Old Black Alkathene Pipe vs Modern Metric MDPE – Sizes & Fittings Guide

Have you uncovered an old black Alkathene water pipe and need to connect it to modern blue MDPE? This is a common problem when repairing or upgrading older water supplies across the UK, particularly on farms, older properties, gardens and agricultural installations.

Older black Alkathene pipe was commonly manufactured in imperial sizes, whereas modern MDPE water pipe is generally manufactured in metric outside diameters. Although the pipes may look similar, their dimensions are different, meaning a standard metric compression fitting will not necessarily fit an old imperial pipe.

In this guide, T-mex Express explains how to identify old black Alkathene pipe, the differences between Class C and Class D, how imperial pipe sizes compare with modern metric MDPE and what to check when connecting old pipework to a new water supply.

What Is Old Black Alkathene Pipe?

Alkathene is a long-established trade name associated with polyethylene pipe. In the UK, the term is still widely used to describe older black plastic water supply pipes found in domestic, agricultural and commercial installations.

Older imperial Alkathene pipe was used extensively for underground water supplies, farm water systems, garden supplies and connections to outbuildings.

Many older pipes were manufactured to specifications associated with BS 1972, with common sizes described in inches and different pressure classes.

Today, the name Alkathene is sometimes used informally to describe modern MDPE pipe as well. However, the important distinction when purchasing fittings is whether the existing pipe is an older imperial size or a modern metric size.

What Is Modern Metric MDPE Pipe?

MDPE stands for Medium Density Polyethylene. Modern MDPE pipe is widely used for cold water distribution, underground service connections, agricultural water supplies and irrigation systems.

Unlike older imperial Alkathene pipe, modern metric MDPE is specified by its nominal outside diameter in millimetres.

Common metric water pipe sizes include:

• 20mm MDPE: Smaller water services and distribution connections.
• 25mm MDPE: Common domestic underground water supply connections.
• 32mm MDPE: Larger property services, agricultural supplies and distribution systems.
• 40mm and 50mm MDPE: Larger water supply and irrigation applications.
• 63mm MDPE and above: Commercial, agricultural and water distribution pipework.

Modern blue MDPE is commonly used for potable water supplies in the UK, while black polyethylene pipe is also widely used for suitable water distribution and irrigation applications. Pipe colour alone does not establish suitability for drinking water: always check the pipe markings, product specification and relevant approvals.

View Blue MDPE Water Pipe Coils at T-mex Express

Old Imperial Alkathene Pipe Sizes – Class C and Class D

One of the most important things to understand about old Alkathene pipe is that its imperial size description does not equal its measured outside diameter.

For example, a pipe described as 3/4 inch Alkathene does not measure 19.05mm outside diameter. A common old imperial 3/4 inch Alkathene pipe measures approximately 26.9mm outside diameter.

Older Alkathene pipes were commonly supplied in different classes, including Class C and Class D. These classes have different wall thicknesses and internal diameters.

Old Alkathene Class C and Class D Size Chart

Note: These are approximate historical dimensions for the imperial pipe types shown. Other older polyethylene pipe specifications and sizes exist. Always verify the pipe markings and measure the actual outside diameter before ordering fittings.

What Is the Difference Between Class C and Class D Alkathene Pipe?

The main difference between Class C and Class D Alkathene pipe is the pipe wall thickness.

For the historical dimensions shown above, both classes have the same nominal outside diameter for a given imperial pipe size, but Class D has a thicker wall and smaller internal diameter.

For example, old 3/4 inch Alkathene pipe has an approximate outside diameter of 26.9mm in both classes. However:

• Class C: Approximately 19.5mm internal diameter.
• Class D: Approximately 17.5mm internal diameter.

This difference is particularly important when selecting pipe inserts or fittings that depend on the internal diameter of the pipe.

The class designation also relates to historical pipe specifications and pressure performance. Do not assume an old pipe remains suitable for a particular operating pressure solely from its original class marking.

Old Alkathene vs New Metric MDPE Pipe Size Comparison

When replacing an old black water pipe, many customers assume that the closest metric pipe size will accept the same fittings.

Unfortunately, this is not necessarily correct.

The table below compares common old imperial Alkathene outside diameters with nearby modern metric MDPE sizes.

Important: The nearby metric sizes are shown for dimensional comparison only. They are not recommended replacement sizes or direct fitting equivalents. The correct replacement pipe size depends on the water flow requirements, pressure, installation and system design.

