Which gasket to use for brass fittings – hemp fiber, teflon or O-ring

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Sealing Brass Pipe Fittings: Hemp, Teflon or O-Ring – What Really Works and When

When a plumber or a handy DIY enthusiast holds a brass pipe fitting in their hand, they usually first consider the thread type, diameter, and pressure class. Sealing? That is often treated casually – one just grabs what happens to be at hand. And this seemingly minor detail is behind most of the problems we encounter in practical work: leaking connections, clogged valves, damaged seals after a year of operation, or on the contrary, connections that cannot be disassembled after years due to being "secured" with completely unsuitable glue.

This guide examines the three most common sealing methods for brass threaded fittings – hemp (rope), PTFE tape (Teflon) and O-rings – from the perspective of the physics of the joint, material compatibility, working medium, pressure, temperature, and practical assembly. This is not a marketing overview, but a technical analysis with real values and scenarios from practice.

Why the sealing of a pipe fitting joint depends more on the seal than on the thread

A brass threaded pipe fitting joint is not airtight by itself. Neither the BSP thread (Whitworth) nor the NPT conical thread are designed to retain pressure without the help of a seal. The difference is that NPT (American conical thread) can approach self-sealing with sufficient tightening, but it is still unreliable without sealing compound or tape. The BSP parallel threads (BSPP, G-thread) are even more dependent on sealing – their function is purely mechanical – the thread holds the fittings together, but without a seal, the medium would freely pass through the gap in the thread profile.

In other words: the thread holds, the seal seals. Both functions are essential, but they are fulfilled by different parts of the joint. Choosing the wrong seal is therefore not just an aesthetic mistake – it is a direct cause of leakage or failure.

Three basic types of seals – overview and principle of operation

1. Hemp fiber (rope) with sealing compound

Hemp fiber is the oldest method and, in the hands of an experienced plumber, still belongs to the most reliable solutions for threaded fittings. It is a natural or semi-synthetic fiber (historically hemp, today often with added synthetic fibers), which is applied to the thread together with sealing compound – traditional oil-based (e.g. Fermit) or modern synthetic (e.g. Loctite 55 fiber, Unitec Hanf).

The principle is mechanical-compressive: the fiber is wound onto the thread in the direction of the thread pitch, and during tightening, the fiber is pressed into the thread profile and the compound impregnates it, creating a cohesive sealing mass that fills all the irregularities of the profile. When sufficiently compressed, the fiber "flows" into the gaps – this is its main advantage over rigid tape.

Advantages of hemp fiber:

Very resistant to vibrations – the fiber dampens mechanical vibrations that could loosen the joint
Usable on BSP and NPT threads, cylindrical and conical profiles
Excellent for larger diameters (1", 1¼", 1½" and more), where Teflon tape does not hold evenly
Easy to adjust position – the fitting can be turned backward after winding the fiber (unlike anaerobic seals)
Affordable, easily available in every hardware store
With the use of the right compound, resistant to temperatures up to 130–150 °C (depending on the compound)
Long service life in installed systems – decades without the need for replacement

Disadvantages and limitations:

Requires skill – improperly wound fiber (too little, too much, wrong direction) causes leaks
Wet fiber without compound deteriorates quickly – must always be impregnated
Not suitable for potable water systems in some countries (legislative restrictions on biologically active materials)
After prolonged standing in a dry environment, the fiber may dry out and the joint needs to be retightened

2. PTFE tape (Teflon)

Polytetrafluoroethylene (PTFE) tape, colloquially "Teflon", is today the most popular seal for brass threaded joints in domestic installations. Its popularity is understandable – it is clean, easy to apply, does not require compound, and works well on small diameters.

Physically, it is a thermoplastic fluoropolymer fiber pressed into a tape of thickness 0.07–0.1 mm (standard tape) or up to 0.2 mm (thick/plumber's tape). When wound around a thread and tightened, the tape deforms mechanically and is pressed into the thread groove similarly to fiber, but without compound – PTFE is itself an isotropic material with exceptionally low friction and good compressibility.

