Blanking an unused waste connection under a sink: which cap fits?

Choose a blanking cap by the connection you are closing—not simply by the size of the waste pipe nearby. A threaded fitting, a plain pipe end and a small appliance hose nozzle need different closures.

If an unused opening has been left under your sink, stop using the connected sink or appliance until that opening is safely sealed. A bucket underneath is not a repair.

Quick shop: caps for threaded waste fittings

These are caps with nuts for matching BSP threads. They are not push-in stoppers for bare pipe and not universal dishwasher-nozzle caps. Identify the fitting first.

McAlpine S23 — 1¼-inch BSP

For closing a compatible 1¼-inch BSP threaded waste connection. Use the fitting's thread specification to select it; a nearby 32mm pipe alone does not establish a match.

View S23 cap at APM

McAlpine T23 — 1½-inch BSP

For closing a compatible 1½-inch BSP threaded waste connection. It is not a slip-over cap for plain 40mm pipe or a closure for the small end of an appliance nozzle.

View T23 cap at APM

Check current availability on the product pages. For another fitting type, browse APM blank caps after identifying the connection.

First identify what has been left open

1. An external thread on a waste fitting

A cap with a retaining nut may be the right type here. Confirm the manufacturer, fitting reference, BSP thread size and sealing arrangement. McAlpine describes the S23 and T23 as blank caps with nuts for BSP threads. Their size labels refer to that thread, not a measurement across the outside of the plastic nut.

2. A plain pipe end with no thread

You need an end closure intended for that pipe system and its actual outside diameter. A nut cannot secure itself to a smooth pipe just because the nominal sizes sound similar. Identify push-fit, solvent-weld or compression construction before ordering. Our waste pipe sizing guide covers the wider pipe-size question.

3. The small nozzle that held a dishwasher or washing-machine hose

This is an appliance connection, not automatically the same size as the trap's main inlet or outlet. Depending on the trap, the closure may be a blank cap, a plug behind the nozzle or a blank nozzle. Find the trap model and its spare-parts instructions; do not assume either threaded cap above fits it.

Why “a cap for 40mm waste” can be the wrong order

The trap body, branch pipe and appliance nozzle are separate interfaces. A sink can have a nominal 40mm waste run while the unused connection you need to close is a smaller appliance port. Equally, the inch designation on a BSP thread is a nominal thread size, not its directly measured outside diameter.

For a useful identification photo, include the whole trap first, then the open connection and any moulded brand or model reference. Photograph a ruler beside the exposed connection—not just across the grip of a loose nut. Measurements help identify a fitting, but should be checked against its manufacturer's specification rather than converted into a cap size by guesswork.

Removed an appliance? Keep the sink usable safely

McAlpine's domestic-appliance-trap instructions explain that a supplied blank cap, plug or blank nozzle lets the trap work as a normal sink trap when no appliance is connected. The instructions also tell installers to retain the blank caps on the illustrated TeeVent arrangements for use if the appliance is removed. That is a reason to keep the original closure, not evidence that all trap caps interchange.

If the original closure is missing, identify the exact trap before buying its replacement. A nozzle that was cut open for installation does not become sealed again when the hose is removed. Do not substitute a rag, adhesive tape or an improvised plug. If the connection cannot be identified or the trap body is damaged, a plumber may recommend replacing the trap rather than improvising a cap.

The appliance's pressurised water inlet is a separate job. Closing the waste opening does not isolate the water supply; these waste caps are not specified here for a water-supply valve.

Fit and test without hiding a problem

1. Prepare: keep the connected sink and appliances out of use, put a container beneath the joint and make the area accessible. Do not handle plugs or sockets if they are wet.
2. Inspect: check the fitting for damaged threads, cracks and a dirty or damaged sealing face. Confirm the cap, nut and required seal arrangement against the fitting instructions. Do not force a nut that starts crooked or will not engage freely.
3. Assemble as specified: follow the manufacturer's sealing and tightening instructions. Do not try to make a wrong-size fitting work by overtightening or packing the joint with tape or sealant.
4. Test while visible: dry the joint, run a small amount of sink water and inspect. If dry, test normal sink discharge and any remaining connected appliance while watching the closure and disturbed joints. Stop immediately if water appears.

A dry cap with water appearing at another joint points to a different fault. Persistent leakage, poor drainage or odour needs investigation; fitting a larger cap is not a cure.