Internal thread vs external thread - differences and use
Internal and external thread on gas supply hoses - why the whole installation depends on it
When a customer in a store or e-shop says "I need a hose for the stove," the technician will almost automatically ask: "What thread do you have on the valve and on the appliance - internal or external?" This seemingly small question determines whether you take home exactly what you need or end up standing in the kitchen with a hose you can't screw onto anything. The difference between internal and external threads is fundamental to any proper gas connection installation - and despite that, it is one of the most common causes of complaints and unnecessary second trips to the store.
In this article, we will thoroughly examine what internal and external threads actually mean, how these types are combined on gas supply hoses, what dimensions and standards are used in practice, how to recognize the thread on-site without special measuring instruments, and what mistakes are most commonly seen when selecting and installing. The goal is for you to be able to confidently say after reading what type of hose you should order - without guessing and without unnecessary returns.
Basic principle - what is internal and what is external thread
A thread is a helical groove that allows two parts to be joined by rotational movement and create a solid and ideally gas-tight connection. With pipe (threaded) threads used also on gas supply hoses, we distinguish two basic types depending on where the thread is located:
- External thread (male, "external") - the thread is cut into the outer surface of the part, typically at the end of the pipe, coupling or hose connector. The part is inserted into the other, the thread is visible and palpable from the outside of the part.
- Internal thread (female, "internal") - the thread is cut into the inner hole of the part. This part is screwed onto the external thread of the other part, the thread is "hidden" inside, you have to look into it or feel it with your finger.
Gas supply hoses almost exclusively use cylindrical (pipe) threads of type G according to the EN ISO 228 standard (formerly known as "Whitworth" inch threads), most commonly in sizes G1/2" (pipe inner diameter approx. 1/2 inch, thread outer diameter approx. 20.9 mm) and G3/4" (thread outer diameter approx. 26.4 mm). Less frequently, we also encounter G3/8".
Why both ends are mentioned in the hose name
On gas supply hoses, the combination of thread types on both ends is commonly mentioned in the product name, for example, "internal - internal". This designation indicates that the hose has an internal thread on both sides, which is screwed onto the external thread of the gas valve on one side and the external thread of the appliance connector on the other side. This combination is most common in households and with most appliances with a fixed threaded outlet, because standard gas taps (ball valves) and stove or cooktop inlets typically have an external thread of G1/2".
Types of connectors on gas supply hoses
In practice, we encounter three basic combinations of connectors on hoses:
- Internal - internal (II) - both ends of the hose have an internal thread. This configuration is the most universal for connecting a gas stove or cooktop, where the distribution side has an external thread of the ball valve and the appliance side has an external thread of the inlet.
- Internal - external (IE) - one side is screwed onto an external thread (e.g., a valve), the other side itself provides an external thread onto which a reducer, adapter or directly an appliance with an internal inlet thread is screwed.
- External - external (EE) - both ends of the hose have an external thread, which is used when both the valve and the appliance have an internal thread (typical for example with some gas heaters or industrial applications).
In our category gas supply hoses, you will find the most popular variant of internal - internal in various lengths, specifically 500 mm, 750 mm, 1000 mm, 1250 mm and 1500 mm. This configuration covers the vast majority of standard home installations, as almost all ball valves intended for connecting appliances have an external thread and the inlets of stoves, ovens and cooktops also do.
Standards and dimensions - which threads actually occur
The majority of gas supply hoses and fittings for household use in Europe are manufactured according to the cylindrical pipe thread G (EN ISO 228-1), which is marked in inches, even though it is a metric standard with an inch-based origin. In the table below you will find the most common dimensions you will encounter with gas connections:
| Designation | External diameter of the thread (mm) | Typical use |
|---|---|---|
| G3/8" | ~16.7 mm | Smaller cooking plates, some older appliances |
| G1/2" | ~20.9 mm | Cookers, hobs, gas taps - the most common dimension in households |
| G3/4" | ~26.4 mm | Higher capacity gas boilers, industrial and commercial applications |
It is important to understand that the number itself (1/2", 3/4") refers to the nominal bore of the connection, not the hose diameter or wall thickness - this is a matter of the flow cross-section, which is dealt with separately and in detail in the article "How to choose the diameter of a gas hose according to the type of appliance". Here we focus exclusively on the type and orientation of the thread.
How to distinguish internal and external threads in practice
In the field, we often encounter situations where the customer does not have a caliper or thread gauge at hand, yet still needs to quickly and reliably determine what type of thread their valve or appliance has. There are several simple rules:
- Visual inspection - look directly into the connection opening. If you see grooves inside the opening (on the walls of the cavity), it is an internal thread. If the thread is visible on a protruding "neck" or collar, it is an external thread.
- Finger touch - if visibility is poor (for example, under a cooker in dim lighting), carefully run your finger along the inner wall of the opening. If you feel grooves inside, it is an internal thread.