Advantages of PTFE tape:

Chemical inertness – resistant to most media including aggressive chemicals, acids, and alkalis
Suitable for potable water systems (certified according to WRAS, NSF, ACS standards)
Operating temperature up to 260 °C (PTFE itself), but for safety up to 120 °C in standard installations
Does not require curing time – the joint is immediately functional
Most cost-effective solution
Suitable for small diameters (1/4", 3/8", 1/2") with the correct number of windings

Disadvantages and limitations of PTFE tape:

Easy to tear during careless application – thin tape can break on threads
On larger diameters (over 1") insufficient thickness of standard tape
Vibrations can cause the joint to loosen – PTFE has no gripping properties like fiber
Too many layers can crack the internal brass thread of the fitting (hence the importance of the correct number of turns)
Not suitable for repeated disassembly and reassembly – the tape must always be removed and a new one applied
On damaged or worn threads, it does not hold reliably

3. O-ring (rubber seal, face seal)

An O-ring is a ring-shaped rubber or elastomeric seal of circular cross-section that seals not in the thread profile, but on the face of the fitting – so-called face-seal or flat-face sealing. It is used exclusively on parallel threads (BSPP, G-thread), where one side of the joint is terminated with a flat flange with a groove for the O-ring, or a flat face.

The principle is different from the previous two methods: the thread serves only for mechanical tightening and compression, and the sealing occurs via the compressive effect of the O-ring squeezed between two surfaces. It is therefore a surface seal, not a thread seal – the thread carries the mechanical load, while the O-ring carries the sealing load.

Advantages of O-rings:

Excellent repeatability – the joint can be disassembled and reassembled multiple times without replacing the seal (if not damaged)
Fast installation – no winding, no paste
Sealing does not depend on the quality of the thread – ideal for valves, instruments, and fittings
Available in many materials (NBR, EPDM, Viton, silicone) for various media and temperatures
Reliable under fluctuating pressures and thermal cycles
Suitable for hydraulic systems where regular checks and maintenance are required

Disadvantages and limitations of O-rings:

Not suitable for conical threads (NPT) or where a flat face is not provided
O-ring can be extruded if the groove is not of the correct dimensions
Rubber materials age – long-term exposure to heat and UV degrades the elastomer
During installation, the O-ring must be properly seated in the groove – a skewed position causes leakage
Not every brass fitting has a groove for an O-ring – it depends on the design

Material compatibility of seals with media, pressure, and temperature

Choosing a seal is not just a matter of the joint design – compatibility with the medium flowing through the fitting connection is crucial. In practice, we most commonly encounter the following media: cold water, hot domestic water, heating water, natural gas, propane-butane, compressed air, antifreeze (glycol), bathroom steam, and hydraulic oil.

Medium	Hemp fiber + paste	PTFE tape	O-ring NBR	O-ring EPDM
Cold water (drinking)	⚠ (legislation)	✔ excellent	✔ excellent	✔ excellent
Heating water (up to 110 °C)	✔ excellent	✔ excellent	⚠ depends on temperature	✔ excellent
Natural gas	⚠ only certified paste	✔ (yellow PTFE)	✔ NBR excellent	✗ not suitable
Propane-butane (LPG)	⚠ special paste	✔ yellow PTFE	✔ NBR excellent	✗ not suitable
Compressed air	✔ excellent	✔ excellent	✔ excellent	✔ excellent
Glycol (antifreeze mixture)	✔ excellent	✔ excellent	⚠ depends on concentration	✔ excellent
Hydraulic oil	✗ not suitable	✗ not suitable	✔ NBR excellent	✗ not suitable

✔ suitable | ⚠ conditionally suitable | ✗ not suitable. This table is for guidance only – always check the specifications of the specific seal manufacturer.

When to use which seal – specific practical scenarios

Scenario 1: Installation of extensions in central heating distribution systems

This is by far the most common case we encounter during renovations and new constructions. Extensions – for example, Extension 3/8" – 12 mm or longer Extension 3/8" – 30 mm – are installed in combination with valves, shut-off ball valves, or thermostatic valves on heating radiators.

