How to Replace a Mixer Tap Cartridge

Photo by Jacek Dylag on Unsplash (https://unsplash.com/photos/1542013936693-884638332954)

By Tomo — licensed plumber, Brisbane QLD.

Legal note up front: replacing the cartridge inside an existing mixer tap is homeowner-legal under the maintenance provisions of the Plumbing Code of Australia, as long as you don’t disturb the body of the tap or the supply pipework. You’re swapping a wear part inside a fitting that’s already installed. What is NOT homeowner-legal: replacing the entire tap body (that’s licensed plumber work), modifying the supply lines, or doing any work where you’ve cut into a wall or floor to access pipework. Stay inside the cartridge swap and you’re fine.

Every guide on the internet — and almost every old plumbing manual — still teaches you how to replace a tap washer. The thing is, the modern Aussie home has barely any washer taps left. Mixer taps (the single-lever ones that do hot, cold, and temperature blend on one handle) have replaced washers as the dominant tap type, and they don’t have washers — they have cartridges. The cartridge IS the wear part. When your mixer drips, leaks at the spout, or won’t shut off properly, you’re swapping the cartridge, not a washer.

The catch — and this is the bit that sends people back to Reece twice in one afternoon — is that mixer tap cartridges aren’t a universal part. There are at least four common cartridge sizes (35 mm and 40 mm being the most common, with 25 mm and 28 mm appearing in some older Methven and Phoenix taps). They’re not interchangeable across brands. You can’t take a Caroma cartridge out of one tap and put a Methven cartridge in. You have to identify what you’ve got, then buy that specific part.

What you’ll need

Allen key set (the handle grub screw is usually 2 mm or 2.5 mm — exact-fit matters or you’ll round it)
Adjustable shifter (250 mm)
Pliers — long-nose for fishing out broken bits if it comes to that
Replacement cartridge — brand-specific, sized to your tap
Old towel and bucket
Vernier caliper or a ruler with mm markings (for measuring the old cartridge if you can’t find brand info)
Phillips #2 screwdriver
Small smear of plumbers silicone grease (Selleys or Permatex) — NOT vaseline, NOT WD-40
Camera phone — take photos of every step before you disturb anything

Step 1: Identify the tap brand

Look on the underside of the spout, on the body, or on the back of the lever — there’s usually a brand mark (Methven, Caroma, Phoenix, Mizu, Dorf, Mondella, Posh, Sussex). Note the model name if there is one. Brand-specific cartridges are stocked at Reece, Tradelink, and Bunnings — but you need the brand to find the right part.

Step 2: Turn off the water

For kitchen and bathroom mixers, there should be isolation valves under the sink or in the cabinet — small chrome valves on the hot and cold flexi hoses. Turn both clockwise to off. If there are no isolation valves, you’ll need to turn off the main water at the meter. Open the tap to bleed pressure and confirm both sides are off.

Step 3: Remove the lever handle

Most mixer levers have a small grub screw on the underside or back of the lever, hidden under a plastic plug or coloured cap (red/blue indicator). Pry the cap off with a fingernail, then loosen the grub screw with the right-size Allen key. Don’t fully remove the grub screw — just loosen it enough to slide the lever off the cartridge spindle. Lever lifts straight up.

Mixer tap cartridge anatomy: handle screw, decorative cover, brass clamping nut, then the cartridge with O-rings and ceramic discs.

Step 4: Remove the decorative cover and dome

Under the lever you’ll see a chrome dome or skirt covering the cartridge. Some unscrew by hand, some need a strap wrench or a soft-jaw spanner. Take a photo before unscrewing so you can see the order of components. Don’t use ordinary pliers — they leave teeth marks on chrome you can’t polish out.

Step 5: Unscrew the cartridge clamping nut

The cartridge is held in place by a brass nut threaded around the cartridge body. Use the shifter to undo it — anti-clockwise. This nut can be tight after years in service. If it’s seized, a small splash of CRC penetrating spray and a few minutes’ wait usually frees it. Don’t go gorilla on it — the brass tap body underneath cracks.

