How to measure and specify a replacement tap valve

Index > Tap and shower spares > How to measure and specify a tap valve

What sort of tap valve and how to measure it

A tap is simply a device with the means to control the flow of water through it. This is achieved opening, closing or restricting the path along which the water will flow.

There are countless designs of tap (also known as a faucet) to accommodate personal taste and application, but they all share a similar function.

There are several ways to achieve such control. but many domestic taps with either one or two knobs or levers use one of two types of valve shown below. Single lever taps tend to use a different type of cartridge - please refer back to previous page or call if you require advice.

Compression washer tap valve

These are more commonly seen in older and traditional style brassware.

They operate by multiple turns of the tap head that screws a plunger in and out, in turn placing pressure on or releasing a soft washer located over an orifice in the tap body. This action constricts the flow over water through the orifice.

Although there can be the odd exception, these can normally be identified prior to dismantling if the tap head requires multiple turns to operate fully.

Compression type tap valve

Ceramic disk tap valve

These are more commonly used in modern brassware.

A ceramic disk valve offers full flow control (fully on to off) within a quarter turn of the tap head.

This makes possible lever control - something that could prove difficult if multiple turns were necessary.

Ceramic disk valves (also referred to as ceramic cartridges) tend to last longer than their traditional 'compression' counterparts - especially in hard water areas.

Ceramic cartridge tap valve

Replacing a tap valve

Replacing either type of valve should be a fairly simple task. The valve (or cartridge) simply screws into the body of the brassware, so extraction is a matter of unscrewing. For a few simple tips on this process please refer here.

For reasons too numerous to list tap valves and cartridges are available in a considerable range of sizes. A Bath tap may have a larger valve than basin brassware in order to facilitate higher flow rates. Quirky and traditional brassware my require a longer stem. Many tap stems have 20 teeth on the spline where others may have 18, 24 or other number. These are just a few examples of possible differences you may encounter.

In short, take nothing for granted. Many tap valves can look similar but small differences can mean that they will not interchange.

As tap manufacturers tend to buy in the internals (the valves) it is usually possible to source replacements whether or not you know the make and model of the brassware. You just need to establish a few key dimensions.

How to measure a tap valve...

Measuring a tap valve is fairly easy. There aren't too many component parts to consider, so it's just a matter of measuring carefully and supplying the necessary information.

1. Replacement tap valve size

This is determined by the thread which screws into the tap body. Happily there aren't too many variables in most domestic brassware.

The most common conventions for domestic taps nowadays are 1/2" and 3/4" BSP.

Don't concern yourself too much with these sizes and units. BSP (which stands for British Standard Pipe) is a rather archaic standard established donkey's years ago and one that continues to be used in the plumbing industry.

A valve specified as a 1/2" BSP will actually measure close to 21mm across the thread. A valve specified as 3/4" BSP will measure close to 26mm across the thread. Confusing?

Just think of them as large or small if it helps. The large one is likely but not exclusively used in a bath tap, the smaller is normally used in a basin tap and sometimes in modern bath mixers.

Do not assume, mixer taps can use either.

Where to measure the thread on a tap valve

Maybe you can see the importance of measuring. By doing so we avoid falling into a trap by making erroneous assumptions.

The valve illustrated is a ceramic disc type, but the convention remains the same for compression washer valves.

2. The depth of a replacement tap valve

Once you have established the valve size, how far it reaches into the body of the tap is probably the next most important measurement.

When you screw the valve into the body of the tap or fitting it is important that the valve is the correct length.

When fully installed the valve must be able to make contact with the 'seat' inside the body.

The seat can be seen when the valve is removed. It often appears as a smooth ring with a large hole through the centre at the bottom of the receptacle.

A ceramic disk valve will 'seat' and seal against this ring when installed whereas a compression washer type valve will screw down onto it.

When in use, water from the supply pipework will enter the tap through the centre of the seat. By opening and closing the hole you control the flow. I hope you can imagine therefore how important getting this dimension is. If the valve depth is for example too short then it cannot seal against the seat or close the hole.

Where to measure the depth of a tap valve

There are no example measurements to be offered and these will vary. While this dimension can be measured accurately on a ceramic disk valve a compression washer type valve is more tricky by nature as the washer is on the end of a plunger that screws in and out. Therefore, depending on the position of the plunger the dimension will change. With a compression type it's probably best to take a mid point reading (from central collar to base of washer).

