How to identify a faucet cartridge.

Most cartridge identification goes wrong in the same place. Someone pulls a cartridge, looks at it, and starts searching for a part number that is not printed on it. Then they are scrolling photo grids trying to eyeball a match, and every cartridge in the grid looks like the one in their hand.

The fix is to stop searching and start describing. A cartridge carries six or seven physical traits that narrow it down fast: how it controls water, how long it is, what the splines look like, how it is retained in the body, and what the stem end is shaped like. Read those traits correctly and you can characterize almost any cartridge in under a minute, even a worn one with no markings at all.

This guide walks through that method the way a specialist does it. It will not hand you a part number. It will teach you to describe what you are holding well enough that a part number becomes findable.

Before anything else, confirm the part category. Four different mechanisms live behind faucet handles and they are not interchangeable, but people call all of them cartridges.

Everything after this section assumes you are holding the fourth type. If you are holding one of the first three, the traits still matter, but the terminology shifts.

Count what the cartridge has to do inside the valve body.

Single function. Controls volume only, or temperature only. Common in two-handle setups where one cartridge handles hot and another handles cold, and in dedicated volume controls on multi-handle shower trim.

Mixing. Blends hot and cold from one lever. The cartridge will show at least three port openings on its body, sometimes four, because it takes in two supplies and delivers one output.

Pressure balancing. A mixing cartridge with a spool or piston inside that reacts to supply pressure changes, so the shower does not scald when a toilet flushes. Often has a visible secondary chamber or a separately moving internal element. Shake it gently. A loose internal spool is frequently audible.

Thermostatic. Holds a set output temperature using a wax element or bimetallic coil that expands and contracts. These are physically larger, usually have a distinct temperature-limit collar, and are noticeably heavier than a pressure-balance cartridge of similar length.

Port count and arrangement is the fastest read here. Lay the cartridge on the bench and count the openings around its circumference, then note whether they sit at one height or at two different heights along the body. Two rows of ports almost always means it is mixing rather than single function.

Length is the single most useful number you can report, and it is the one most often measured wrong.

Measure the whole assembly end to end. Stand it up and run the measurement from the tip of the stem down to the bottom face that seats in the valve. The stem is included. The retainer is included. You are measuring the part as it comes out, not the body alone.

Take the measurement with calipers if you have them. If you are using a tape, stand the cartridge on a flat surface and sight straight across the top rather than at an angle, because a quarter inch of parallax error is enough to point at the wrong part.

Be precise. "About three inches" is not a usable number when several distinct cartridges live between three and three and a half inches. Two cartridges can share every other trait on this page and differ only in length.

If the cartridge is broken and you are holding pieces, measure the sleeve or the cavity in the valve body instead and say that is what you measured.

The broach is the shaped end of the stem where the handle attaches. It is the trait that most reliably separates cartridges that otherwise look identical, and most people skip it because they do not know the word.

Look at the top of the stem straight down the axis. You will see one of these:

Splined. A ring of small teeth around the circumference, like a tiny gear. Count them. Common counts cluster in recognizable ranges, and the count is a hard discriminator: a twenty-tooth broach and a twenty-four-tooth broach will not accept the same handle even if the cartridges are otherwise the same size.

Flatted, sometimes called D-shape. The stem is round with one or two flat faces machined into it. Note whether there is one flat or two, and whether two flats sit opposite each other or at an angle.

Square or hex. Self-explanatory, but measure across the flats. A square broach comes in more than one size.

Threaded. The handle screws on rather than pressing on. Note the thread direction and whether there is a shoulder below the threads.

Something holds the cartridge in the valve body against line pressure. What that something is tells you a lot about the family of valve you are working on.

Retainer clip. A U-shaped or horseshoe-shaped metal clip that slides down through slots in the valve body and captures a groove in the cartridge. If you had to pull a clip straight up with pliers before the cartridge would move, this is your style. Note whether the clip is brass or stainless, and whether it is one piece or two.

Retainer nut, sometimes called a bonnet nut. A threaded collar that screws into the valve body over the cartridge shoulder. Note whether it takes a wrench, a plastic spanner, or has ears for hand tightening.

Threaded body. The cartridge itself threads directly into the valve body with no separate retainer. Less common, but distinctive.

Press fit with an O-ring stack. Held by friction and sealed by O-rings, usually with a cap or escutcheon providing the mechanical stop.

Count the O-rings on the body while you are looking, and note roughly where they sit. Three O-rings evenly spaced reads differently from two clustered at the bottom.

Note what the cartridge is made of and what the wear looks like. Both narrow things down.

Brass bodies and plastic bodies were used in different eras and different product tiers. A brass cartridge with a plastic top cap is its own signature. Ceramic discs feel glassy and are usually visible as two pale plates if you can see into the ports.

Then read the wear. Scale buildup, a green or white crust, tells you about water chemistry and often about age. Scored O-ring grooves tell you it was leaking past. A cartridge that came out in two pieces tells you the plastic became brittle, which is itself an age indicator. Discoloration on one side only can indicate it was installed in a specific orientation.

None of this identifies the part on its own. All of it narrows the field, and it is the kind of detail a specialist uses to rule things out quickly.

Do not throw away the context. Before the cartridge goes in a bag, look at what it came out of.

Note the handle count and the trim style. Note whether there are screw holes or set screws and where they sit. Note any casting marks, letters, or numbers on the valve body inside the wall, which survive long after the trim finish has been replaced. Note the number of ports in the body and whether there is a diverter involved.

Photograph the open valve body before you walk away. Cartridge identification frequently comes down to a detail in the body rather than on the part, and going back into a wall for a second look costs more than the photo did.

This method lets you describe a cartridge precisely. It does not tell you which replacement part fits, because the mapping from physical traits to a specific orderable part is not something you can derive from the part in your hand. That mapping lives in manufacturer cross-reference data, decades of superseded part numbers, and aftermarket equivalents that were never printed in any catalog a plumber has access to.

That is the gap between describing and identifying. Experienced plumbers close it from memory on the brands they see every week. Nobody closes it from memory across every brand, every era, and every discontinued line, and that is the situation most identification requests come from: an older fixture, an unmarked part, a worn broach, or a line that stopped being made before the cross-reference was digitized.

If you are sending the cartridge to us or to anyone else, the photos do most of the work. Four shots cover it.

Practical notes: wipe the part first, because scale and grease hide detail. Use plain indirect light rather than a direct flash, which blows out the metal and erases the broach. Fill the frame with the cartridge. Shoot on a contrasting background, so a dark part on a light surface or the reverse. If any markings exist anywhere on the body, get a sharp close-up of them even if they look meaningless, because superseded part numbers and mold codes often are meaningful.

Shower valves come apart differently and often leave a piece behind. Our step-by-step identification process covers those angles in more detail, and shower cartridge identification has notes specific to tub and shower valves.

Send it rather than working it yourself when any of these are true:

The broach is worn or damaged enough that the count is uncertain. The fixture is old enough that the line may be discontinued. There are no markings anywhere on the part or the body. The cartridge came out in pieces. You have a trait combination that does not match anything you have seen. Or you are on the clock and the identification is not the part of the job you are being paid for.

That last one is the real reason most pros send them in. Knowing how to read a cartridge and choosing to spend twenty minutes on cross-reference research are different decisions, and the second one is usually not worth the truck time.