If you have spent any time searching for undercarriage parts for compact excavators recently, the number 320-4870 has almost certainly come up. It is the track roller specified by Caterpillar for the two most popular models in its small excavator line, the 305E2 and the 306E, and it has quietly become one of the most searched undercarriage part numbers in the 5-6 tonne mini excavator category. Owners, mechanics and rental fleets all arrive at the same page when they take a machine out of service to rebuild its running gear, which is exactly why this guide exists.
A track roller sits in the centre of the whole pushing and hauling argument that mini excavators win every day. It carries the loaded weight of the machine, keeps the track chain aligned and takes constant hammering on hard ground. When it fails, the machine stops earning. Below we explain what the 320-4870 actually is, which machines it fits, how to tell it apart from the look-alike parts around it, how to catch wear before it becomes a bill, and exactly what to check when you buy a replacement.
What is the CAT 320-4870 track roller?
The 320-4870 is a genuine Caterpillar track roller, part of the undercarriage running gear that supports a mini excavator on its track chain. On most compact excavators the undercarriage uses a set of bottom rollers that ride on the rail of the track chain and transfer the machine’s weight to the track shoes and then to the ground. The 320-4870 is one of those bottom track rollers, and it is machined, hardened and sealed to survive years of abrasive dust, water and impact.
Every track roller is essentially two parts in one. There is the hardened steel wheel with its precision-ground running surface, and there is the sealed bearing assembly at its core that allows the roller to turn smoothly as the track chain drives past it. The seal keeps grease in and grit out. If a seal fails, the bearing dries out, the roller begins to knock and the running surface starts to wear flat, which drags the whole undercarriage down with it.
Why 320-4870 is searched so heavily right now
There are three reasons this specific number has become a hot search. First, the 305E2 and 306E are two of the bestselling compact excavators in their class, so the installed base is enormous and growing. Second, undercarriage parts have a natural service life — the more machines are working, the more sets of rollers are being replaced. Third, and most importantly, the 305E2 and 306E share running gear that is also found on a family of similar machines, which means one part number addresses a very large fleet. When rental companies and municipal fleets start stocking the same roller for a whole group of machines, demand for that part number takes off.
Which machines does the CAT 320-4870 fit?
The 320-4870 track roller is most commonly associated with the Caterpillar 305E2 and the 306E mini hydraulic excavators, the two models it was original equipment on in the popular 5-6 tonne segment. Because compact excavator frames are often shared across closely related models, the same roller and its sister part 320-4871 also appear on a range of other machines in the family, as well as on a number of OEM-label compact excavators built around the same undercarriage platform.
It is important to understand the relationship between the two roller numbers. On the machines that use this running gear, there is normally a pair of bottom rollers per side, and the front and rear positions can carry different part numbers. The 320-4870 and 320-4871 are designed as the matched set for this platform, so when you rebuild an undercarriage you should confirm the position each roller sits in and order both numbers rather than assuming a single number covers every position.
The 305E2 and 306E platform
The 305.5E2, 306E2 and related models form one of the most successful compact excavator families that Caterpillar has produced. These small machines do the work that larger excavators cannot reach — urban utilities, drainage, landscaping, demolition in tight spaces and foundation work — and they are run by everyone from big contractors to one-machine owner-operators. The huge number of these machines working in the US, Europe, the Middle East and Asia is the reason their undercarriage parts have such a broad and continuous search demand.
Because these machines spend a large share of their working hours on hard surfaces such as concrete and asphalt, their track rollers take a beating that straight-earth machines never see. This makes the track roller one of the first undercarriage components to need attention and one of the most reordered parts in the whole machine, which is why service shops keep the 320-4870 in their parts cannon.
How a track roller works and why alignment matters
The job of the bottom track roller is to support the vertical load of the machine and hold the track chain down against the track frame as the sprocket drives it around. As the machine travels, each roller turns at the track speed and continuously redistributes the machine’s weight across the track shoes. The rollers also help keep the chain tracking centrally, stopping it from riding off the idler and the sprocket.
Because of that job, everything about the roller is about controlling friction. The running surface is hardened so it does not wear quickly against the rail, the bearing is preloaded to remove play, and the seal is designed to keep contamination out for thousands of hours. Any deviation — a flat spot, a loose bearing or a scored surface — puts extra drag and vibration into the chain and turns a minor maintenance item into a full running-gear rebuild.
Track roller versus carrier roller
It is worth distinguishing the 320-4870 from a carrier roller, because parts suppliers receive a surprising number of wrong orders. Bottom track rollers carry the machine’s weight on the ground side of the chain and are normally the larger, more heavily loaded components. Carrier rollers, by contrast, sit on the top of the track frame and simply support the upper return run of the chain. If a parts list or diagram does not make the position clear, verify which type the number refers to before you order, because they are not interchangeable.
