What diameter and type of connecting material do I need for my pump
What diameter and type of connecting material do I need for my pump?
One of the most common practical questions I encounter when selecting accessories for pumps is quite simple: "What diameter of thread or pipe should I buy to make it fit?" Unfortunately, the answer is not as simple as it might seem – it depends on the type of pump, the pipe material, the operating pressure, the medium the pump is transporting, and a whole series of overlapping standards and norms used in practice. In this article, we will look at the entire issue systematically – from the basics of thread systems and pipe diameters, through the specific dimensions of common circulation, submersible, and sewage pumps, to recommendations for selecting the correct sealing and fastening elements.
If you are looking for a step-by-step installation guide, I recommend reading the article Step-by-step installation of accessories for a circulation pump, which you can find in the same Knowledge Center. Here, we primarily focus on dimensions, thread types, and material recommendations.
Why is the correct selection of diameter so important?
It may seem trivial, but an incorrect diameter of connecting material is one of the most common causes of water leaks, corrosion, and pump failures. From practical experience, I know that people often confuse the outer diameter of the pipe with the inner diameter, the nominal diameter DN with the inch designation of the thread, and also add confusion about which threads are tapered and which are not. The result is an installation that appears to "hold" at first glance, but after a few weeks it starts to leak or – in the worst case – causes a breakdown.
The correct selection of diameter also affects hydraulic losses in the system. A pipe that is too small increases the flow speed, creates turbulent flow, increases resistance, and raises the energy consumption of the entire system. On the other hand, a pipe that is too large can cause problems with fastening and sealing. There is also the aspect of cost – oversized accessories are not cheaper just because "bigger is better".
Thread systems used in practice – what do those abbreviations mean?
Before we move on to the specific diameters for different types of pumps, it is essential to understand the thread designation system. In plumbing and heating practice, we encounter several standards that are partially compatible and partially not.
G – Pipe thread (ISO 228)
The designation G (from the German "Gewinde") denotes a cylindrical pipe thread according to the ISO 228 standard. This is by far the most common designation you will encounter in practice for heating and water pumps. The G thread is cylindrical – this means that it does not seal by the thread itself, but by a seal (O-ring, Teflon, hemp fiber, or flat gasket). Examples: G½, G¾, G1, G1¼, G1½, G2. The number after G indicates the nominal inch size – but note, it is not the actual pipe diameter! G1 does not mean a pipe with a diameter of exactly 1 inch (25.4 mm), but a pipe with an outer diameter of 33.7 mm and a specific thread pitch.
R/Rc – Tapered pipe thread (ISO 7)
The designation R (external tapered) and Rc (internal tapered) is used where the tapered shape of the thread itself partially ensures sealing. The pitch is the same as for the G thread, but the thread is conical – when tightened, it wedges and partially seals even without a separate seal. In practice, you will encounter R½, R¾, R1, etc. The tapered R thread can be combined with the cylindrical internal thread Rp (previously designated Rc), but it is not an ideal solution in terms of long-term sealing.
NPT – American tapered thread
NPT (National Pipe Thread) appears mainly in pumps of American or Asian origin. It has a different pitch than European G/R threads, and although it may look similar, it is not compatible! NPT and G threads must not be combined without special reducing fittings. In Slovak practice, you will encounter it mainly in submersible pumps and some sewage pumps imported from Asia.
M – Metric thread
The metric thread (M8, M10, M12, etc.) is used in the area of pumps primarily for fastening elements – screws, nuts, clamps, flanges. It is not commonly used for pipe connections themselves, but for accessories such as cable clamps, clamps, or pump anchoring in a pit, it is a standard designation.
Standard dimensions of connections for circulation pumps
Circulation pumps for heating systems (boilers, floor heating, radiators) are among the most commonly installed pumps in households and industry. The typical size of the connections depends on the pump's power and the dimensions of the pipe into which it is mounted.
Small circulation pumps (single-family homes, apartments)
For standard single-family homes with boiler power up to 20–25 kW, circulation pumps with a G1½ or G2 connection are typically used. This is the most common format for pumps with an axial distance of 130 mm (also known as "Euroformat" or "Wilo-Grundfos dimension"). The most common connection dimensions include:
- G 1½" (DN 40) – the most widespread format for small circulation pumps, with a connection outer diameter of approximately 47.8 mm
- G 2" (DN 50) – for pumps with higher flow, larger homes, or multi-circuit systems
- G 1" (DN 25) – small pumps for utility circuits, solar systems, or zoning
Important detail: most modern circulation pumps (Grundfos Alpha, Wilo Stratos, Salmson, etc.) are delivered with a set of adapters that allow connection to various pipe diameters. So if you have purchased a pump with G1½ and your pipe is G1¼, the solution is a reducing fitting, not the replacement of the entire pump.