How Do I Identify Old Black Alkathene Pipe?

If you have discovered an old black water supply pipe and are unsure what size it is, the best approach is to identify it before purchasing any replacement fittings.

Step 1: Check the Pipe Markings

Clean a section of the pipe and look for printed or embossed markings.

Older Alkathene pipe may show information such as:

• The manufacturer's name.
• The imperial pipe size.
• Class C or Class D.
• BS 1972 or another historical specification.

Not all older pipes will have visible markings, especially after many years underground.

Step 2: Measure the Outside Diameter

Use a vernier caliper to measure the outside diameter of a clean, undamaged section of pipe.

For example:

• Approximately 21.5mm may indicate old 1/2 inch imperial Alkathene.
• Approximately 26.9mm may indicate old 3/4 inch imperial Alkathene.
• Approximately 33.7mm may indicate old 1 inch imperial Alkathene.

These measurements should be treated as identification clues, not definitive proof of the pipe specification.

Step 3: Check the Pipe Condition

Inspect the pipe for cracking, deep scratches, deformation, excessive ovality and signs of ageing.

Older polyethylene pipe may have been exposed to years of ground movement, pressure changes and environmental conditions.

If the pipe is badly damaged or degraded, replacing a longer section may be more reliable than installing a repair fitting.

Step 4: Identify a Compatible Fitting

Once you know the actual outside diameter, pipe specification and condition, select a fitting specifically approved for the existing pipe.

Do not assume that a standard 20mm, 25mm or 32mm MDPE compression fitting will seal correctly on old imperial pipe.

How Do You Connect Old Black Alkathene Pipe to New Blue MDPE?

Connecting old imperial Alkathene pipe to modern metric MDPE is possible, provided a suitable transition fitting is available for the particular pipe sizes and materials.

One common solution is an imperial-to-metric polyethylene compression adaptor.

These fittings are designed to accommodate the dimensional differences between older imperial pipework and modern metric polyethylene systems.

Example: Connecting Old 3/4 Inch Alkathene to 25mm MDPE

Suppose an existing underground water supply uses old 3/4 inch Alkathene pipe measuring approximately 26.9mm outside diameter, and you want to connect it to new 25mm blue MDPE pipe.

You would need a transition fitting with:

• One connection specifically compatible with the old 26.9mm imperial pipe and its specification.
• One connection designed for modern 25mm metric MDPE pipe.
• The appropriate pressure rating and water-use approvals.
• Any pipe inserts or supporting components required by the fitting manufacturer.

Do not force the old 26.9mm pipe into a standard 25mm metric compression fitting. An incorrect fitting may damage the pipe, fail to seal or become disconnected under pressure.

Always follow the fitting manufacturer's installation instructions and pressure-test the completed connection appropriately.

Can I Use Plasson Fittings on Old Alkathene Pipe?

Plasson manufactures a wide range of polyethylene compression fittings, including products for metric MDPE systems and certain specialist transition applications.

However, a standard Plasson metric compression fitting should not automatically be used on old imperial Alkathene pipe.

Compatibility depends on the exact fitting model, pipe outside diameter, pipe material, wall thickness and any required inserts or adaptors.

Where old pipework is involved, check the manufacturer's technical information before installation.

Explore our PE Pipe and Fittings range for modern polyethylene pipework products.

Is Black Alkathene Pipe Still Used Today?

Yes, but the terminology can be confusing.

Modern black polyethylene pipe is still widely used for water distribution, irrigation and industrial applications. However, modern black metric polyethylene pipe is not necessarily the same size or specification as old black imperial Alkathene pipe.

For example, a modern black 32mm polyethylene pipe is normally specified by its 32mm outside diameter. An old 1 inch imperial Alkathene pipe may measure approximately 33.7mm outside diameter.

Although both pipes are black and made from polyethylene, they cannot automatically use the same compression fittings.

When ordering replacement pipe or fittings, always specify whether your pipe is imperial or metric.

Should I Repair Old Alkathene or Replace It with Modern MDPE?

Whether to repair or replace an old Alkathene water supply depends on the condition of the existing pipe, the length of the run and the nature of the fault.

A localised repair may be appropriate where the existing pipe is in good condition and a compatible repair fitting is available.

