A rubber part with no legible number is still a part you can identify. Four measurements do most of the work: outer diameter, bore or spline, height, and bolt-hole spacing. Send those with the machine model and serial range, and a supplier who knows the range can usually tell you what fits before you pay.
You have the old part on the bench. The number is worn off, or it was never stamped on, and searching the machine name returns a wall of mounts that all look the same. Order one at random and you lose a week, plus the return.
There is a way out of that loop, and it is not a better search term. It is measuring the part you already have.
Key Takeaways
- Every part family is identified by the same four numbers: outer diameter, bore or spline, height, and bolt-hole spacing. Measure all four, because three right and one wrong is still a wrong part.
- Spline count outranks every other coupling measurement. An 11-spline shaft will not accept a 13-spline coupling, and the two look nearly identical on a bench.
- A photo with a ruler lying in the same plane as the part removes the millimetre-versus-inch argument before it starts.
- Size codes describe the part, not the machine. One coupling or cushion often fits many models, which is why the same part appears under a dozen machine names.
- The machine model alone is not enough. Production changes mid-series, which is why suppliers ask for the serial range as well.
Why the Part You Want Is Unsearchable
Search fails on this kind of part for a structural reason, not because you used the wrong words.
Three naming systems are in play at once. The machine manufacturer has its own part name and number. The aftermarket seller lists the part by its size code. And you, standing in the yard, know it as whatever your crew calls it. None of those three vocabularies overlap.
A buyer on HeavyEquipmentForums put the problem plainly: "you can't search Luagandong 3 ton blade ram and find it. You gotta measure the ram and search 1.5" bore 30" stroke..."
That is the whole method in one sentence. When the name fails, the measurement works, because measurements are the one vocabulary that both sides already share.
So stop trying to name the part. Describe its dimensions.
The Four Numbers That Identify Almost Any Rubber Part
Every bonded-rubber part on an excavator, whether it is a mount, a coupling or a cushion, is pinned down by the same set of dimensions.
| Measurement | What it is | Why a wrong value kills the fit |
|---|---|---|
| Outer diameter or footprint | The overall size of the rubber body or the steel plate | Too large fouls the surrounding casting; too small leaves the mount loose in its seat |
| Bore or spline | The hole through the centre, or the internal teeth | Wrong bore will not go on; wrong spline count goes on and strips under load |
| Height or thickness | Distance from mounting face to mounting face | Changes where the engine or cab sits, which loads whatever it is bolted to |
| Bolt-hole spacing | Centre-to-centre distance between mounting holes or studs | Off by a few millimetres and it simply will not bolt up |
Add two pieces of context and the picture is usually complete. The machine model tells the supplier which family to look in. The serial range tells them which production variant of that model you have.
Measure in millimetres and write the numbers down before you clean the part. Grease and rust hide the reference edges you need.
How to measure without a caliper
A vernier caliper is the right tool and most workshops have one. If you do not, a steel rule and two straight edges will get you close enough for identification, provided you measure across the widest point and keep the rule square to the part.
For a bore, measure the opening in two directions at right angles and average the two. A worn rubber bore is rarely perfectly round, and the diameter you report should be the one the part was made to, not the one wear has left behind.
When the part is still on the machine
Some mounts and cushions are easier to measure in place than to remove, particularly when the surrounding pipework has to come off first. Measure what you can reach, then say plainly which numbers you could not take. A supplier who knows the family can often identify the part from the mounting pattern and the machine model with one measurement missing.
Do not guess a value you could not reach. A wrong number is worse than a missing one, because it sends the search down a false branch and it arrives looking authoritative.
Engine Mounts: Measure Before You Match a Number
An engine mount is not a single piece of rubber. It is a rubber block bonded between steel plates, usually with a stud on one face and a bolt hole or a second stud on the other. That construction is why a mount that looks right can still be wrong.
A bonded engine cushion: measure the rubber body and the steel hardware separately.
Measure it in two parts. First the rubber body: length, width and height. Then the hardware: stud diameter and thread, stud length, and the centre-to-centre spacing of the fixing points. Note whether the mount is stud-to-stud or stud-to-plate, because the two are not interchangeable.
