.png)
Compression Fittings Q&A
A practical knowledge base covering the most common compression and tube fitting questions - leaks, over-tightening, ferrule (olive) make-up, material selection, and maintenance. Applies to brass and stainless steel (SS304/SS316) compression, tube, flare and instrumentation fittings.
Fitting Basics
What is a compression fitting and how does it work?
A compression fitting joins a tube or pipe without soldering, welding or flaring. It has three parts: a body with a tapered seat, a compression nut, and a ferrule (also called an olive). As you tighten the nut, it drives the ferrule into the tapered seat, squeezing it tight around the tube. This creates a leak-proof mechanical and metal-to-metal seal that also grips the tube against pull-out.
What are the three parts of a compression fitting?
Every compression fitting has: (1) the fitting body with an internal tapered seat and often a tube stop; (2) the compression nut that provides the tightening force; and (3) the ferrule or olive that deforms to form the seal. The ferrule is the working part — the nut and body simply drive and hold it in place.
What is a ferrule or olive and what does it actually do?
The ferrule (olive) is a soft metal ring that sits between the nut and the body. When compressed, it bites into the tube and wedges into the body's taper, sealing the joint and anchoring the tube. In a double-ferrule design the front ferrule seals while the back ferrule grips the tube — separating the two jobs for better vibration and pressure performance.
What is the difference between single-ferrule and double-ferrule fittings?
A single-ferrule fitting uses one ring to both seal and grip — simple and economical, common in general brass compression work. A double-ferrule fitting splits those duties: the front ferrule creates the seal and the back ferrule provides the tube grip and vibration resistance. Double-ferrule (twin-ferrule) designs are the standard for instrumentation and high-pressure gas/hydraulic service.
See our stainless steel tube fittings for double-ferrule instrumentation types.
What is the difference between a compression fitting and a flare fitting?
A compression fitting seals by squeezing a ferrule onto an unmodified tube. A flare fitting requires the tube end to be cone-flared with a flaring tool, then clamped between a male and female cone. Flare joints handle vibration and higher pressures well and are common in refrigeration, gas and automotive lines, while compression fittings install faster with no special tube prep. Explore our brass flare fittings range.
Do compression fittings need PTFE tape or thread sealant?
The compression seal itself — where the ferrule meets the tube — needs no tape or sealant; it is a mechanical metal seal. Never wrap the ferrule or tube. Thread sealant or PTFE tape is only used on tapered pipe threads (like NPT or BSPT) on the other end of the fitting, applied to the male threads only. Parallel threads (BSPP) seal on a face or washer, not the thread.
Rule of thumb: seal the pipe thread, never the compression end.
Installation
How do I install a compression fitting correctly?
1) Cut the tube square and deburr inside and out. 2) Slide the nut onto the tube first (threads facing the fitting), then the ferrule with its tapered edge toward the body. 3) Push the tube fully into the body until it bottoms on the tube stop. 4) Hand-tighten the nut, then hold the body with one wrench and turn the nut with another to the specified make-up. 5) Pressure-test before putting the line into service.
How tight should a compression fitting be? How many turns?
For a general single-ferrule/olive fitting, tighten to finger-tight, then roughly one-quarter to one-half turn past hand-tight with a wrench. For double-ferrule instrumentation fittings, the standard is 1⅓ turns (1-1/4) from finger-tight. Over-tightening is a leading cause of leaks — when in doubt, tighten in small steps and test. Make fine adjustments no more than 1/8 turn at a time.
Which way does the ferrule or olive face?
The ferrule's tapered (sloped) end faces into the fitting body, toward the tapered seat; the flat or barrel end faces the nut. Installing an olive backwards is a very common cause of leaks because the taper can't seat correctly. On double-ferrule sets, keep the front and back ferrules in their correct order and orientation.
Do I really need two wrenches to tighten a compression fitting?
Yes, for a reliable joint. Use one wrench to hold the fitting body steady and a second to turn the nut. If you turn the nut while the body rotates, the tube can twist, the ferrule can gall, and you'll transfer torque into the connected components — all of which cause leaks or damaged threads.