However, replacing the pipe with modern metric MDPE may be worth considering if:

• The existing pipe has suffered repeated leaks.
• The pipe has become brittle, cracked or heavily damaged.
• Suitable imperial fittings are difficult to obtain.
• The water supply needs upgrading for greater demand.
• New buildings, agricultural equipment or irrigation systems are being connected.
• The ground is already being excavated for other work.

Modern metric MDPE offers the advantage of readily available standard pipe sizes and a broad selection of compatible compression and electrofusion fittings.

For a new water supply, select the correct pipe material, diameter and pressure rating based on the installation requirements rather than simply matching the old imperial size.

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Common Problems When Connecting Old Alkathene Pipe

1. The Fitting Is Too Small

An old imperial pipe may have a slightly larger outside diameter than the nearest modern metric pipe. A standard metric compression fitting may therefore not accept it.

2. The Fitting Tightens but Still Leaks

A fitting may appear to tighten correctly without providing the necessary seal or mechanical grip. This can happen when the fitting is not designed for the pipe diameter or specification.

3. The Pipe Is Out of Round

Older buried pipework may become oval or distorted. Excessive ovality can prevent compression fittings from sealing correctly.

4. The Wrong Pipe Insert Is Used

Where a fitting requires an insert, it must suit the pipe's internal diameter and wall thickness. Class C and Class D imperial pipes can have different internal diameters despite having the same outside diameter.

5. The Pipe Is Too Damaged to Repair Reliably

Deep scratches, cracks or deterioration near the connection can prevent a reliable seal. In these circumstances, cutting back to sound pipe or replacing the affected section may be necessary.

Frequently Asked Questions About Old Alkathene and Metric MDPE

What size is old 1/2 inch black Alkathene pipe?

Common old 1/2 inch imperial Alkathene pipe manufactured to historical BS 1972-related dimensions has an approximate outside diameter of 21.5mm. It is not the same outside diameter as modern 20mm or 25mm metric MDPE pipe.

What size is old 3/4 inch Alkathene pipe in millimetres?

Common old 3/4 inch imperial Alkathene pipe measures approximately 26.9mm outside diameter. Its internal diameter varies depending on the pipe class, with approximately 19.5mm for Class C and 17.5mm for Class D in the historical dimensions shown.

What size is old 1 inch Alkathene pipe?

Common old 1 inch imperial Alkathene pipe has an approximate outside diameter of 33.7mm. This is different from modern 32mm metric MDPE pipe.

Can I connect old black Alkathene pipe to blue MDPE?

Yes, provided a suitable transition fitting is available and approved for both pipe specifications. Measure the old pipe outside diameter and identify its class or specification before selecting a fitting.

Is old black Alkathene pipe imperial or metric?

Many older black Alkathene water pipes installed in the UK were manufactured in imperial sizes. However, black polyethylene pipe is also available in modern metric sizes, so colour alone cannot identify the pipe.

What is the difference between Class C and Class D Alkathene?

For the common historical pipe dimensions shown in this guide, Class D has a thicker pipe wall and smaller internal diameter than Class C for the same nominal imperial size.

Can I use a 25mm MDPE fitting on old 3/4 inch Alkathene?

Not unless the specific fitting is designed and approved to accommodate that old imperial pipe. Standard 25mm metric compression fittings are intended for compatible 25mm outside diameter pipe, whereas common old 3/4 inch Alkathene measures approximately 26.9mm outside diameter.

Is old Alkathene pipe suitable for drinking water?

Suitability depends on the original pipe specification, approvals, installation history and present condition. Do not assume an unidentified old black polyethylene pipe is approved for potable water. For new drinking water installations, use products with the appropriate UK water regulations compliance and approvals.

Can I replace old 1/2 inch Alkathene with 20mm MDPE?

Possibly, but the replacement size must be selected according to the required water flow, pressure loss and installation conditions. A 20mm metric pipe is not a direct dimensional replacement for old 1/2 inch imperial Alkathene.

Buy MDPE Pipe and Fittings from T-mex Express

At T-mex Express, we supply polyethylene pipe, compression fittings and associated water supply components for domestic, agricultural, commercial and industrial applications throughout the UK.

Whether you are replacing an old black Alkathene water main, repairing an agricultural supply or installing a completely new metric MDPE system, choosing the correct pipe and fittings is essential.

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• PE Pipe and Fittings
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