Here is the part people miss. Two mounts can share a footprint and differ in stud length or rubber height. Bolt the wrong one in and the engine sits at a different height than the manufacturer intended, which changes the angle on the coupling that drives the hydraulic pump. You have not just fitted a bad mount; you have started a second failure.
If the machine also shakes at idle, that is a separate diagnosis with its own checks, and the engine mount range lists what we hold by machine.
Pump Couplings: Spline Count Comes Before Everything
For a coupling, one measurement outranks the rest. Count the internal teeth or splines in the bore, and count them twice.
The spline bore is the measurement that decides whether a coupling will drive at all.
A buyer on HeavyEquipmentForums described the trap exactly: "The old pump is an 11 spline drive shaft... Everything I am finding for a new coupler is for a 13 spline drive shaft."
Eleven and thirteen splines are close enough in appearance that people order the wrong one from a photograph. Then either it will not slide onto the shaft, or it does and the teeth fail early because they were never fully engaged.
After the spline count, record the spline major diameter, the overall outside diameter of the coupling, and its height. If the coupling is the bolted-flange type rather than a splined hub, measure the bolt circle instead of the spline.
If your machine clunks when you shut it down, that is the classic sign this part has failed, and the pump coupling range is listed by size with the element type shown.
Radiator Hoses: Two Diameters and a Shape
A hose is the one part on this list where a single diameter is not enough, because the two ends are frequently different sizes.
Measure each end separately. Upper and lower hoses rarely share an internal diameter.
Measure the internal diameter at each end separately, then the overall length along the centreline of the hose, not the straight-line distance between the two openings. Finally, describe the shape: how many bends, roughly what angle, and which way they face.
The shape is not cosmetic. A hose with the correct diameters and the wrong bends will sit against a neighbouring component and wear through, or kink and restrict flow. That is the mechanical reason a length of straight hose cut to fit is a false economy, and it is why the correct part is moulded rather than adapted.
If you already know the number, our radiator hose range is grouped by brand and the OEM numbers are listed with each part.
Roller and Cab Rubbers: Diameter, Bore, Height, Bolt Centres
These are the simplest parts to measure and the easiest to get wrong, because they are often sold by a size code that hides the dimensions.
Roller and cab cushions are sold by size code. Measure anyway, and confirm the code matches.
Measure the outer diameter, the bolt-hole or stud diameter, the height, and the centre-to-centre bolt spacing. For a round cushion, note that the diameter is the governing figure; for a square or rectangular one, both dimensions matter.
The trap with this family is assuming that a part sold as, say, a 165 round is the same across every machine that uses it. Measure yours and confirm. Worn rubber distorts, so take the measurement across the least damaged part of the cushion and say so when you send it.
Both families carry their own diagnosis once fitted, and the cab mount range and roller shock rubber range are listed with the size codes we hold.
How to Photograph a Part So the Supplier Ships the Right One
Measurements get you most of the way. A photograph closes the rest of the gap, but only if it is taken in a way that carries scale.
The buyer checklist that circulates on r/heavyequipment asks for exactly this: "machine model plus serial range," an "OEM number, part number, or casting/tag photo," and "old part photos and key measurements."
Four shots do the job:
- Flat, square-on, with a ruler in the same plane as the part. Not behind it, not propped at an angle. If the ruler sits in a different plane, the photo cannot be read for scale.
- The bore, lit from the side. Shine a light into the bore at an angle so the splines or the hole edge cast a shadow. A head-on flash fills the bore and hides the teeth.
- Any number, tag or casting mark. Even a partial number narrows the search faster than any measurement.
- The mounting face. This shows whether the fixing is a stud, a bolt hole, or a threaded plate.
Take the photos before you throw the old part away. That sounds obvious until the machine is back together and the part is in the skip.
What a Size Code Actually Tells You
Once you have measurements, a size code becomes useful rather than mysterious.
Coupling codes, cushion diameters and mount codes describe the part, not the machine. That is why the same coupling turns up under several model names: machines that share a pump share a coupling. The code is a lookup key into the supplier's list, and it only works after you have confirmed your own dimensions match it.
The practical sequence is: measure the old part, then look for a code that matches those measurements, then check the code against your machine model and serial range. Any listing that asks you to skip the first step is asking you to guess.
When You Have a Number, Read It Off the Part
If a number survives on the old part, use it, but read it off the part rather than off a listing.
