M8 Thread vs. Other Sizes: What Mounting Kit Do I Need

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Thread M8 and other dimensions: how to navigate mounting accessories for heating and plumbing

When a customer comes into the store and says "I need a bolt to mount the expansion tank," they usually think it's a five-minute job. The opposite is true. Behind that simple term "mounting accessory" lies a whole world of dimensions, standards, thread systems, and material specifications, where every millimeter matters. And when you make a mistake – which we see regularly in practice – it results in a leak, a crack, or in the worst case, damage to the entire installation. In this article, we will go through what the thread M8 actually means, how it differs from other common dimensions, how to recognize which accessory you need for your specific application, and what you need to check before tightening anything.

This topic continues from other articles in our Knowledge Center – for example, How to choose the right connecting and fastening elements for the bathroom and plumbing or Comparison of types of fastening systems for plumbing, where you can find a broader context for the entire category. Here we focus specifically on metric threads, their comparison, and practical selection of accessories.

What does the designation "M8" mean and how is the metric thread system read

The letter M in the designation M8 means a metric thread according to the ISO 68-1 standard (or DIN 13). The number after it – in this case 8 – indicates the nominal outer diameter of the thread in millimeters. That's all most people know. But there's much more to it.

Each metric thread has an assigned standard pitch (thread lead, in English "pitch"), which is the distance between adjacent thread lines measured in millimeters. For M8, the standard pitch is 1.25 mm. There is also a fine thread M8×1.0 – used where more precise adjustment is needed or where the component walls are thinner. In everyday plumbing and heating practice, we almost exclusively encounter the coarse (standard) thread, i.e. M8×1.25.

What else do you need to know? Strength class. On metric thread bolts you will find numbers like 4.6, 8.8, 10.9. The first number expresses a hundredth of the tensile strength (in MPa), the second number after the decimal point is a tenth of the yield strength ratio to the tensile strength. For common plumbing applications at temperatures up to 90–95 °C, the class 8.8 in galvanized or stainless steel is sufficient. For more aggressive environments (swimming pools, chlorinated water), A2 or A4 stainless steel is required.

Common metric thread sizes in heating and plumbing: M6, M8, M10, M12

In the practice of bathroom installations, heating systems, and equipment mounting, we encounter several metric sizes. Each has its typical applications:

Thread M6

M6 (outer diameter 6 mm, standard pitch 1.0 mm) appears in plumbing installations mainly for mounting light objects – for example, plastic covers, consoles for smaller bathroom lights, connections of light plastic components, or mounting strips. The load that M6 can take is significantly lower than M8 – with standard steel of class 8.8, we are moving around 9–10 kN in tension. For suspended devices or expansion tanks, M6 is undersized.

Thread M8

M8 is the workhorse in this area. Most consoles for bathroom equipment, mounting elements for expansion tanks up to a volume of about 35–50 liters, and mounting accessories for medium-weight plumbing equipment use exactly M8. The tensile strength for class 8.8 reaches about 18–20 kN. The key is the correct bolt length, drilling depth, and the correct wrench – a metric M8 nut has a wrench width of 13 mm (SW13), which is the most common question in practice.

For the most common case of mounting an expansion tank on the wall, we recommend the mounting accessory M8 directly – it is a complete, precisely dimensioned part for this application, which eliminates the need to search for individual components and ensures compatibility.

Thread M10

M10 (outer diameter 10 mm, pitch 1.5 mm, wrench width SW17) appears in the mounting of heavier devices – large expansion tanks over 50 liters, water storage heaters, load-bearing boiler structures. The tensile strength for class 8.8 is 28–30 kN. If you see a hole with a diameter of about 10–11 mm and a drilled depth of about 15–20 mm on the device or console, it is most likely M10.

Thread M12

M12 (pitch 1.75 mm, SW19) is the domain of heavy equipment – massive consoles, industrial storage tanks, load-bearing steel structures. In a standard apartment installation, you won't encounter it, but in technical rooms with larger boiler rooms, it is quite common. The reason we mention it: if you confuse M12 with M10 (which can happen in poorly lit technical rooms), the thread will "catch" in the hole, but it is not properly engaged – and under vibrations or thermal stress, it will loosen.

