An excavator pump coupling is identified by a size number, and that number is usually molded into the rubber or stamped on the hub. In our range the sizes run from 4 up to 500. Read the number off your old coupling, confirm the spline count, and you have almost everything needed to order.
The reason people get this wrong is not carelessness. It is that two different vocabularies are in play, and only one of them is printed on the part.
You need a coupler. You look at the old one, and there is a number molded into the rubber face, or there is not. The supplier's listing says "Coupling A" and yours said "size 50" when you asked on a forum. Nothing lines up, so you order the one that looks closest, and it comes back wrong.
Key Takeaways
- Coupling size is a number, not a letter. Our range runs 4, 8, 10, 16, 22, 25, 28, 30, 50, 80, 90, 140, 200, 250 in the A and AS families, and separately 16 to 500 in the H family.
- The number is often molded into the part. A buyer on HeavyEquipmentForums put it plainly: "Should be a number on the coupler molded in for size, or measure the OD of the center hub."
- Size is necessary but not sufficient. The spline count in the bore has to match the pump shaft, and an 11-spline shaft will not take a 13-spline coupling.
- Letter families are not interchangeable. A 50 in one family is not a 50 in another, so the family code has to match too.
- Oil and fuel destroy some coupling rubber. If yours turned soft and sticky rather than cracking, that is a different failure with a different cause.
Why "Coupling 50" Is the Wrong Thing to Say
Ask for "a coupling 50" and you will get a part that is the right size and possibly the wrong everything else.
Couplings are described by two things that are independent of each other: the size number, which sets the physical dimensions, and the family, which sets what the part actually is. Saying "size 50" answers the first question and leaves the second one open.
The vocabulary problem runs the other way too. In the threads where mechanics actually solve these problems, nobody says "Coupling A" or "Coupling H". They say the number, sometimes with an AS suffix. A mechanic diagnosing a coupler wrote "Coupler appears to be size 50", and in the same thread another posted photos named 140AS.jpg and 50AS coupler 002.jpg.
So both vocabularies are real. The number is what the buyer knows and what the part carries. The family is what the catalogue is organised by. The article you are reading exists to join them up.
The Size Number Is Molded Into the Part
Before you measure anything, clean the old coupling and look at it under a light.
The size is commonly molded into the rubber face, or stamped or cast into the metal hub. It may be a bare number like 50, or a number with letters like 50AS, 25H or 110K. Take a photograph of whatever you find, even if it is only partly legible, because a partial code narrows the search faster than any measurement.
If the part carries no readable code, you are not stuck. Measure the outside diameter of the centre hub and the thickness of the element, and send both with a photograph. That is enough for a supplier who knows the range to place it, and it is the method mechanics on the forums use when the moulded number has worn away.
One practical warning from the same threads: 50 and 80 are the pair that gets confused. A mechanic measuring one wrote that it "could also be an 80 with a 100mm OD hub, but the 80 is 65mm thick where the 50 is only 58." Two sizes that differ by seven millimetres of thickness will not sort themselves out in a photograph unless the ruler is in the frame.
The Size Range We Hold, by Family
This is our range as it actually stands. Every line is a live part, and each size exists as an element on its own.
| Family | Sizes | What it is |
|---|---|---|
| A | 4, 8, 10, 16, 22, 25, 28, 30, 50, 80, 90, 140, 200, 250 | 14 sizes, the widest family |
| AS | 4, 8, 10, 16, 22, 25, 28, 30, 50, 80, 90, 140, 200, 250 | The same 14 sizes again, as a distinct part |
| H | 16, 25, 30, 35, 40, 45, 50, 90, 100, 110, 130, 140, 160, 240, 300, 500 | 16 sizes, with assembly versions available |
| K | 25, 50, 110, 165, 240 | 5 sizes, assembly versions available |
| SS | 25, 29, 32 | 3 sizes |
| AM, E, EB, SH | 50 (16T), 320 (15T), 200/300 (14T, 16T), 60/280 (14–20T) | Sizes listed with the spline count in the code |
Two things in that table cause more wrong orders than anything else. The first is that A and AS both run the same 14 numbers. A 50A and a 50AS are different parts with the same nominal size, so "size 50" on its own does not settle which one you need. The second is that H is a different series, not a bigger A. The H family starts at 16 and runs to 500, and its sizes do not line up with the A numbers.