Signs that a 320-4870 track roller needs replacing
- Flats or eccentric wear visible on the roller’s running surface when you look along it from the side.
- Oil or grease weeping from the roller seal, which tells you the seal has failed and dirt is getting in.
- A rumbling, scraping or grinding noise from the undercarriage while the machine tracks.
- Excessive wobble or play when the roller is lifted and shaken by hand.
- Visible scoring, pitting or rust patches on the hardened surface.
A healthy roller spins freely and smoothly with firm, even resistance, and it shows a clean, round, polished running surface. The moment you see flats, roughness, an oil mark on the seal or any play in the bearing, that roller is due for replacement. Worn rollers are not just a comfort concern — they accelerate wear on the track chain, the pins and the bushes, and they can eventually cause tracking problems that cost far more to fix.
How to inspect a track roller properly
Start by cleaning the running surface so you can see defects clearly. Then measure the roller diameter at several points around the circumference with a caliper, comparing the edges with the middle. A difference between the ends and the centre of more than a fraction tells you the roller has worn conical, which means the surface is not supporting the rail evenly. Check the seal area for any oil film, and rotate the roller by hand to feel for notchiness in the bearing.
When a roller is worn out of round, it is not worth repairing. The cheap option of refacing a worn roller rarely restores the case hardness or the original geometry, and a roller that has already worn once will wear again far more quickly. For a part this central to the machine’s reliability, a fresh matched roller is almost always the better investment.
Rebuilding an undercarriage: replace rollers as a set
If you are taking a mini excavator out of service to replace a track roller, think about the rest of the running gear at the same time. Undercarriage wear is usually distributed unevenly, but replacing a single badly worn roller while the remaining rollers on the same side are also worn creates an imbalance. New rollers with full diameter will carry proportionally more load while the older ones bed in, which shortens the life of the new parts.
The practical approach on a machine that has reached the roller-replacement stage is to inspect the idler, the sprocket and the chain in the same pass, and to replace the rollers on a side as a matched set where the budget allows. Over a full undercarriage cycle this almost always turns out to be cheaper than rationing parts one at a time.
Buying a replacement 320-4870: what to check
Track rollers are bought on trust, so shop with your eyes open. Check that the replacement matches the original in roller diameter, face width, bore size and bolt pattern so it fits the track frame without rework. Insist on the same case depth and hardness on the running surface, and confirm that the bearing, seal and any mounting hardware are included or available. Ask for the OEM cross-reference so you know exactly what you are getting.
A reputable supplier will be able to tell you the machines their 320-4870 replacement covers, show you the dimensional data and explain how their part is manufactured. If a price looks too good for a load-bearing part that has to keep a track chain aligned under a working excavator, treat the low price as a warning rather than a bargain.
Where to source the CAT 320-4870
Racer Machinery supplies the CAT 320-4870 track roller and the matching 320-4871, along with the wider set of undercarriage parts for the 305E2, 306E and related compact excavators. We provide genuine, cross-referenced and fully interchangeable rollers that match the OEM dimensions and hardness, and we carry the fast-moving mini excavator undercarriage numbers in stock so you are not held up waiting for parts.
We ship worldwide to owner-operators, service workshops and rental fleets, and because we handle the whole undercarriage family we can put together a complete set — rollers, idlers, sprockets, track chain and shoes — in a single order. That saves you from juggling multiple suppliers and guards against mismatched parts arriving from different directions.
Undercarriage maintenance tips for mini excavators
Small excavators live on hard ground more than their larger cousins, so a few habits stretch every undercarriage component. Keep the track frames clean of packed dirt that hides wear and holds moisture. Check roller seals during the daily walk-around. Maintain the correct track tension — too loose and the chain slaps, too tight and it eats rollers and bushes. Work on abrasive surfaces with reasonable care, and rotate the machine instead of dragging it whenever possible.
A few minutes of inspection on a regular schedule is incomparably cheaper than a mid-season undercarriage failure. The track roller is one of the easiest parts to check and one of the most important, which is why keeping an eye on the 320-4870 is such a high-value habit.
Reading part number 320-4870
Caterpillar part numbers are not random, and 320-4870 fits the familiar structure of a Cat number. The first group identifies the part as belonging to a family or platform, while the later digits narrow it down to a specific component variant. For the person buying parts, the practical value of understanding this is that two Caterpillar numbers that look superficially similar can be completely different components, so it pays to read the full number rather than the memorable part of it.
It is a common mistake to order from a partial or misread number, and undercarriage parts are among the most frequently misordered because several rollers in the same running gear use close but different numbers. Always confirm the complete 320-4870 against your machine’s parts book and the number stamped on the removed component, and never abbreviate a number on an order with the assumption that the supplier will ‘know what you mean’.