Overview table: metric threads and key values

Thread	Outer diameter	Pitch (coarse)	Wrench width (SW)	Tensile strength (8.8)	Typical application
M6	6,0 mm	1,0 mm	SW10	~9,5 kN	Light consoles, covers, plastic parts
M8	8,0 mm	1,25 mm	SW13	~18–20 kN	Expansion tanks, sanitary consoles, mounting brackets
M10	10,0 mm	1,5 mm	SW17	~29–31 kN	Heavy storage tanks, larger boilers
M12	12,0 mm	1,75 mm	SW19	~47–50 kN	Industrial structures, massive storage tanks

Where exactly do we encounter M8 in the installation of expansion tanks and sanitation

Let's move from theory to practice. In real installations of apartment and house heating systems, M8 appears in several specific situations:

1. Mounting the expansion tank to the wall or console

Standard membrane expansion tanks with a volume of 8–35 liters – those we see in the vast majority of apartment gas boiler installations – have a connecting thread of G¾" (for the hydraulic system), but the mechanical mounting of the tank to the structure (console, wall anchor point) uses exactly M8. Manufacturers of consoles and mounting kits dimension the entire system so that the wall–console–tank connection safely handles the total weight of the tank filled with water (for an 18-liter tank, this is about 20–22 kg with water) plus dynamic loading from pressure shocks.

If you are looking for a comprehensive solution for an expansion tank with a G¾" thread, we recommend looking at the mounting kit for a 3/4" expansion tank – this is a complete set where the dimensions are mutually coordinated, which will save you from the tedious search for compatible components.

2. Anchor points for sanitary fixtures and consoles

Consoles for sinks, wall-mounted toilets, shower bars – here M8 appears at the anchor points into mounting frames (Geberit, Viega, Tece, etc.). Anchor screws into concrete, aerated concrete, or drywall are dimensioned for M8 with the appropriate washers in the case of medium-heavy fixtures.

3. Connection of console parts and load-bearing structures

Modular console systems are assembled from parts joined mostly by M8 screws and nuts. The advantage of M8 here is also practical – the wrench width of SW13 is common in every basic wrench set, so the assembly does not require special tools.

How to recognize which thread size you have in front of you – without a caliper

In practice, it happens that an installer comes to a device and a threaded location and does not have a caliper with them. There are three reliable methods for quick identification:

Visual comparison with a coin: The diameter of a 1 euro cent coin is 16.25 mm. The diameter of M8 is 8 mm – a bit larger than half of a euro cent. M10 is approximately 60% of the diameter of this coin. A rough estimate, but helpful in the field.
Comparison with common tools: If the nut or screw head fits onto an SW13 wrench – it is M8. On SW10 – M6. On SW17 – M10. This is the fastest method in practice.
Thread gauge: A cheap tool for a few euros, where the thread is placed against the tooth of the comb and the one is searched where there is no gap between the teeth. Every service technician should have one.
Drilling through the nominal hole diameter: If you know the diameter of the pre-drilled hole (e.g., from the technical documentation of the device), you can deduce the thread: a 6.5 mm hole → M8, an 8.5 mm hole → M10.

You will read more about choosing the correct size for a specific application of an expansion tank in the article Which thread and connector diameter to choose for an expansion tank in this Knowledge Center.

What makes up a complete mounting kit for M8 and why just buying a bolt isn't enough

This is one of the most common mistakes we see in practice. A customer buys a pack of M8 bolts from a hardware store, goes home and realizes they are still missing at least four other components. A complete mounting kit for M8 in the context of securing equipment (e.g., an expansion tank) includes:

M8 bolt (correct length according to the thickness of the bracket + depth of the drill hole or length of the nut)
M8 nut (solid nut, optionally self-locking Nyloc nut for vibrating environments)
M8 flat washer (pressure distribution, preventing material damage)
M8 spring (locking) washer or serrated locking washer (prevents self-loosening due to vibrations and temperature changes)
Bracket profile or anchor plate with the appropriate hole
Anchor plug matching the wall material (concrete, brick, aerated concrete, drywall – each requires a different type)

That's why it makes sense to use ready-made mounting kits from trusted manufacturers. M8 mounting kit on atria.sk is designed precisely to include mutually compatible parts for the given application – you don't have to worry about thread compatibility, washer thickness or bolt length separately.

Mounting kit material: galvanized, stainless steel or something else?

In bathrooms and technical rooms with increased humidity, material choice is critical. Experience from practice: standard carbon steel without surface treatment corrodes in a bathroom within two years. What are the options?

Galvanized steel (electrolytically or hot-dip galvanized): Standard for most internal applications where there is no direct contact with chlorinated water. Hot-dip galvanizing (HDG) lasts longer than electrolytic. Fully sufficient for a typical boiler room and technical room.
Stainless steel A2 (304): For bathrooms, shower areas, spaces with condensation. Resistant to common corrosion, but not to chlorides (pools, seawater).
Stainless steel A4 (316): For pool environments and direct contact with chlorinated water. Significantly more expensive, but the only correct choice where chlorides are present.
Brass: Appears in some connecting elements for water work (plugs, adapters). Less common for mechanical fastening elements due to its lower strength at comparable weight.