For heating systems with operating temperatures up to 90 °C and pressures up to 6 bar, hemp fiber with sealing compound is the ideal choice. The reason is simple: the piping is firmly embedded, vibrations are minimal, but thermal expansion of the pipes causes micro-movements in the joints – and exactly the plastic fiber compensates for these micro-movements without loss of tightness. PTFE tape is also acceptable, but it may gradually lose compression after a larger number of thermal cycles (seasonal switching on and off of the boiler).

For extensions with a diameter of 3/8", I recommend 5–7 tightly wound layers of PTFE tape in the direction of the thread (for a right-hand thread – clockwise when viewed from the front). For hemp fiber, about 20–25 cm of fiber with compound applied in the same direction is sufficient.

Scenario 2: Gas piping – there is no room for experiments

Gas pipe fittings are the most strictly regulated in terms of sealing. Laws and standards (STN EN 1775, STN 38 6420) require the use of certified sealing materials. For threaded gas connections (BSP, G-thread), the following applies:

PTFE tape must be marked as suitable for gas (yellow tape for gas, thicker, certified for natural gas and LPG)
Sealing compounds must have gas certification (e.g. Fermit Gaz, Loctite 577 with appropriate certification)
Hemp fiber without compound is not acceptable; with compound certified for gas it is allowed, but installation must be carried out by a qualified person
O-rings on gas connections are common for battery connections and valves – the material must be NBR (nitrile rubber), EPDM is unsuitable for gas

In practice, we often encounter the mistake of using standard white PTFE tape on gas piping. Standard white tape is not certified for gas and may have serious consequences during inspection or in the event of an insurance incident.

Scenario 3: Drinking water piping – PTFE or special fiber

For drinking water piping (hot and cold water), the requirement for certification of sealing materials according to hygiene standards is becoming increasingly common in the SR and throughout the EU (directive ES No. 852/2004, KTW methodology in Germany, WRAS standard in the UK). Traditional hemp fiber with traditional compounds based on zinc oxide or linseed oil may not meet these certifications.

The safest choice for drinking water is PTFE tape with NSF 61 or WRAS certification. An alternative is synthetic fiber Loctite 55, which meets European hygiene standards. For larger diameters (over 3/4") or repeated assembly, it is suitable to use an EPDM O-ring (EPDM is certified for drinking water).

Scenario 4: Extensions during installation of measuring instruments and valves

Extensions such as Extension 3/8" – 15 mm or Extension 3/8" – 20 mm are commonly used for feeding flow meters, pressure gauges, air vents, or for extensions during installation of thermostatic heads. It is important here that the device will likely need to be disassembled in the future (battery replacement, calibration, malfunction). Therefore, it is more advantageous to use an O-ring (if available on the valve) or PTFE tape instead of hemp fiber with compound – the latter hardens over long-term installation and disassembly then requires more force, which can lead to cracking of the brass fitting body.

Correct application technique – step by step

Application of hemp fiber with compound

Preparation of the thread is key. The thread must be clean, free of oil, dust and old sealing residue. For cleaning, we use a wire brush or compressed air. Procedure:

We tear off about 20–35 cm of fiber (depending on the diameter – smaller diameter, less fiber).
We grasp the fiber and start winding it from the end of the thread towards the base – that is, in the direction of the thread pitch, clockwise for a right-hand thread.
We wind the fiber with moderate tension – it should be pressed into the thread profile, not just hang on the surface.
We apply a thin layer of sealing compound to the wound fiber – a finger movement along the entire thread is sufficient.
We screw the pipe fitting by hand as far as possible, then tighten it with a wrench to the required torque (for brass G1/2" typically 30–40 Nm, G1" about 60 Nm – do not over-tighten, brass can crack).

Application of PTFE tape

The thread must be dry and clean – PTFE tape does not adhere well to wet or oily threads.
We place the tape at the beginning of the thread (from the end of the thread) and press it firmly with our finger.
We wind it in the direction of the thread pitch, with slight tension, 4–6 layers for diameters 1/4"–1/2", 6–10 layers for 3/4"–1".
We press the end of the tape and tear it off – do not cut it with a knife, as the edge can scratch the thread.
We press the tape into the thread with our finger before screwing – this helps even distribution.
We screw by hand and then tighten with a wrench. IMPORTANT: after tightening, do not turn the fitting back – the PTFE will shift and the joint will not seal.