Larger circulation pumps (commercial buildings, industry)
In commercial applications, you will encounter pumps with flanged connections DN 50, DN 65, DN 80, DN 100 and more. Here the situation is more straightforward, as flanges are standardized according to PN6 or PN10 and are dimensionally and shape-wise prescribed. The connecting material – bolts, nuts, and gaskets – must be dimensioned for the system's operating pressure and the temperature of the medium.
Diameters and types of connections for submersible pumps
Submersible (well) pumps have their own logic. The threaded connection on the pump's discharge port corresponds to the pump's diameter and the expected flow. The larger the well diameter and the higher the flow, the larger the connection diameter.
- 4" submersible pumps (body diameter approx. 95–100 mm): commonly G1" or G1¼" discharge thread
- 6" submersible pumps (body diameter approx. 145–150 mm): G1½" to G2" discharge thread
- 8" and larger pumps: G2" and larger, sometimes flanged connections
For submersible pumps, it is particularly important to secure the discharge pipe in the well casing. Here, special connecting material is used – primarily stainless steel cable clamps, which are used to secure the pump's suspension cable. Cable clamps are standardized according to cable diameter: the most commonly used are for cables with a diameter of 4 mm, 5 mm, 6 mm, and 8 mm. Never combine a clamp for a different cable diameter than it is intended for – in the case of a pump falling into the well, it can lead to a financial disaster and safety risk.
More about the correct securing and sealing of the pump's piping can be found in the article How to properly seal and secure the pump's piping – cable clamps and couplings.
Sewage pumps – connection diameters and special features
Sewage pumps (pumps for pumping contaminated water, sludge, and sewage) generally have larger connections than circulation pumps of the same power, as they must transport a medium containing solid particles without clogging. Common connection diameters for sewage pumps:
- G 1¼" (DN 32) – small sewage pumps for households, basements, garden ponds
- G 2" (DN 50) – medium sewage pumps for construction sites, sumps, warehouses
- G 3" / DN 80 and larger – industrial sewage pumps, construction sites, sewer applications
In the case of sewage pumps, you will also encounter hose or flanged connections directly on the hose. The hose is connected using a hose clamp or a flanged fitting. The inner diameter of the hose must match the outer diameter of the pump's outlet – for example, if the pump has an outlet with an outer diameter of 51 mm, the hose must have an inner diameter of 51 mm as well. Do not confuse the hose diameter with the threaded connection diameter!
More about the selection of accessories for different types of pumps can be found in the article Selecting accessories according to pump type – circulation, submersible, sewage.
Materials of connecting accessories – what can withstand what?
Dimensions are only half of the decision. The other – and sometimes more important – part is the choice of material. From the perspective of corrosion, pressure, and temperature, the following basic groups are used in practice:
Brass and bronze
The most common material for valves, fittings, and connections in heating systems. Brass resists hot water up to about 110 °C, has good machinability, and relatively low cost. Caution: in systems with soft or demineralized water, dezincification of brass (leaching of zinc) can occur, leading to material porosity. For such applications, I recommend special "DR brass" (dezincification-resistant) or bronze.
Stainless steel (AISI 304 / AISI 316)
Stainless steel is practically the standard in the field of submersible pumps – the entire body of the pump is often made of stainless steel. For connections and accessories to pumps, stainless steel is ideal wherever corrosion is a risk: garden applications, wells, drainage, brine in heat pumps. AISI 316 (with molybdenum) is more resistant to chlorides than AISI 304. For cable clamps and suspension elements of submersible pumps, stainless steel is an absolute requirement – stainless steel cable clamps are the standard solution for these applications. More about the comparison of stainless steel and galvanized accessories can be found in the article Stainless steel vs. galvanized accessories for pumps – which is better.
Galvanized steel
Galvanized fittings and threaded nipples are cheap and mechanically durable, but have limited lifespan in environments with aggressive water or in conditions with alternating humidity. They can be used in closed heating circuits, but I do not recommend them for contact with potable water or in outdoor operations with high humidity (garden pumps, wells).
Plastic (PP, PVC, PE)
Plastic fittings and adapters are mainly used in drainage technology, garden pumps, and cold water distribution systems. The limitation is temperature – PP usually withstands up to 70–80 °C, PVC even less. In central heating, plastics have no place in most cases, but in low-temperature systems (floor heating, heat pump with a low-temperature circuit), plastic distribution lines and fittings are commonly used.
How to measure or determine the thread diameter of an existing fitting?
The most common problem customers come with is: "I don't know what thread I have on the pump." Here are proven methods from practice:
Method 1: Reading from documentation
Every pump has a technical datasheet, where the dimensions of the connections are listed. Simply search for the pump model online or in the attached manual. Look for markings such as "Connection G1½" or "Rp1" or "DN 50 PN6 flange". This is the most reliable method.
