How Do You Replace a CAT 303 Carrier Roller?

A CAT 303 that has started rubbing its own track is usually reporting one small part. The upper run of the belt has begun to sag, and instead of riding clear it touches the frame - which grinds the belt's inner surface and, once started, gets worse. The part responsible is the carrier roller: a top roller whose only job is to hold the upper run at the right height, and the cheapest component in the undercarriage to replace before it damages anything else.

Last updated: October 5, 2026

This guide covers what the carrier roller does, the signs it is failing, what to check before replacing it, the replacement sequence, how to confirm the right part, and how to keep the new one alive.

What the carrier roller does

On a tracked machine, the track runs around the drive sprocket at the back and the idler at the front, and the section between them - the upper run - is a length of flexible belt with nothing under it. Left alone it droops. Carrier rollers, also called top rollers or upper rollers, are mounted above the track frame between the sprocket and the idler, and they hold that upper run at a set height.

Their job is not to carry the machine's weight - that is the bottom rollers' work. It is to set the height and line of the upper run so the belt does not sag onto the frame and does not wander sideways. That is why a failed carrier roller shows up as a rubbing track rather than as an obvious mechanical failure, and why the damage usually appears on the belt before the operator notices the roller.

The signs it is failing

Four observations, all made with the machine raised and the track slackened. Turn the roller by hand: it should rotate smoothly with no flat spot, roughness or free play - a roller that will not turn has already stopped holding the belt up. Look for a weep: oil or grease streaked along the undercarriage after the machine has stood is the earliest warning a sealed roller gives, and it is the only symptom that arrives while the part is still usable. Look at the height of the upper run: with the belt correctly tensioned it should sit on the rollers, not sag between them or press against the frame. Look at the belt's inner surface for scuffing or polished patches, which is where a sagging run has been touching.

If the run is sagging but the rollers turn freely, the cause is usually tension rather than the rollers - and that is a different fix. Checking the tension first avoids replacing a part that was never the problem.

What to check before the new roller goes on

Two things, and both are cheap while the machine is already up. First, inspect the parts the roller works with: the idler face for scoring or uneven wear, the sprocket teeth for hooks, and the bottom rollers for flat spots and weeping seals. A worn neighbour will wear the new roller in the same pattern as the old one. Second, check the mounting area - clean it properly, because a roller seated on packed material never sits square, and look at the frame for distortion if the failure followed an impact.

Replacing it

The sequence on a compact machine is short. Park on firm, level ground and chock the machine so it cannot roll. Raise the side being worked and support it on stands rather than on the jack. Release the track tension through the grease cylinder - slowly, and with your hand clear of the fitting's discharge path - then support the belt and, if the roller cannot be reached with the track in place, break the master link and roll the belt clear of the roller position.

Unbolt the old carrier roller, keeping the mounting hardware together so it can be inspected - fasteners that have stretched are replaced, not reused. Fit the new roller, confirming it seats flat on the cleaned mounting face, and tighten the hardware to the manufacturer's torque figure. Refit the track, set tension to the figure in the machine's manual, measured where the manual specifies and on both sides, then run the machine briefly and watch the upper run: it should sit on the roller, not touch the frame. Re-check the mounting hardware after the first hours of work.

Confirming the right part

Carrier rollers are model-specific, so fitment comes down to four checks, all taken from the roller you are removing: the part number cross-referenced to your machine's model and serial number; the roller diameter and width, which set the height of the upper run; the bore or shaft diameter, measured with the hardware off; and the mounting arrangement, including how the roller is retained. The CAT part for this machine class is listed as carrier roller 2668794 for the Cat 303CCR.

Ask for the seal design and the bearing type as well, because those decide how long the replacement lasts in mud and dust - and add the warranty terms, stated as a period plus an operating-hours cap, whichever comes first. Where a machine is in for several parts, doing them in one job with the belt off costs less than repeating the access work; the wider carrier roller range is organised by machine for that comparison, and the same reasoning applies to parts as different as releasing track tension safely before any of the work begins.

Keeping the new one alive

Three habits decide whether a replacement reaches its design life. Keep the belt's tension to the manual's figure on both sides: an over-tensioned track loads the carrier rollers harder than any ground condition, and an under-tensioned one lets the belt slap and ride. Clean the undercarriage after muddy or abrasive work so grit is not held against the seals. And check the top rollers at each tension check rather than at the annual service - on a compact machine it takes a couple of minutes, and a weep caught early is the difference between one roller and a roller plus a belt.

This article is part of the carrier rollers guide, which covers what the top roller does, how to confirm fitment and how to read wear.

FAQs

What does a carrier roller do on a CAT 303?

It holds the upper run of the track at the right height between the drive sprocket and the front idler, keeping the belt off the undercarriage frame and holding its line as the machine travels.

How do I know when it needs replacing?

The track rubbing the frame at the top, a track that will not hold tension, roughness or a flat spot when the roller is turned by hand, and oil or grease weeping at the seal. The weep means replace now - a sealed roller cannot be re-greased.

Can I replace it without taking the track off?

On some compact machines the roller can be reached with the belt slackened and moved aside, which saves the job of breaking the master link. If it cannot be reached, release tension, support the belt, and roll it clear of the roller position.

Why did the old roller fail?

Usually the seal: grit or water gets past it, the bearing loses its lubrication, and the roller stops turning. Less often it is misalignment or tension, which is why checking the belt's line and tension before fitting the replacement matters.

Should I replace more than one at a time?

If the top rollers share the same hours and ground and one has failed, the rest are close behind, and doing them in one job - with the belt off and tension released - costs less than repeating the labour.