Some of our couplings are named after the machine rather than a size, because that is how they are traded. Those carry the spline count in the code, which is genuinely useful: a sh280-20t is a 20-tooth coupling, and a yanmar-13t is a 13-tooth one.
A coupling named with a tooth count, like the 13T on this one, tells you the spline count before you measure.
Size Alone Is Not Enough: The Spline Count
If the coupling has internal teeth, the tooth count outranks every other measurement, because a coupling with the wrong count will not drive at all.
This is not a rare mistake. A buyer on HeavyEquipmentForums described it 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."
Count the teeth in the bore, and count them twice. Photograph the bore from an angle with a light so the teeth cast shadows; a head-on flash fills the bore and hides them. If the coupling is a bolted-flange type with no spline, measure the bolt circle and the bore instead.
An element with no spline is identified by bore, hub diameter and thickness rather than tooth count.
What the Letter Families Mean on Your Order
Here is the honest version, and it is worth reading because it is where suppliers and buyers talk past each other.
The family code identifies a distinct part. Within a family, the number is a size, and the sizes progress in a way that is consistent across the range. Across families, the same number does not mean the same part, and elements are not interchangeable between families even when the moulded number matches.
What we will not do is invent a rule for you. If your old coupling carries a readable family code, that code plus the size is the fastest possible route to the right part, and we will match it. If it carries only a number, the number narrows the field and the measurements decide it.
What a successful size match still does not tell you
Getting the size and family right makes the part fit. It says nothing about how long it will last.
Two couplings can match on every dimension and differ completely in the rubber compound, in how well the element is bonded to its hubs, and in how consistent the manufacturing batch is. Those three things decide the service life, and none of them appear in a size code.
So when a supplier quotes you a size and stops there, you have half a specification. Ask what the element is made of and how the metal-to-rubber bond is produced. The comparison between OEM and aftermarket rubber parts covers what to listen for in the answer, and it is a separate question from the one this page answers.
That is why sending a photograph of the old part matters more than sending a description of it.
The family code and the size are both molded in, which is why the whole part is worth photographing.
Why Coupling Rubber Fails Early: Oil, Fuel and Heat
Couplings fail in two visibly different ways, and they point at different causes.
Rubber that has cracked and hardened has aged out. Heat cycles, time and ozone do this. It is the ordinary end of a wear part's life, and the noise it makes is the shutdown clunk described in our guide to the shutting-down clunk.
Rubber that has gone soft, sticky or jelly-like has been attacked chemically. A mechanic on HeavyEquipmentForums was blunt about this failure mode: "whose bright idea was it to make these flex couplers out of a rubber material that turns to jelly when a little bit of oil or fuel hits it?"
That distinction matters when you are choosing a replacement. If your coupling died of oil exposure, a like-for-like replacement will die the same way, and the useful question is whether the leak above it has been fixed. Send us a photograph of the failed part and we will tell you which failure we think you are looking at.
When the Coupling Is Fine and Something Else Is Wrong
Not every knock at the pump is the coupling, and replacing a good coupling does not fix a bad pump.
Rule these out before you order. A loose pump mounting bolt will rock under changing load and knock, and it is a ten-minute check. A worn pump drive gear produces a similar sound and needs a pump, not a coupling. And engine mounts at the end of their life let the whole power pack move further than it should, which adds its own noise on top of everything else.
If the machine also shakes while it is running rather than only clunking when it stops, you are looking at a different part, and our engine mount symptom guide covers how to separate the two.
Element or Assembly: Check Which One You Are Removing
Some couplings are sold as a bare rubber element, and some as a complete assembly with the metal hubs already fitted. The two are not interchangeable as purchases, and the difference shows up on the invoice rather than in the fit.