Method 2: Measuring the outer diameter of the thread with a caliper
If you don't have documentation, measure the outer diameter of the external thread with a caliper. Then compare it with a standardized table:
- Outer diameter of the thread approx. 20.9 mm → G ⅜"
- Outer diameter of the thread approx. 26.4 mm → G ½"
- Outer diameter of the thread approx. 33.2 mm → G ¾"
- Outer diameter of the thread approx. 41.9 mm → G 1"
- Outer diameter of the thread approx. 47.8 mm → G 1¼"
- Outer diameter of the thread approx. 53.7 mm → G 1½"
- Outer diameter of the thread approx. 59.6 mm → G 2"
Note: the measurement must be accurate to tenths of a millimeter. Thread G1 has an outer diameter of 33.249 mm – if you measure 33.7 mm, you are measuring the outer diameter of the pipe itself (OD), not the thread. Be sure to distinguish exactly what you are measuring.
Method 3: Thread gauge (thread comparator)
A thread gauge is a set of metal "combs" with different thread pitches. You place them on the thread and look for the one that fits without play. For pipe threads G, the pitch (number of threads per inch, TPI) is as follows: G½ = 14 TPI, G¾ = 14 TPI, G1 = 11 TPI, G1¼ = 11 TPI, G1½ = 11 TPI, G2 = 11 TPI. By combining the measurement of diameter and pitch, you can uniquely identify any thread.
Sealing materials – what for which thread?
Choosing the sealing material is just as important as choosing the diameter. Wrong sealing = leakage. From practice, I know these combinations:
- Cylindrical thread G – always requires a separate sealing: flat gasket (for fittings with a sealing surface), O-ring (for fittings with a groove), hemp fiber with sealing paste or PTFE tape (Teflon) for threaded connections without a sealing surface.
- Conical thread R – self-sealing, but for certainty, it is recommended to wrap PTFE tape or apply sealing compound (Loctite 577, Tangit Uni-Lock, etc.).
- Flanged connections – use a flat gasket made of rubber (EPDM, NBR) or spiral-wound metal. The thickness and material depend on the operating pressure and temperature.
Important practical note: Teflon tape is always wound in the direction of the thread (clockwise when viewed from above). If wound in the opposite direction, it will unwind during tightening and not seal. Also – more layers of Teflon do not mean better sealing, usually 3–5 turns are enough for small threads (up to G1") and 5–8 turns for larger ones.
Practical examples – specific situations and their solutions
Situation 1: Replacement of a circulation pump in a family house
The customer replaced the old Wilo RS 25/6 circulation pump with a more modern energy-efficient pump. The old pump had G1½ connections with an axial distance of 130 mm. The new pump (Grundfos ALPHA) has the same format. It is therefore sufficient to replace the gaskets (flat rubber gasket 1½"), unscrew the old pump and screw in the new one on the same threads. No reduction is needed.
Situation 2: Garden pump with a hose
The garden pump has a discharge thread G1". The customer wants to connect a 1-inch garden hose (inner diameter 25 mm). A transition piece is needed: G1" thread to a hose coupling for 25 mm hose. The hose clamp (hose fitting) must be for the range 23–27 mm – the standard range for a 1-inch hose.
Situation 3: 4" borehole pump with discharge G1"
The customer has a 40 m deep borehole and installs a 4" borehole pump. The discharge thread is G1", the discharge PE pipe has an outer diameter of 32 mm (1" PE hose for the borehole). A transition piece is needed for the discharge pipe, and above all a correctly dimensioned stainless steel rope clamp for a stainless steel rope with a diameter of 6 mm. The rope carries the entire weight of the pump (8–12 kg) plus the weight of the water column in the discharge pipe. With 40 meters of PE pipe G1", this can be an additional 2–4 kg. The clamp must be dimensioned at least for double this load.
Situation 4: Sewage pump and incorrect hose connection
The customer bought a sewage pump with a 50 mm (2") flanged outlet. He had a hose with an inner diameter of 47 mm in stock. The 3 mm difference caused the hose to stretch but not seal – at full pressure, the hose was pushed out. Solution: the inner diameter of the hose must always be the same as the outer diameter of the flange (or 0–1 mm smaller for an elastic hose), the hose clamp must tightly press along the entire circumference.
Most Frequently Asked Questions (FAQ)
How can I determine the thread diameter of my circulation pump if I don't have the manual?
The most reliable method without the manual is to measure the external thread diameter using a caliper. Most common household circulation pumps have G1½" connections (external thread diameter approx. 53.7 mm) or G2" (approx. 59.6 mm) with a center distance of 130 mm. If you measure a diameter of approx. 53–54 mm, it is G1½", and approx. 59–60 mm means G2". If you are still unsure, take a photo of the connection with the measuring tool and send it for evaluation – from experience, such a photo is usually sufficient to clearly determine the dimensions.