How a track roller wears and what determines its life
The life of the 320-4870 comes down to how it is worked and how it is looked after. Rollers wear fastest on hard, abrasive ground, under heavy side loading, and when track tension is wrong. On machines that work predominantly on concrete or asphalt, the running surface wears more quickly than on machines working in soft ground, because the hard surface accelerates the normal wear process and hammers the seal and bearing.
Moisture and mud also play their part. A roller that is packed with mud, or one whose seal has been damaged by spinning in deep mud, will fail much sooner than one kept clean and dry. Operating site conditions are therefore the single biggest variable in undercarriage life, and fleet owners who understand this schedule their inspections accordingly.
Wear measurement and service limits
If you want to be precise about when a roller needs attention, measure the running surface diameter in several places and compare it with the original specification. Undercarriage components have defined service limits, and a roller that has lost enough diameter to approach those limits should be scheduled for replacement rather than left until it fails. Recording measurements at each inspection builds a history that makes the replacement decision objective instead of guesswork.
Step-by-step: replacing a track roller on a mini excavator
Replacing the 320-4870 is a job most competent technicians can do on site with the right tooling. Start by supporting the machine securely and relieving the track tension so the chain can be handled safely. Raise the machine to lift the weight off the roller, then remove the bolts and mounting hardware that hold the roller to the track frame.
Once the old roller is free, clean the mounting pads and bore, inspect them for damage, and fit the new roller with fresh hardware torqued to spec. Before lowering the machine, rotate the new roller by hand to confirm it turns freely and smoothly, then re-tension the track and check the whole run for alignment. A clean fit and a correctly tensioned chain are the two things that determine whether the new roller gets a long life.
Replacing a pair or a full set
Mini excavator undercarriages usually carry a set of bottom rollers per side, and when one roller has worn out it is a strong signal that the other rollers on the same side have also been working. Replacing a single badly worn roller while the rest of the side is worn creates an uneven load distribution in the chain, with the new full-diameter roller carrying more than its share until the others bed in.
The cost-effective approach is to inspect every roller on the machine and to replace the ones that are worn as a matched set, ideally on both sides if they are at similar life stages. This protects the track chain, the idler and the sprocket, all of which wear faster when the rollers are not supporting the machine evenly.
Common undercarriage problems seen in compact machines
- Track chain running out of line when one roller is worn flatter than its partners.
- Oil leaks showing as dark wet marks under the track frame, telling you roller seals have failed.
- Uneven shoe wear caused by rollers that are no longer carrying the machine evenly.
- A thrown track, which is far more likely when rollers and idlers are badly worn.
- Excessive track noise during travel, which in most cases comes from worn running gear.
Almost every one of these problems can be traced back to the running gear being allowed to wear past its service limits. The roller is at the centre of it, because a healthy, true-running set of rollers keeps the whole chain and frame working as designed.
Genuine, aftermarket and replica rollers
When the 320-4870 is bought in the aftermarket you will see it offered as genuine, aftermarket cross-referenced, and replica parts. A genuine Caterpillar roller is the reference standard but carries a premium price. A quality aftermarket replacement is manufactured to match the OEM dimensions and hardness and offers reliable performance at a better price. A low-cost replica may be built to a looser spec, and on a load-bearing, wear-critical component like a track roller, that risk is usually not worth saving.
The key to a safe aftermarket purchase is insisting on dimensional data and hardness documentation. A reputable supplier will tell you the roller’s diameter, face width, bore and case hardness, and will explain the cross-reference to the OEM number. Buying on price alone, without those details, is how a cheap roller turns into a costly failure.
Frequently asked questions about the 320-4870
- Is the 320-4870 the same as a carrier roller? No — it is a bottom track roller that carries the machine’s working weight on the ground side of the chain.
- Does the 320-4870 fit more than the 305E2 and 306E? It is original equipment on those models and appears on related machines in the same compact excavator family; confirm your serial number.
- How many 320-4870 rollers does a machine have? It varies by model, so check the parts book for the number of bottom rollers on each side and which positions use 320-4870 versus 320-4871.
- Can I replace just one roller? You can, but replacing them as a matched set on a side protects the chain and the other components from uneven wear.
- How often should I inspect track rollers? At least at every regular service, and more often when the machine works on hard or abrasive ground.
Conclusion
The CAT 320-4870 track roller carries a surprisingly large share of responsibility for such a small component. It keeps the track chain aligned, supports the machine’s weight and takes years of abuse on hard ground, which is why it is one of the most searched undercarriage numbers in the compact excavator market. Knowing what it does, which machines it fits and how to catch wear early keeps your 305E2 or 306E working without surprise bills.
Whether you need a single roller, the matching 320-4871, or a complete undercarriage set for your mini excavator, Racer Machinery offers the parts, the cross-references and the shipping speed you can depend on. Call on us for a durable, correctly specified replacement that keeps your compact excavator tracking straight and earning.