- Test with an old component - if you have an old hose or fitting available, place it against the connection. A part that can be "slid into" requires an external thread on the hose; a part that is to be "placed on top" requires an internal thread on the hose.
- Photograph with a ruler - take a photo of the connection with a ruler or a coin of known diameter placed next to the thread, and send the photo to the seller. An experienced technician can almost instantly determine the size and type based on the diameter and type.
In practice, the safest rule is: gas ball valves almost always have an external thread on the side where the appliance connects (the inlet from the wall or distribution system often has an internal thread, onto which the valve is screwed with an external thread from its other side). Similarly, the inlet neck of most cookers and hobs has an external thread. This is why the hose configuration internal-internal is the most popular in our range - it simply fits both sides of the most common installation.
Why mixing thread types is a problem - real scenarios from practice
Over the years of selling and installing gas connections, the same situations repeat themselves almost constantly, when the customer buys the wrong combination:
Scenario 1: Replacing an old cooker with a new one
The customer buys a new cooker, the old valve remains in place. They assume that the old hose will fit the new appliance, because "after all, it's a cooker like any other". In reality, some older appliances have an internal thread on the inlet neck (especially older Italian or other older models), while most modern cookers have an external thread of G1/2". If the old hose was of the external-internal type, it will not fit the new appliance and a new hose with an internal-internal configuration must be purchased.
Scenario 2: Kitchen renovation and relocation of the valve
During renovation, the gas distribution location is changed and a new ball valve is installed. The plumber installs a valve with an external thread (a common standard), but the customer buys a hose with an external fitting on both ends without checking what type of thread the valve has. The result - the hose does not fit, it must be returned or an adapter must be purchased, which delays the installation by several days while the correct part is sourced.
Scenario 3: Connecting a gas grill or terrace stove
With portable appliances (grills, terrace stoves, LPG cooking units), atypical threads or even a different type of connection (e.g., quick coupling) often occur, not the classic G-thread. In such cases, it is essential to first verify the type of connection directly in the appliance's documentation, otherwise you risk buying a completely incompatible hose.
In all these cases, a simple rule applies, which we recommend to every customer: always check the type of thread on both sides of the connection before ordering the hose - on the valve and on the appliance. A detailed guide on how to choose a specific hose according to the appliance can be found in the article "How to choose a gas supply hose for an appliance."
Sealing of threaded connections - the difference between internal and external connections
The type of thread (internal/external) not only determines how the parts are mechanically connected, but also affects the sealing method. With cylindrical G-threads (unlike conical threads such as NPT or R, which are sealed by the cone itself), the seal of the connection is usually ensured by a separate sealing element:
- Flat gasket - the most common solution for gas supply hoses. A rubber, fiber or graphite gasket is placed between the front face of the external thread and the front face of the internal thread. When tightened, it compresses and creates a gas-tight connection.
- String with sealing compound - used mainly for rigid metal distribution systems, less so for hoses with a ferrule.
- Teflon tape (PTFE) - more suitable for conical threads, not usually required for cylindrical threads with flat sealing, but some fitters use it as an additional protection on the thread itself (not on the sealing surface).
Important practical warning: standard plumbing Teflon tape or classic plumbing sealant without certification for gas must not be used on gas lines. Always use only materials and seals specifically intended by the manufacturer for gas connections, ideally original seals supplied with the hose.
Adapters and reductions - when types don't match
If you find that your connection and appliance have incompatible thread types or dimensions (e.g., a valve has an external G1/2" thread and the appliance has an internal G3/4" thread), the solution is a reduction or adapter. In most cases, however, it is simpler and safer to choose a hose with the correct combination of fittings directly, rather than chaining multiple adapters together - each additional connection in the chain represents another potential leak point and another point that must be checked during inspection. Experts recommend using a maximum of one adapter between the valve and the hose, ideally none at all.
When choosing the correct combination, we recommend following this order:
- Determine the type and size of the thread on the gas valve.
- Determine the type and size of the thread on the appliance's inlet.
- Select a hose whose fittings exactly match both sides - most commonly internal-internal for the combination of two external threads on both sides of the installation.
- Verify that the size (G1/2" vs G3/4") fits on both sides - the type (internal/external) and the size are two separate properties and both must match simultaneously.
- Choose the appropriate length - refer to the article "What length of gas hose is suitable for your appliance."
Stainless steel vs rubber hoses and the impact on fitting type
The type of thread (internal/external) is independent of the material of the hose itself - both stainless steel and rubber gas connection hoses are produced with the same types of fittings, since the fitting is a metal fitting that is either slipped over or screwed onto the hose, regardless of whether the flexible part is made of stainless steel corrugation or rubber with reinforcement. The differences between these two materials (lifespan, flexibility, resistance to mechanical damage) are discussed in detail in the article "Comparison of stainless steel and rubber gas hoses." From the perspective of thread selection, however, the same rules apply to both types.