Tightening torque for M8: numerical values and practice

Another area where customers improvise and it can go wrong. The correct tightening torque (torque value) for M8 depends on the bolt strength class and whether the threaded surfaces are greased or dry:

M8, class 8.8, dry threads: Recommended torque 25 Nm
M8, class 8.8, greased threads (with grease or vaseline): Around 18–20 Nm (lower torque due to reduced friction)
M8, class 10.9, dry threads: Around 35 Nm

In a typical installation of an expansion tank, we are in the range of 20–25 Nm for M8×1.25 class 8.8. This is important for two reasons: an over-tightened bolt can shear the thread in the bracket or even break during temperature changes, while an under-tightened bolt will gradually loosen due to vibrations in the heating system. More about typical mistakes during installation can be found in the article Common mistakes when installing connecting elements in bathrooms and how to avoid them.

Step-by-step guide to selecting the correct mounting kit

Based on what we have covered above, we can create a simple decision-making process for anyone who is selecting a mounting kit:

Detailed description of steps:

Step 1 – Device weight: Check the technical specification of the device (expansion tank, tank, bracket). You are looking for the weight of the empty device plus the weight of the liquid at full capacity. For an 18-liter expansion tank, it is about 22 kg, for a 35-liter tank about 40 kg. For devices up to 35 kg, M8 is the correct choice at a single anchoring point; at two anchoring points, M8 can safely handle 50–60 kg.

Step 2 – Thread in the device: Physically measure the thread in the bracket or on the device (see the identification procedure above). Never rely solely on visual estimation – M8 and M10 look similar at first glance, especially if the threads are slightly worn or dirty.

Step 3 – Material according to environment: Galvanized steel is sufficient for dry technical rooms. Choose A2 stainless steel for bathrooms and shower areas. A4 is the only option for pool areas.

Step 4 – Assembly: Either assemble the kit from individual components (bolt + nut + flat washer + spring washer), or reach for a ready-made mounting kit, where compatibility is guaranteed by the manufacturer.

Typical mistakes when choosing a mounting kit – what we’ve seen in practice

Over the years of sales and technical support, we’ve seen just about everything. Here are situations that keep repeating:

Mixing up M8 with G¼" (or other pipe threads): A customer comes with a tank that has a hydraulic connection G¾", and assumes that the wall mounting bracket must have the same thread. G¾" is a pipe thread for hydraulic connections, M8 is a metric thread for mechanical mounting – these are two completely different things. For more on the differences between metric and pipe threads, see the article How to mount an expansion tank: choosing a set and the correct fitting size.
Too short a bolt: A bolt must have at least 1.5× the diameter of the thread in the solid material (i.e., at least 12 mm for M8 in metal). In practice, we see bolts with only 5–6 mm of thread, where the thread holds on just two or three turns – it will pull out under load or vibration.
Missing lock washer in a vibrating environment: Heating systems vibrate. Without a spring or toothed lock washer, the nut gradually loosens. The customer notices when the tank bracket is visibly tilted – or when the tank falls down.
Wrong anchor for wall material: A plastic anchor for solid masonry used in hollow brick or drywall won’t hold. Each wall type requires a different type of anchor. The most common disaster: a chemical anchor used without properly cleaning the hole – it holds only a fraction of the nominal strength.

Pipe threads vs. metric threads: basic distinction

Since this question comes up repeatedly, we’ll briefly address it here too. In heating and plumbing installations, we encounter two types of threads at the same time:

Metric threads (M-series): Used for mechanical mounting and joining structures. Marked as M + number (diameter in mm). The thread profile is an isosceles triangle with a 60° angle. These are the threads discussed throughout this article.

Pipe threads (G-series / Whitworth, BSP): Used for hydraulic connections (water, gas, heating). Marked as G + number (the number is historically in inches, not the actual hole diameter). The thread profile is a Whitworth triangle with a 55° angle. G¾" is the most common thread for the hydraulic connection of an expansion tank. It is smaller than M8, despite the reference to ¾ inch!

To sum up this section: never mix metric and pipe threads. They may sometimes look similar from the outside (especially with smaller sizes), but their profiles are incompatible – the thread may screw in, but it won’t hold properly and will fail under pressure or load.