Mounting of O-ring

We check the condition of the O-ring groove – it must not be damaged or cut.
We slightly grease the O-ring (for water: clean vaseline or silicone grease; for gas: special gas grease; for oil: hydraulic oil of the system in use).
We insert the O-ring into the groove so that it is evenly distributed around the entire circumference – it must not be twisted or folded.
We tighten the threaded connection by hand, then with a wrench. The O-ring will be compressed – do not over-tighten, otherwise the O-ring will extrude from the groove.
After tightening, we visually check that the O-ring is not visibly pushed out of the groove.

Most common sealing errors with brass fittings

From the experience of dozens of customers and plumbers, the same mistakes are repeated over and over. The most common ones are:

Winding PTFE tape in the wrong direction – when tightening, the tape unwinds instead of being pressed into the thread. The result is no sealing and the leak is not prevented.
Too few layers of PTFE on larger diameters – 2–3 layers on G3/4" are insufficient; we need 8–10 layers of thick tape or we should use hemp thread.
Dry hemp thread without paste – dry thread quickly disintegrates, and within a few months the connection will start to drip or leak.
Rotating the fitting back by a few degrees for better positioning – with PTFE tape, this is a fatal mistake. The tape shifts and a gap remains in the thread profile. With hemp thread and paste, a slight rotation (max. 30° back) is acceptable, but only if the thread is still pliable (freshly applied).
Dry O-ring – a dry O-ring rotates during tightening and can be damaged by the thread grooves before the connection is sealed.
Over-tightening brass fittings – brass is a softer material than steel. Excessive torque will break the internal threads and the body of the fitting. For standard G-threads, the rule is "hand + ¼ turn with a wrench" – not "hand + 3 turns with a wrench".
Mixing sealing materials – for example, hemp thread layered over old PTFE tape, or vice versa – the result is unpredictable.

For more information on installation errors and their diagnostics, see the article Installation of brass threaded fittings – procedure, sealing and common errors and Common faults of brass fittings – why the connection leaks and how to fix it in this Knowledge Center.

Sealing on special types of brass extensions and fittings

Alongside standard cases, there are situations where the choice of sealing material requires additional consideration. Extensions such as Extension 3/8" – 40 mm are used, for example, when installing thermostatic valves on radiators, where the space geometry is limited. In such cases, PTFE tape is advantageous (thicker layer, easier handling in confined spaces), but for repeated seasonal installation/removal of the valve, we prefer an O-ring connection if the valve is structurally suitable.

With combined fittings (e.g., extensions with reducing threads), the rule applies: each sealing point must be treated separately. If a fitting connection has both internal and external threads, each threaded connection must be sealed individually during installation. It is not acceptable for the sealing from one connection to "stretch" to another.

Details on how to choose the correct combination of extensions, reductions, and other components can be found in the article Extensions vs. bushings vs. reductions – when to use which type of fitting. For correct dimensional matching of threads, we also recommend reading Thread standards BSP, NPT and metric thread – what size fitting do I need.

Certification, standards and legislation

In professional plumbing practice, the choice of sealing material is not only a technical decision – it is also a matter of compliance with standards. In Slovakia, the following key regulations apply to sealing in installations:

STN EN 10226 – thread standards for pipe threads (corresponds to ISO 7-1), relevant for sealing BSP connections
Decree of the Ministry of Health of the SR No. 508/2009 – safety when working with pressure equipment, pressure classes and requirements for seals
Government Decree of the SR No. 354/2006 – quality of water intended for human consumption – indirectly affects the choice of materials in contact with drinking water
DVGW W270 (German standard, but commonly required in SR) – microbiological safety of materials in contact with drinking water
Directive PED 2014/68/EU – pressure equipment, seals for equipment in higher pressure categories

For DIY enthusiasts, these standards are less relevant, but for anyone installing as part of a profession or in properties intended for rental, knowledge of them is essential.