Installation and tightening - specifics according to thread type
When installing a hose with an internal thread onto an external thread valve, it is important to follow several guidelines that vary slightly depending on which side of the connection has the internal and which has the external part:
- Screwing in - the internal thread (a nut or hose fitting) is slipped over the external thread of the valve and manually screwed in a few turns to prevent misalignment (so-called "cross-threading"), which can damage the threading.
- Tightening - after manual installation, the connection is tightened with a suitable flat or box wrench to the recommended torque (usually specified in the hose's instructions; as a rough guide, this means tightening "until snug plus approximately a quarter to half a turn," not over-tightening with universal pliers).
- Seal check before installation - during each installation, check that an undamaged seal is inserted into the fitting with the internal thread. A missing or compressed, hardened seal is one of the most common causes of gas leaks, as discussed in the article "Common causes of gas leaks in connection hoses."
- Do not use pliers on the metal corrugation - when using stainless steel hoses, the tightening wrench or pliers must not be applied to the flexible corrugation itself, only to the solid fitting (hexagonal or angular part of the fitting).
A complete installation procedure, including recommended tightening torques and leak testing, can be found in the separate article "Step-by-step installation of a gas connection hose." To verify the result, we also recommend reading "How to check the tightness of a gas connection."
Practical table - quick overview of combinations
| Valve (supply side) | Appliance (connection side) | Required hose |
|---|---|---|
| external thread | external thread | internal - internal (most common) |
| external thread | internal thread | internal - external |
| internal thread | internal thread | external - external |
| internal thread | external thread | internal - external (reversed orientation) |
As the table shows, the internal-internal combination is repeated in the vast majority of domestic installations, since the long-standing standardization of ball valves and appliance inlet nozzles in Slovakia and Europe has settled on an external G1/2" thread on both sides of the connection. Therefore, this configuration forms the core of our product range.
When is it time to replace the hose regardless of thread type
Regardless of whether you have correctly selected the type and combination of threads, the hose itself has a limited lifespan. Rubber hoses should be replaced approximately every 5 years (according to the production date marked on the hose), while stainless steel hoses have a longer lifespan, but they also need to be replaced immediately if there are signs of corrosion, deformation, or damage. Details on exactly when to replace the hose can be found in the article "Lifespan and replacement of gas hoses - when is it time for service."
Common questions about internal and external threads in gas hoses
How do I know whether my gas valve has an external or internal thread?
Look directly at the valve's end where the hose connects. If you see threading on the protruding cylindrical cap, it is an external thread - this is the most common case for standard ball valves in households. If the threading is inside the opening, it is an internal thread. In case of uncertainty, we recommend placing the old hose against it or consulting a seller with a photo.
Can I buy an external-external hose even if I have an external thread on both the valve and the appliance?
No, in this case you need the exact opposite - a hose with internal threads on both ends (internal-internal), because the internal thread of the fitting is screwed onto the external threads of the valve and the appliance. A hose with external fittings would not fit at all on external threads.
Is there a difference in sealing between internal and external connections?
The principle of sealing for the cylindrical G-thread is the same regardless of whether the internal or external part is on which side - sealing is done with a flat gasket between the mating faces of both parts, not by the thread itself. The difference is only in which part physically holds the gasket (it is usually seated in the fitting with the internal thread).
Can I combine different sizes, for example G1/2" internal on one side and G3/4" on the other?
Yes, there are hoses and reducer fittings with different sizes on both ends, but it must be a deliberately manufactured and certified combination, not an improvised connection of mismatched sizes by force. Before purchasing such a hose, always verify the exact size on both sides of your installation, ideally with the help of the article "What diameter gas hose to choose according to the type of appliance."
What if my appliance has a completely different type of connection than a thread (e.g., a quick-connect coupling)?
Certain modern appliances or portable devices (camping stoves, portable burners) use quick-connect couplings or other types of fittings instead of the classic G-thread. In such cases, you need a hose or adapter specifically designed for that type of coupling, and a classic internal/external G-thread combination cannot be simply screwed on. Check the type of connection in the appliance's manual before ordering.
Can an external thread be "converted" to an internal one if I bought the wrong hose?
No, the type of thread is a fixed structural property of the fitting and cannot be modified at home. The only solution is to purchase the correct hose with the appropriate combination of fittings or a certified adapter from a professional seller.
Conclusion
The difference between internal and external threads is a simple concept, but in practice, incorrect identification of the type and combination of fittings is the most common reason why customers have to replace or purchase additional adapters. The rule to remember is clear: always verify the type of thread on both sides of the connection - on the gas valve and on the appliance's inlet - and choose a hose with an exact matching combination of fittings. For most domestic installations with a standard ball valve and a common stove or cooktop, this practically means a hose with an internal-internal configuration, which you can find in our category in lengths from 500 mm to 1500 mm. If you are unsure, it is always better to consult a seller or installation technician before ordering, rather than risk unnecessary replacement, lost time, or, in the worst case, improvised "solutions" on the gas line, where safety is always the priority.
Do you have a question on this topic?
Not sure what to choose or facing a specific situation in your home? Write to us – we will be happy to help.