Maintenance and inspection: when to check mounting kits

Even a properly dimensioned and correctly installed mounting bracket requires periodic inspection. The recommended interval is once a year – typically during the autumn inspection of the heating system. What to check?

Visual inspection of bolts and nuts for corrosion
Check the tightness of nuts (by hand – if the nut moves without a wrench, there is a problem)
Check the bracket and anchor points for visible cracks or deformations
Check whether the device (expansion tank) shows any unusual tilt or movement

For more on systematic maintenance, read the article Maintenance and inspection of fastening elements in plumbing installations.

Frequently asked questions (FAQ)

Can I use an M8 bolt instead of M10 if the hole is a bit bigger?

No. If the hole is dimensioned for M10, an M8 bolt will not be properly engaged – the threads won’t be in full contact, and the bolt won’t carry the load along its full length. It might not be a problem immediately under static load, but under dynamic load or vibration, the connection will loosen or fail much faster. Always use the correct size. If the hole is damaged and enlarged, use a thread insert (Helicoil) – not a randomly larger bolt.

What wrench do I need for an M8 nut/bolt?

The standard M8 nut and the head of a hexagonal M8 bolt have a 13 mm wrench size (SW13). You need a spanner or socket wrench of 13 mm, or a 1/4" or 3/8" ratchet with a 13 mm socket. Set a torque wrench to 25 Nm for a dry thread of class 8.8.

How long should an M8 bolt be when mounting a bracket to the wall?

It depends on three things: the thickness of the bracket (or other part the bolt goes through), the thickness of the washers, and the depth of the drilled hole (or the height of the nut). Basic rule: the thread in metal should be at least 1.5× the thread diameter, i.e., at least 12 mm for M8. If the bolt goes through a bracket of 10 mm thickness, then through a flat + spring washer (total of about 4 mm), and into a nut of 6.5 mm height, you need a bolt of at least 10 + 4 + 6.5 = 20.5 mm, i.e., practically 25 mm (always round up to the nearest standard size).

Is there a difference between coarse and fine M8 thread in a plumbing application?

For the vast majority of plumbing and heating applications, the coarse thread M8×1.25 is used. The fine thread M8×1.0 appears only in precise engineering applications or in thin-walled parts. If you accidentally buy a mounting bracket with a fine thread and try to screw it into a bracket with a coarse thread (or vice versa), the thread will be damaged immediately. When ordering, always check the pitch if the manufacturer specifies it.

What does the marking A2-70 on a bolt mean?

A2 indicates the type of stainless steel (austenitic 18/8, i.e., approximately 18% chromium and 8% nickel – AISI 304 standard). The number 70 after the dash indicates the strength class: minimum tensile strength is 700 MPa. For comparison: a standard 8.8 bolt made of carbon steel has a tensile strength of 800 MPa – A2-70 is slightly less strong but significantly more corrosion-resistant. For plumbing applications, A2-70 is the right choice in most cases.

Can I combine a stainless steel bolt with a galvanized nut?

Technically yes, but in practice it’s better to avoid it. Different metals in contact in the presence of an electrolyte (i.e., water with minerals) create a galvanic cell and electrolytic corrosion occurs, with the less noble metal corroding much faster. Galvanized steel in contact with stainless steel and moisture will corrode faster than if it were alone. Always combine the same materials: stainless with stainless, galvanized with galvanized.

Conclusion: the right mounting kit is not a small detail

A mounting bracket may look like a minor detail at first glance – just a few bolts, washers, and nuts. In reality, it is an element on which the long-term reliability of the entire installation depends. A poorly dimensioned or improperly installed bracket can cause the device to fall, hydraulic leaks, or damage to the wall and structure. On the other hand, a properly chosen and correctly installed bracket works quietly for decades without any problems.

For most standard apartment and single-family house installations (expansion tanks up to 35 l, medium-weight plumbing fixtures, brackets and load-bearing elements), the M8 mounting kit is the right choice – in terms of size, load capacity, and tool availability. For a complete solution for both hydraulic and mechanical mounting of an expansion tank, we recommend the expansion tank mounting set 3/4", where both aspects are solved in one compatible package.

If after reading this article you are still unsure whether M8 is the right choice for your specific situation, check out other articles in this Knowledge Center – for example, Mounting fastening elements for expansion tanks and plumbing step by step or Common questions about connecting and fastening systems for the bathroom and plumbing, where you’ll find more context and specific procedures.

Do you have a question about this topic?

Still unsure or dealing with a specific situation in your home? Write to us – we’ll be happy to help.