and one run tight loads every roller and the idler bearing continuously, which is why the manual's figure is the only correct setting. Clean the undercarriage after work in mud, clay or abrasive spoil, because packed material holds the track off its seat and carries grit into seals. And inspect the steel whenever the belt is off, turning each roller by hand and looking for weeping seals, play on the shaft and hooks on the sprocket teeth.

Two records make the next purchase measurable: the part numbers fitted to each machine, with the hours at which they went on, and the tension figure each machine is set to. Together they replace a seller's claim with your own history - and they are the first thing a supplier will ask for if a part fails early. Two related questions that come up often on this subject are whether tracks are directional and what actually extends track life.

Where to start, by part

If you already know what is due, go straight to the guide for that component: rubber tracks, track rollers, carrier rollers, idlers, drive sprockets, track shoes and pads or repair link kits. Each guide explains what the part does, how to confirm fitment and how to buy it; the ranges behind them are rubber tracks, track rollers, carrier rollers, idlers, sprockets, track shoes and pads and repair link kits. Each range lists the machines and part numbers a component covers, which is the fastest way to see whether your number is one of them.

If you are not sure what is wrong yet, start from the inspection rather than the parts list: raise the machine on level ground, release the tension, and work along the undercarriage one component at a time. The part that is due will usually be obvious - a weeping seal, play on a shaft, hooks on the teeth, a groove in the running surface - and ordering from that observation is what stops a machine returning to the same failure twice.

What changes with machine size

The same five components appear on a 1.7 tonne mini and on a 30 tonne machine, but the balance between them changes, and that changes what is worth keeping in stock.

On a compact excavator, the belt and the front idler do most of the work. The belts are narrow - from about 230 mm to 350 mm - the track frame is short, and machines that pivot in tight spaces load the idler and the rollers at the front of the frame hardest. The consumables are the belt, the idler and the bottom rollers, and the sprocket usually follows the belt rather than leading it.

On a compact track loader, the belts are wider - roughly 320 mm to 450 mm - and the load per roller is higher because the machine carries a bucket or a fork on a short frame. Rollers and sprockets wear faster relative to the belt than on an excavator, and tension is the setting that decides whether the belt or the steel fails first.

On a dozer or a large excavator, the undercarriage is a set of steel components and the "belt" question becomes a chain, shoe and roller question. The discipline is identical - part number against serial number, then the dimensions off the part being replaced - but the parts list is longer, and the penalty for getting a flange type or a mounting pattern wrong is measured in days of downtime rather than hours.

FAQs

How do I know which undercarriage part needs replacing?

Work along the assembly with the machine raised and the track slack: turn each roller by hand, look for weeping seals and play on shafts, check the sprocket teeth for hooks and the idler face for a groove, then the belt for cuts and exposed cord. The order they fail in is rollers first, then idler, then sprocket, then the belt.

How do I confirm a part fits my machine?

By the part number cross-referenced against your model and serial number, then confirmed against the dimensions of the part you are removing - bore, diameter, flange and mounting for a roller or idler; width, pitch and link count for a track.

Are aftermarket undercarriage parts suitable for excavators?

On wear parts, a documented aftermarket component does the same job, and the price difference is real money. What decides it is the specification, the part-number cross-reference and the warranty terms - not the badge on the part.

Should I replace rollers and tracks at the same time?

Where they share the same hours and the same ground, yes. The access work is the same, and a new belt running on worn rollers wears into the pattern the old set already had. Ordering the parts in one job is cheaper than taking the track off twice.

How do I keep an undercarriage alive in Canadian conditions?

Keep the tension at the manual's figure, clean packed mud and abrasive spoil out of the frame, and inspect the steel whenever the belt is off. In winter, check the compound's published temperature band on any track you buy, because abrasion resistance and cold flexibility are traded against each other in the rubber.