Can I combine a G (cylindrical) thread on the pump with an R (conical) thread on the fitting?
Technically yes – the threads will fit into each other, as the pitch is the same. However, in terms of sealing, this is problematic: the conical R thread will wedge into the cylindrical G thread and may damage it or wedge prematurely and not fully. For a temporary solution with sufficient PTFE tape, it may work, but for a permanent installation, I recommend always using the same type of thread on both sides, or using an intermediate fitting with the correct transitions.
How many layers of PTFE tape are sufficient for a G1" threaded connection?
For a G1" thread, 4–6 layers of PTFE tape wound in the direction of the thread are usually sufficient. For larger threads (G1½" and G2"), I recommend 6–8 wraps. More layers do not proportionally increase the seal – with too many layers, the PTFE may "float" and the connection may not be tightened to the correct torque. An alternative is sealing thread compounds (Loctite 55, Tangit) or hemp fiber with paste – these are more reliable for larger diameters and I recommend them for permanent heating installations.
What diameter rope and clamps do I need for a 4" submersible pump weighing 10 kg in a 60 m borehole?
For a standard 4" submersible pump up to 15 kg, I recommend a stainless steel rope with a minimum diameter of 4 mm (tensile strength approx. 1,000–1,200 N), but preferably 5–6 mm (tensile strength 1,500–2,500 N). Also consider the weight of the discharge pipe filled with water: for 60 m of PE pipe G1", this is an additional approx. 5–7 kg. The total load is thus approx. 17–22 kg (170–220 N). A 4 mm rope has theoretically sufficient strength, but I recommend a safety factor of at least 5–10×, so a 5 mm rope is minimum, ideally 6 mm. Always use stainless steel rope clamps of the appropriate diameter (for a 6 mm rope → clamp for 6 mm rope).
What if the pump has an NPT thread and I have European pipe fittings with G threads?
In this case, a special NPT-to-G transition fitting is necessary. These adapters are commonly available, but you must ensure the correct combination: external NPT (male) to internal G (female) or vice versa. Never attempt to drill an NPT thread directly into a G thread – the threads have different pitches and under force, the connection will either leak or damage the thread in the pump body or fitting. If you are unsure, it is better to remove the entire connection and have it evaluated by a professional.
What is the difference between the "nominal diameter DN" and the inch designation of the G thread?
DN (Diameter Nominal, nominal inside diameter) is a European standardized designation for the actual internal diameter of the pipe, rounded to whole millimeters. G denotes a pipe thread, where the number after G is a historical "inch" dimension that does not correspond to the actual pipe or thread diameter. For example, G1" does not mean a pipe with a diameter of exactly 1 inch (25.4 mm), but a pipe corresponding to DN25, i.e., with an external diameter of 33.7 mm and an internal diameter of approx. 26–27 mm (depending on the wall thickness). In practice: when a seller or technician refers to a "G1" pipe" or "G1 thread", they mean the same thing – a pipe thread according to ISO 228 for a nominal diameter of 25 mm. DN25 = G1" = "one-inch thread" – all three designations refer to the same standard dimension.
Summary – Quick Guide for Selecting Diameter and Type of Connecting Material
To conclude this section, I offer a practical overview to help you quickly orient yourself when selecting the correct connecting material:
- Circulation pump, family house, boiler up to 25 kW: G1½" (DN40), brass connections, flat rubber gasket, or PTFE tape
- Circulation pump, larger house or commercial building: G2" (DN50) or flanged DN50/DN65, material according to the medium
- 4" submersible pump: G1" or G1¼" discharge thread, 32 mm PE discharge pipe, stainless steel rope 5–6 mm, stainless steel rope clamps for the corresponding diameter
- 6" submersible pump: G1½" or G2" discharge, PE pipe 40–50 mm, stainless steel rope 6–8 mm
- Domestic sewage pump: G1¼" or G2" thread or hose output 32–50 mm, hose clamps, plastic or stainless steel according to the environment
- Garden surface pump: G1" thread, reduction to garden hose 25 mm, hose clamp 23–27 mm, brass or plastic
If you are unsure, it is always better to measure the connection before ordering, rather than relying on estimates. Correctly dimensioned connecting material is the foundation for safe, quiet, and low-maintenance operation of your pump for many years. For further information on selecting and combining accessories, I also recommend the article How to Choose the Right Accessories for Your Pump – What to Pay Attention to, as well as Common Faults and Water Leaks in Pump Accessories – Causes and Solutions and How to Avoid Corrosion and Damage to Pump Accessories in Heating Systems, which you can find in the same Knowledge Center.
Do you have a question on this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.