Note: Inspect the seat of your valve

It is important that the seat onto which the valve will seal is in good condition. If it isn't this might be the reason why your tap is dripping rather than a problem with the valve.

I might be fair to say that the seat in a tap fitted with a ceramic disk valve is less likely to suffer damage through use than its counterpart in a tap with a compression washer type valve.

Damage to a tap seat is not too difficult to spot. It will often appear as a dark blemish on a relatively shiny ring. You can also feel for damage or pitting on the seat with your finger. If you can detect pits roughness or holes when you run your finger or finger nail around it then you have a damaged seat. (NB: Be careful not to get your finger stuck in the hole in the process! Beware of sharp edges.)

If your tap seat is damaged it might be possible to re-cut the seat. To do so you will need to buy or hire a gadget called 'tap re-seating tool' or a 'seat cutting tool'. We can provide info on these if you'd care to call.

If your tap seat is damaged then changing the valve is unlikely to fix your problem.

Other tap valve variables -

3. Decorative cover shroud on a tap

There are numerous other things that set valves apart. You might for example have a decorative shroud that covers the valve when in situ. You will of course have removed this in the process of exposing the valve.

Manufacturers can have a range of methods by which a shroud is secured to the tap. One of the most common methods is where the shroud screws onto the top of the valve body.

If this is the case then the replacement valve must have an appropriate threaded section often located just above the central collar.

This will be a much finer thread that the one located below the collar.

Maybe your tap has a different method of securing a shroud, or maybe has no shroud at all whereby a threaded section is simply unnecessary. Whatever yours looks like this is a feature to consider and report when sourcing a replacement.

Where a shroud screws onto a tap valve

4. The 'Nut' on a tap valve

In order to screw and tighten your valve into the body of your brassware you will probably need a spanner. You will therefore probably find a nut-like section near the base of the stem.

Again, manufacturers and different designs will determine different methods of achieving this, so don't be surprised if you find something slightly different when you start to inspect.

Just because a valve has or hasn't got a nut-like arrangement does not necessarily mean it is unsuitable for use where once there was or wasn't, but it is important to ensure that you can tighten the valve into the body of the tap and that the nut doesn't interfere with a decorative shroud if fitted.

Nut type flats on a tap valve where a spanner fits

5. The stem on a tap valve

This is probably the most easy feature of a tap valve to spot and is obviously important to get right or the tap head will not fit or locate correctly.

The stem length becomes more critical the longer it gets. Most short valves are what they are - a valve with a splined spigot sticking out the top. It is likely you needn't concern yourself about obtaining measurements for short valves and a visual confirmation will confirm suitability.

The stem is relevant to your valve in more than one way. It has a certain visible length (1). It determines the distance from the central collar that the tap head will locate (2), and it determines the over all dimension of the valve (3).

Dimensions 1 & 2 are important as this dictates the position of the tap head.

Dimension 3 helps confirm the identity of the correct valve when the valve depth is factored in.

The shaft diameter must be known if it is to pass through a cover plate or shroud.

6. Teeth on a tap valve spline

The number of teeth on the spline is most important. Get this wrong and your tap head simply won't fit. The most common number of teeth on the spline is 20. However, there are examples with 18, 24, 32, 40 etc, but the latter are less common.

Key dimensions on a tap valve

Note: It should be mentioned that some tap shafts utilise a square or 'flat' profile onto which the tap head fits rather than a spline. This may well be the case with very old brassware. Should this be what you require please make us aware.

Tip: The easiest way to count the number of teeth on a spline is to place a pencil mark in one of the grooves, then place your fingernail in that groove and rotate the shaft. Count how many 'clicks' there are until you return to your pencil mark. Hey Presto....

Help and Overview when identifying a tap valve

If you wish to enquire about the availability of a valve for your application you now know what we need to know. To help, use one of the following diagrams to check you have taken all the necessary measurements, then eith give us a call or send a copy of the diagram with measurements to us by email or fax.

Click on the image below that looks closest to the valve you seek?

This will open a form on which you can fill in all the key information we need to know...


Ceramic Disk Quarter Turn Valve with short spindle		Compression washer type valve with short spindle


Ceramic Disk Quarter Turn Valve with long spindle		Compression washer type valve with short spindle
Print the form. Once you have entered your details, re-scan (or photograph) the form and send a copy by email to support@hants1.co.uk

You are always welcome to call and we will endeavour to be of assistance.

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