Find the right faucet aerator: housing, thread size and service key

Identify what is actually being replaced

Find the faucet’s full model number, including a variant or production code where the manufacturer uses one. A receipt, installation sheet or model label is stronger evidence than a photograph of a similar faucet. Open its parts diagram and find the outlet assembly. Save the replacement code and the diagram revision together.

Look underneath the spout with the water off. A visible ring may hold a separate insert. A recessed keyed insert may carry its own thread. A pull-out spray head can have an integral outlet or an inlet screen that needs a different procedure. If the parts drawing calls for a complete spray head, do not treat its face as a removable standard aerator.

If you are still deciding whether the outlet is the cause of weak flow, use the linked pressure diagnosis first. This guide starts at the ordering decision; it does not establish that a replacement is needed.

Describe the part before shopping

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Use male and female to describe the replacement part

In an aerator listing, male describes thread on the outside of the replacement housing or insert; female describes thread on its inside. Record which part the description refers to. A faucet spout with an external thread receives an internally threaded housing, so copying the spout’s description into the replacement field can reverse the order.

Neoperl’s US housing list includes Regular female 55/64″–27 and male 15/16″–27 versions. Its Junior housing list instead includes female 3/4″–27 and male 13/16″–27. A size name such as Junior therefore needs its product family and full thread alongside it.

For an insert-only replacement, the insert must also match the existing housing and washer arrangement. The housing thread describes how the assembly meets the spout; it does not, by itself, identify the insert inside. Photograph separated parts in their original order if the manufacturer’s removal procedure permits it.

Separate CACHÉ sizes from ordinary housing sizes

Neoperl’s US CACHÉ page lists the four model-dependent sizes below. These are integrated-thread products that fit directly into the faucet. This is a family-specific identification table, not a complete list of all recessed faucet outlets.

Neoperl CACHÉ identification examples

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A diameter measurement narrows the search; it does not verify a thread

If the old part can be removed using its documented procedure, record its markings first. A caliper reading across an external thread can help a supplier identify candidates; write down that it is a measured outside diameter, not a confirmed nominal thread. An inside opening or the smooth rim of a housing is a different measurement.

Keep the entire designation. In M24×1, the nominal diameter is 24 mm and the pitch is 1 mm. In 15/16″–27, the nominal diameter is 15/16 inch and the pitch is expressed as 27 threads per inch. Converting only the diameter throws away the pitch information.

Here is an arithmetic comparison, not a test of interchangeable parts: 15/16 × 25.4 = 23.8125 mm, close to 24 mm; 25.4 ÷ 27 ≈ 0.941 mm per thread, which differs from 1 mm. A ruler can make the diameters look alike while missing the thread difference. These numbers do not authorize substituting one for the other.

If neither the markings nor the faucet diagram identifies the thread, mark it UNKNOWN and ask the manufacturer or supplier to confirm it from the model and old part. Do not trial-fit an assortment by tightening each one into the faucet. Resistance at the start is a reason to stop and check alignment and identity.

Match the flow specification and key as well

The same CACHÉ size can be supplied with different flow rates, stream patterns and functions. Copy the original or manufacturer-approved replacement’s flow marking, including units, and the exact product code. Matching the screw thread only passes the mechanical connection check; it does not preserve the faucet’s intended water use or stream.

Use the service key specified for that part. Neoperl identifies different keys for integrated-thread models and separate keys for vandal-proof housings; conventional keys do not remove those tamper-resistant versions. Coin Slot products are a distinct design. An outlet that cannot be turned by fingers is not evidence that a coin will work.

For an unfamiliar, rectangular or unusually small outlet, return to the faucet’s parts drawing. Do not enlarge notches or grip a finished spout with unprotected pliers to make a generic tool fit.

Make one complete replacement order

Write one order line containing the faucet model, replacement part number, thread, intended flow, seal and required key. State whether you want the complete assembly or just the insert. Check what the package includes; a matching insert does not replace a damaged outer housing.

Hypothetical candidate screen for an outlet documented as CACHÉ M21.5×1

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Check the seal and the result during installation

Use the faucet and replacement part’s instructions for removal and installation. Protect the drain opening so a loose component is not lost. Keep the water off while handling the outlet, and do not force a scaled, brittle or damaged part to turn.

Neoperl notes that a housed aerator’s old washer can remain in the spout. Account for it before fitting the replacement so an old seal is not accidentally left behind. For its CACHÉ replacement procedure, the maker specifies careful key engagement, protection of the O-ring, and fitting only flush with the faucet rather than driving the part farther in. That instruction is not a torque rule for every aerator.

Restore flow and inspect the joint as well as the stream. If water appears around the attachment, stop the water and revisit the part identity, seal and installation. Repeated tightening is not a substitute for correcting the wrong thread or damaged seal. A leak-free new outlet with persistently weak flow sends you back to the supply diagnosis.