Why is deburring the tube important before installing a fitting?
A burr or ragged edge left by cutting can gouge the ferrule's sealing surface as it seats, leaving a leak path. Burrs also restrict flow and can shed debris into the system. Always cut with a proper tube cutter (not a hacksaw where avoidable) and deburr inside and outside before assembly.
How do I make up a double-ferrule tube fitting correctly?
Insert the tube fully until it bottoms on the fitting shoulder, hand-tighten the nut, then mark the nut at the 6 o'clock position and rotate it 1-1/4 turns (to 9 o'clock) while holding the body. This consistent make-up ensures both ferrules set correctly. On reassembly of a previously made-up joint, only 1/8 to 1/4 turn is needed.
Can I use a gap gauge to check if a fitting is tightened enough?
Yes — for double-ferrule instrumentation fittings, a gap inspection gauge is the professional way to confirm sufficient pull-up. After tightening, if the gauge will not fit into the gap between the nut and body, the fitting has been tightened enough. It's a quick, tool-free way to verify make-up without over-torquing.
Troubleshooting Leaks
Why is my compression fitting leaking?
The most common causes are: under-tightening (ferrule not fully deformed), over-tightening (crushed ferrule or damaged tube), a ferrule installed backwards, the tube not bottomed in the body, a scratched or reused ferrule, dirt/burrs on the sealing surface, or a size/material mismatch. Work through these in order before replacing parts.
My compression fitting still leaks after tightening — what now?
First, snug it in small increments — no more than 1/8 turn at a time — and retest. If it still weeps, depressurize, disassemble and inspect the ferrule and tube for scratches, cracks or a crushed ferrule. Replace a damaged ferrule, confirm the tube is bottomed and the ferrule faces the right way, then remake. If a pipe-thread end is weeping, that end — not the compression end — may need fresh sealant.
Can over-tightening cause a compression fitting to leak?
Yes — over-tightening is one of the biggest culprits. Excess torque can crush the ferrule past its elastic limit, deform the tube, or damage the nut threads, and none of that can be undone by tightening further. A crushed ferrule must be cut off and replaced. This is why counting turns (1/4–1/2 for olives, 1-1/4 for double-ferrule) matters more than “as tight as it goes.”
How do I fix a slow weeping or dripping compression fitting?
A slow weep usually means the ferrule is slightly under-set. Depressurize, then re-tighten 1/8 turn and test. If it persists, take the joint apart: check for a backwards or damaged ferrule, a burr, or debris on the seat, and confirm the tube is fully inserted. Replace the ferrule if it's scored or flattened, reassemble and pressure-test.
Why does my fitting seal liquid but still leak gas?
Gas molecules are far smaller than liquid, so a joint that holds water can still leak gas through a marginal seal. For gas service you need a properly made-up, undamaged ferrule — ideally a double-ferrule design — and leak testing with soap solution or, for critical systems, a helium leak detector. Never assume a no-drip joint is gas-tight.
Why does my fitting keep coming loose or leaking under vibration?
Vibration can back a nut out or fatigue an under-gripped tube. Double-ferrule fittings inherently resist vibration because the back ferrule grips the tube firmly — make sure it's properly made up. In high-movement zones, support the tube run with clamps, minimize unsupported spans, and consider vibration dampers. Re-check make-up on any joint that has loosened.
How do I detect a leak in a gas or air line fitting?
Brush a soap-and-water solution over the pressurized joint and watch for growing bubbles — the classic, safe field test. A hissing sound also indicates escaping gas. For high-purity or critical systems, use a helium leak detector. Always depressurize before disassembling to fix a confirmed leak.
Why did my new fitting leak right after installation?
Fresh-install leaks almost always trace to assembly: the tube wasn't bottomed in the body, the ferrule was reversed or omitted, the make-up turns were short, or a burr/debris sat on the seat. Recheck each of these. A brand-new fitting rarely leaks from a defect, though a size or thread-standard mismatch (e.g. wrong olive size) will also show up immediately.