Numbers on rubber parts hide in predictable places: moulded into the rubber face, stamped on the steel plate, or on a tag that may have corroded. Clean the part and look under a light before you conclude there is no number. A partly legible number is still a strong lead.
Then check it against a reference you trust. Our excavator parts catalogue maps OEM numbers to machine fitment, including verified numbers like 20Y-03-42250 for the Komatsu PC200-8 lower hose and 135-5680 for the CAT 307B. If your number is there with a match, you are done. If it is not, send the measurements instead.
Frequently Asked Questions
How do I identify an excavator rubber part with no part number?
Measure four things: outer diameter or footprint, bore or spline, height, and bolt-hole spacing. Add the machine model and serial range, photograph the part with a ruler in the same plane, and send all of it to a supplier who knows the range.
What is the most important measurement on a hydraulic pump coupling?
The spline count. If the coupling has internal teeth, count them and confirm the number before anything else. An 11-spline coupling will not drive a 13-spline shaft, and the two are hard to tell apart in a photo.
Can I measure an excavator mount with a tape measure instead of a caliper?
For identification, usually yes, if you measure across the widest point and keep the tape square to the part. A caliper is better for bores and stud diameters. Whichever you use, report the unit you measured in, because a millimetre and inch mix-up is a common cause of a wrong order.
Do I need the serial number or is the model enough?
Send both. The model narrows the search to a family, and the serial range identifies which production variant you have. Manufacturers changed components mid-series, so two machines with the same model name can take different parts.
Why can't I find the part by the name my crew uses for it?
Because your crew's name, the manufacturer's part name and the seller's size code are three different vocabularies that do not map onto each other. Searching by measurement works because dimensions mean the same thing to both sides of the transaction.
How many photos should I send a supplier?
Four covers it: the part flat with a ruler in the same plane, the bore lit from the side, any number or casting mark, and the mounting face. Take them before you discard the old part.
Is a size code enough on its own to order a part?
No. Work in this order: measure the old part, find a code that matches those measurements, then check the code against your machine model and serial range. A code without your own measurement is still a guess.
What if the rubber is so worn that it has changed shape?
Measure across the least damaged area and say in your message that the part is distorted. If the bore is oval from wear, report the rounder of the two readings and mention the wear. A supplier who knows the family can often work from the mounting hardware and the machine model.
Where to Go From Here
Write the four numbers down, photograph the part with a ruler in the frame, and add the model and serial range. That package answers the question a supplier actually needs answered, and it takes about ten minutes.
Do not order from the machine name alone. That is the step where the week gets lost, and it is entirely avoidable once the measurements are on paper.
One boundary worth stating. Measurement identifies the part's dimensions, not its rubber. Two cushions can match on every number and differ entirely in compound and bond quality, which is what decides how long the replacement lasts. Dimensions get you the right part; they say nothing about how good it is. That is a question for the manufacturer side, and it is why we publish what we hold rather than a generic size chart.
Send the model, the serial range and your photographs and we will confirm the match before it ships, the same check we run on wholesale orders and on single units.
Reading Next
- Why Your Excavator Clunks When You Shut It Down, the failure the spline count protects you from.
- Excavator Cab Mount: Diagnose Cab Shake While Tracking, how the cushion measurements above translate into a diagnosis.
- Excavator Vibration at Idle: How to Tell If It's the Engine Mounts, for when the mount is the symptom rather than the part you are replacing.
- How to Replace a Komatsu PC200-8 Lower Water Hose, a worked example of buying against a verified OEM number.
- OEM vs Aftermarket Excavator Rubber Parts: What Actually Differs, how to judge the supplier once you know which part you need..
- Excavator Hydraulic Pump Coupling Sizes: How to Find Yours, how the size and family codes work, and how to read them off your old coupling..
- HS Code for Excavator Rubber Parts: What Actually Applies, how bonded parts and plain rubber goods are classified differently for import..
- Excavator Engine Mount Squeak: Why It Sounds Like a Fan Belt, the noise that gets misdiagnosed as a belt before anyone checks the mounts..
- When to Replace Excavator Shock Absorption Rubber (And When Not To), how to tell a failed cushion from a shake that comes from somewhere else..