Common questions (FAQ)

Can I combine hemp thread and PTFE tape on one connection?

No, combining different sealing materials on one threaded joint is not recommended. Both materials have different hardness, compressibility and thermal expansion behavior. The result is unpredictable and a leak can occur despite the joint being "well wrapped". Always use one material consistently across the entire threaded profile – either thread tape with paste, or PTFE tape, or an O-ring. Never mix them.

How many layers of PTFE tape is the correct number for a G 3/8" thread?

For G 3/8" (one of the most commonly used diameters for extensions and fittings) the standard is 4–6 layers of standard white PTFE tape (thickness 0.07–0.1 mm). If you use a thicker tape (0.2 mm, so-called plumber's tape), 3–4 layers will suffice. The correct number also depends on the quality of the thread – a new, sharp thread requires fewer layers than a worn, rounded thread. After tightening the joint, no white tape should be visible from the side, pushed out of the thread.

My PTFE tape joint is leaking after three years – is it enough to just tighten it?

No. If a PTFE tape joint starts to leak after years, the tape has either aged and lost its elasticity, or the joint has slightly shifted (due to thermal expansion, vibrations). The only correct repair is to completely disassemble the joint, thoroughly remove the old tape from both threads (using a wire), clean the threaded profile and apply fresh PTFE tape. Tightening the joint without disassembling is counterproductive – you may crack the brass body or the thread.

What O-ring to use in a heating system with a glycol-water mixture?

For glycol mixtures (ethylene glycol or propylene glycol) at temperatures up to 90 °C, an O-ring made of EPDM material (ethylene-propylene-diene monomer) is suitable. EPDM is resistant to glycols and heating water and has good thermal resistance up to 130–150 °C. NBR (nitrile rubber) is not suitable for glycol mixtures – glycol swells NBR and it degrades quickly. Viton (FKM) is also compatible, but significantly more expensive and not necessary for this case.

Can I use silicone sealant instead of sealing on threaded fittings?

Absolutely not. Sanitary silicone is not intended for sealing under pressure on threaded joints. Under water pressure or heating, the silicone is quickly squeezed out of the gap and the joint starts to leak. Silicone can be used only for external sealing (e.g. around bathtubs, showers – i.e. at atmospheric pressure), not as a substitute for threaded sealing. Over the years, we see similar "repairs" during service interventions and they are always a source of further problems – including mold growth at the leak site, which was "covered" with silicone.

Why is my joint leaking, even though I tightened it very strongly?

Brass is a relatively soft metal and over-tightening a threaded joint can paradoxically worsen the seal. If the internal thread of the fitting is bent or cracked due to excessive force during tightening, the thread profile deforms and the sealing material (cotton or PTFE) cannot compensate for such mechanical deformation. Moreover, over-tightening PTFE tape causes it to be pushed to the edges of the thread and a hollow space remains in the center. The correct procedure is: tighten with a wrench to the required torque, not to "maximum force". For standard brass fittings G 1/2", the recommended torque is 25–35 Nm, for G 3/4" around 50 Nm.

Conclusion – practical summary for common decisions

Selecting the correct sealing material for brass threaded fittings is not rocket science, but it requires knowledge of three key parameters: type of thread (tapered or parallel), medium (water, gas, air, oil) and the requirement for disassemblability. For most common heating and plumbing systems with BSP threads, a simple choice applies: cotton thread with sealing paste for fixed joints on larger diameters, PTFE tape for potable water systems and smaller diameters, and O-rings where the fitting or valve construction allows it and future service is expected.

For gas – always use certified solutions (yellow PTFE tape or approved paste) and ideally under supervision or directly carried out by a qualified person. For potable water – PTFE with hygiene certification or EPDM O-ring. For heating – cotton thread with paste is a proven and reliable solution that can handle thermal cycles without problems for decades.

If you are unsure about the choice for a specific case, the article How to choose brass threaded fittings – what to focus on before purchase or Common questions about brass threaded fittings in this Knowledge Center will help you, where you will find further scenarios and recommendations.

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.