An assembly costs more because it includes the hubs. If your hubs are sound, buying an element on its own is the cheaper and correct choice. If the hubs are worn or cracked, an element on its own leaves you reusing the worn parts, and the new rubber will not last.
Check the hubs before you decide. Look for cracks at the point where the rubber meets the steel, and check the spline or the bore for wear. A hub that has been running with a failed element has often been taking loads it was not designed for, so inspect it rather than assuming it survived.
In our range, assembly versions are held for the H and K families. The A and AS families are listed as elements. If you are not sure which of the two you are looking at on the machine, photograph the whole coupling including the hubs and we will tell you what you are holding.
What to Send Before You Order
Four things, and the order matters less than sending all four:
- The size code, if any survives on the old part.
- The family code, if it is molded in.
- The spline count, or the bolt circle if there is no spline.
- A photograph of the old coupling, flat and square-on, with a ruler in the same plane as the part.
If the number is gone entirely, the method on our guide to identifying a rubber part with no part number applies here unchanged: measure the hub diameter and the thickness, and send the machine model and serial range alongside.
Frequently Asked Questions
How do I find the size of my excavator pump coupling?
Look for a number molded into the rubber face or stamped on the hub. Common sizes are two-digit, like 50, 80 or 90. If nothing is legible, measure the outside diameter of the centre hub and the thickness of the element and send both with a photograph.
What does the AS in a coupling size mean?
It denotes a distinct part within our range rather than a size variant, and the A and AS families run the same 14 numbers. That is why "size 50" on its own is not enough to order: a 50A and a 50AS are different parts.
Is a size 50 coupling the same as a 50AS?
No. They share the nominal size but are different parts. Confirm which one your machine takes against the old coupling, or send us a photograph of the old part and we will match it.
What is the most important measurement on a pump coupling?
The spline count, if the coupling has one. An 11-spline coupling will not drive a 13-spline shaft, and the two are difficult to tell apart in a photograph.
Can I measure a coupling with a tape measure?
For identification, usually yes, if you measure across the widest point and keep the tape square to the part. Report the unit you measured in. A millimetre and inch mix-up is a common cause of a wrong order.
Why did my coupling turn soft instead of cracking?
That is chemical attack, usually from oil or fuel reaching the rubber, rather than ordinary ageing. Replacing the coupling without fixing the leak means the new one will fail the same way.
How many sizes of coupling do you hold?
Fourteen in the A family and the same fourteen in AS, sixteen in H, five in K, three in SS, plus machine-named couplings that carry their spline count in the code. The full list is in our coupling range.
Do I need the engine code as well as the coupling size?
If the machine model has more than one engine variant, yes. Engine changes mid-series can change the coupling, which is why we ask for the machine model and serial range alongside the size.
Where to Go From Here
Clean the old coupling, photograph it with a ruler in frame, and note the size, the family code and the spline count. That package answers the question in one exchange instead of three.
One boundary worth stating plainly. This guide covers couplings that are identified by a size and a family. If your machine uses a belt-driven or gear-driven pump rather than a coupled one, none of it applies, and the noise you are chasing has a different cause. Send the model and we will tell you which arrangement you have.
Send the model, the serial range and your photographs and we will confirm the size against what we hold before it ships, the same check we run on wholesale orders and on single units. Our coupling range is listed by size with the family code shown on each part, and the numbers we hold are also in the parts catalogue.
Reading Next
- Excavator Clunks When Shutting Down, the symptom that sends most people looking for a coupling.
- How to Measure Excavator Rubber Parts Without a Part Number, the four measurements that work when the code has gone.
- Excavator Vibration at Idle, for when the noise is the mounts rather than the drive.
- OEM vs Aftermarket Excavator Rubber Parts, how to judge the rubber itself once the size is settled.
- Excavator Cab Mount: Diagnose Cab Shake While Tracking, when the shake is coming from the cab rather than the engine or the track..
- HS Code for Excavator Rubber Parts: What Actually Applies, how bonded parts and plain rubber goods are classified differently for import..