Where to Buy Replacement Idlers for Komatsu PC300?

An idler usually announces itself before it fails: the machine starts tracking to one side, the track runs slack no matter how much you tension it, or there is a growl at walking pace that you can feel through the seat. Left alone, a failing idler takes the track with it - and a track is the most expensive part of the undercarriage, so the idler is the cheapest thing on the machine to replace early.

Last updated: October 5, 2026

The idler above is listed for a smaller Komatsu class than a PC300, which is the point worth taking from the photograph: idlers are sold by part number against a machine, not by the model family on the decal.

What the front idler does on a PC300

The front idler is the wheel the track runs around at the front of the undercarriage. It sets the track's line and it carries the tensioning load, which is why it is one of the two components - with the sprocket - that decides whether a track runs true. A healthy idler holds the belt aligned and absorbs shock loads; a worn one lets the track wander, and the misalignment then wears the track's shoulder, the rollers and the sprocket, in that order.

A idler is a guide wheel with a bearing in the middle, and everything that matters about it is in the bearing and the seal: how the bearing is protected, how it is lubricated and how the load is carried from the wheel into the frame.

The three choices that actually differ

Two idlers that fit the same machine can differ in three ways, and each one is worth asking about before you buy.

Bolt-on or welded. A bolt-on idler can be replaced without disturbing the frame; a welded assembly is integral to the frame and is a larger job. Which one your machine uses is fixed at build, so this decides how much labour the replacement involves. Sealed or unsealed bearings. A sealed bearing keeps its lubrication for life and is not re-greased in service; an unsealed design allows maintenance access at the cost of needing it. On a machine working in mud and water, the sealed arrangement is the one that lasts. Cast or fabricated wheel. A cast wheel carries load with less distortion; a fabricated one is lighter and usually cheaper. For a machine that spends its days on rock or demolition debris, the mass is worth having.

Whichever you choose, the part has to be the right one for your build series. Komatsu changed idler part numbers across the PC300 range, so a listing titled "PC300" is a starting point rather than a fit - and the same caution applies to the guide plates that hold the idler in the frame, which are a separate part and wear at the same time.

Three measurements close the fitment question on any idler: the diameter of the wheel and the width of its face, the bore or shaft diameter it mounts on, and the mounting arrangement - the bracket, the bolt pattern and how the assembly is retained in the frame. Take them from the idler you are removing rather than from a catalogue drawing, because a machine that has been rebuilt with a different frame over its life will not match the book.

Reading the idler you are replacing

The condition of the old idler tells you whether replacing it is the whole fix, or whether something around it caused the failure.

What you find	What it usually means
Uneven wear across the face	The track is not running centred - tension, alignment or a worn roller flange
A growl or rumble under load	Bearing damage, often from water or grit past the seal
A weep of oil or grease	Seal failure; the bearing is losing its lubrication and cannot be re-greased in service
Cracks in the wheel or the mounting	Impact or fatigue - replace rather than repair, and check the frame for distortion
A track that will not hold tension	A worn or leaking tensioner as well as the idler, so inspect both before ordering

The pattern matters more than the individual symptom. An idler wearing evenly across its face has lived a normal life and simply reached its end; one worn on a single side is reporting a problem that a new idler will inherit, which makes the alignment check the important part of the job.

When the idler needs to go

Check these during the same walk-round as the rest of the undercarriage. Noise under load, a track that will not hold tension, cracks in the wheel or the mounting, bearing play felt with a bar under the wheel, and an idler face worn unevenly across its width are all reasons to plan a replacement. A weeping seal is the one to act on immediately, in the same way as on a roller: a sealed bearing that has lost its lubrication is already failing.

The parts either side of the idler matter as much as its own condition. A worn sprocket or track rollers will take the new idler out the same way they took the old one, so inspect the whole assembly before ordering - the idler range sits alongside rollers and sprockets for exactly that reason.

Replacing an idler on a PC300

The sequence is the same as any front idler replacement on this class of machine. Park on level ground and chock the machine; lift so the undercarriage is clear; relieve track tension through the grease cylinder and support the track before you break the link; remove the track from the front of the undercarriage; then remove and refit the idler assembly, following the manufacturer's torque figures and using the correct lifting points - an idler and its bracket are heavier than they look.

Two points on that sequence cause most of the trouble. The first is supporting the track before the link opens: a track on a raised machine is heavy, it moves when the tension comes off, and the blocking is what keeps it away from the frame and from anyone working underneath. The second is where the load is taken. Work on a firm, level surface with the machine chocked and the side being worked supported on stands rather than on the jack alone, because the jack is for lifting, not for holding a machine while hands are inside the undercarriage.

Refit the track, set tension to the figure in the manual, measured where the manual says to measure it, and then check the other side. Idler adjustment and track alignment is the step that decides whether the new part lasts, and it is the step most often skipped when a machine is under pressure to get back to work.

What to confirm before you buy

Two things decide whether an idler purchase is straightforward: the part number, cross-referenced to your machine's build series and serial number, and the specification of the wheel and bearing as supplied. Ask for both in writing, along with warranty terms stated as a period plus an operating-hours cap, and the stock position - because a machine waiting for an idler is the most expensive part of the job.

Where a repair turns into more than one part, it is worth deciding between a repair kit and individual parts before ordering, since an undercarriage that has taken one failure usually has a second one close behind it.

This article is part of the idlers guide, which covers what the front idler does, how to confirm the right wheel and how to read wear.

FAQs

What signs indicate a worn idler on a Komatsu PC300?

Noise under load, a track that will not hold tension, cracks in the wheel or mounting, visible bearing play, and an idler face worn unevenly across its width. Any of those mean the idler is no longer holding the track's line properly, and the wear is being passed on to the track and rollers.

How do I confirm an idler fits my PC300?

Cross-reference the part number against your machine's build series and serial number, not against the model name, because Komatsu changed idler part numbers across the PC300 range. Ask the supplier to put that cross-reference in writing before you order.

Can I replace an idler myself?

Yes, with the right lifting equipment and the machine properly supported, because the work is mostly about relieving track tension, supporting the track and following the torque figures. The two risks are lifting something heavier than it looks, and re-tensioning the track without checking alignment - both are avoidable with the manual in hand.

Sealed or unsealed bearing - which should I choose?

Match the design to the conditions. A sealed bearing keeps its lubrication for its service life and suits machines working in mud, water and abrasive dust; an unsealed design allows maintenance access but needs it. Ask which one is being supplied rather than assuming from the price.

What maintenance extends idler life?

Correct track tension, regular inspection of the seal and the wheel face, cleaning the undercarriage after abrasive work, and replacing the parts around the idler - rollers, sprocket and guide plates - before their wear reaches the idler itself.