Step 6: Pull the old cartridge straight out

With the nut off, the cartridge lifts straight up. It may need a wiggle — old O-rings sometimes stick to the tap body. Don’t lever it sideways with a screwdriver because you can score the seating surfaces. If it’s truly stuck, gentle long-nose pliers on the spindle, pulling straight up.

Step 7: Measure the old cartridge — this is where most people go wrong

Now the Aussie gotcha. With the old cartridge in your hand, measure the diameter of the cylindrical body (NOT the spindle) with the caliper. 35 mm and 40 mm cartridges look almost identical to the eye but a 40 mm won’t fit a 35 mm seat and vice versa. Also note: some cartridges have flat sides or a cut-out on the bottom that engages with the tap body — that orientation matters. Take the OLD cartridge to Reece (or your nearest plumbing supplier) and ask them to match it. Don’t try to identify it from a photo — bring the part.

Step 8: Inspect the tap body before fitting the new cartridge

Look down into the tap body where the cartridge seats. Wipe out any mineral scale or grit with a clean rag wrapped around your finger. Smell for any rotten-egg sulphur smell — if present, you’ve got biofilm in the tap body and a quick wipe with metho on a rag clears it. Lubricate the new cartridge’s O-rings with a thin smear of plumbers silicone grease — this stops them tearing as the cartridge slides into place.

Step 9: Install the new cartridge

Drop the new cartridge into the body in the same orientation as the old one (this is where your photos pay off). It should drop straight in until it seats — if it’s not seating, you’ve either got the wrong size or the orientation tab isn’t aligned. Don’t force it. Hand-tighten the brass clamping nut, then a quarter-turn with the shifter — too tight cracks the cartridge body or the tap body.

Step 10: Reassemble and test

Reinstall the dome/skirt, then the lever. Tighten the grub screw and replace the indicator cap. Slowly open the isolation valves under the sink. Operate the lever — should move smoothly through hot and cold, no drips at the spout, no leaks at the cartridge nut. Run hot and cold to full temperature for 60 seconds each. Wipe everything dry, wait 5 minutes, then check again with a dry tissue. Any moisture at the cartridge joint means under-tightened nut or a damaged O-ring.

Common things that go wrong

Where this job tends to derail:

Lever won’t come off. The grub screw is fully out but the lever is corroded to the spindle. Don’t lever it sideways — spray penetrating fluid into the joint, wait 10 minutes, then pull straight up. Persistent ones may need a steam wipe to expand the lever slightly.
Cartridge clamping nut is seized solid. CRC penetrating spray, 15 minutes’ wait, then a soft-jaw spanner with controlled steady force. If it still won’t budge, this becomes a licensed plumber call because the next step would be removing the tap body, which is plumbing work.
New cartridge installed but tap drips at the spout. O-ring not seated, or you bought a cartridge that’s almost-but-not-quite right. Verify the cartridge is fully home, the O-rings are intact, and the diameter and tab orientation match the original. Drips that won’t go away usually mean the wrong cartridge.
New cartridge installed but lever feels stiff or loose. Stiff usually means dry O-rings — pull the cartridge out, smear plumbers silicone grease on the O-rings, reinstall. Loose means worn cartridge spindle (rare on a new part) or a worn lever interface.
Hot and cold are swapped after install. Cartridge is rotated 180 degrees. Pull and reorient. Most cartridges have a small index tab or flat that only fits one way — make sure that’s seated.
Aerator on the spout is also blocked. While you’ve got the tap apart, unscrew the aerator (the small mesh-and-flow-restrictor at the spout tip), soak in vinegar for 5 minutes, brush, reinstall. Often improves flow more than the cartridge swap did.

The Tomo rule

Measure before you buy, or take the old cartridge with you. 35 mm and 40 mm are NOT interchangeable, and there is no universal cartridge across Methven, Caroma, Phoenix, and Dorf. Reece will help if you bring the part. Buying online by guessing the size is how you end up doing the job twice. And the modern equivalent of the washer fix is the cartridge swap — bin the 1980s tap repair guides because they don’t apply to your kitchen mixer.

Got a mixer tap that’s still leaking after a cartridge swap, or a tap body that’s actually cracked? Send us a write-up.