My brass olive is cracked or deformed — can I still use it?
No. A cracked, flattened, scored or previously over-crushed olive will not seal reliably and should be replaced. Cut it off and fit a new one. Trying to reseat a damaged olive is a common reason a “repaired” joint leaks again within days.
Maintenance & Reuse
Can compression fittings be reused?
The body and nut can generally be reused if their threads and sealing surfaces are undamaged. Whether the ferrule can be reused depends on the type and its condition (see the next question). Always inspect every part, clean the sealing surfaces, and replace anything worn, scored or corroded before remaking a joint.
Can I reuse a ferrule or olive after taking the fitting apart?
A ferrule that has already been made up is deformed onto its original tube. A double-ferrule joint can usually be re-made onto the same tube with just 1/8–1/4 turn, because the ferrules are already set. Reusing an olive on a different tube, or reusing any cracked/flattened ferrule, is not recommended — fit a fresh one to guarantee the seal.
How do I remove a stuck compression fitting or olive?
Depressurize and drain first. Loosen the nut with two wrenches; if the olive is seized on the tube, a purpose-made olive/ferrule puller removes it without kinking the tube. As a last resort the tube can be cut behind the olive and re-prepared. Avoid levering with excess force, which distorts the body and tube.
How often should I inspect and maintain compression fittings?
Include fittings in routine plant/system inspections — visually check for weeping, corrosion and loosened nuts, and verify tightness on joints exposed to vibration or thermal cycling. Frequency depends on service severity: critical gas, hydraulic and instrumentation lines warrant more frequent checks than low-pressure water lines.
How do I stop compression fittings from corroding?
Match the material to the environment first — SS316 for corrosive, marine or coastal service, brass or SS304 for milder duty. Keep joints clean and dry, avoid trapping dissimilar metals in wet contact (galvanic corrosion), and address leaks promptly since standing moisture accelerates attack. In chloride-heavy environments, avoid plain brass.
See our stainless steel fittings for corrosion-resistant options.
What maintenance hacks help extend fitting life?
A few field-proven habits: always deburr and blow out tubes before assembly so debris never reaches the seat; count your make-up turns instead of over-tightening; keep a small kit of matching spare ferrules on hand; label made-up joints with a paint mark so you can spot a nut that has backed off; and support long tube runs to keep vibration off the joints.
A paint-pen witness mark across the nut and body makes a loosened fitting obvious at a glance during inspection.
How should I store spare fittings and ferrules?
Keep them clean, dry and sorted by size and material in labelled bins so you never mix a brass olive into a stainless job or grab the wrong size. Protect threads and sealing surfaces from nicks. Keeping brass and stainless separate also avoids cross-contamination that can promote corrosion.
Material Selection
Should I choose brass or stainless steel compression fittings?
Brass is economical, easy to machine and ideal for water, air, gas and general low-to-medium pressure work in non-corrosive settings. Stainless steel handles higher pressures and temperatures and resists corrosion, making it the choice for instrumentation, chemical, marine and demanding industrial service. Media, pressure, temperature and environment drive the decision. Compare our brass fittings and stainless steel fittings.
SS304 vs SS316 — which stainless steel should I choose?
SS304 is a versatile, cost-effective stainless for general industrial and indoor service. SS316 adds molybdenum, giving markedly better resistance to chlorides, salt water and aggressive chemicals — the right pick for marine, coastal, offshore and chemical-process applications. If corrosion is a real risk, the SS316 upgrade is usually worth it.
Are brass compression fittings safe for drinking water?
Brass is widely used for potable water, but in aggressive water chemistries ordinary brass can suffer dezincification (zinc leaching that weakens the fitting). For potable systems, especially where water is soft or high in chloride, specify dezincification-resistant (DZR/CR) brass or stainless steel, and follow your local potable-water standards.
Can I mix brass and stainless steel fittings in the same system?
Mechanically you can thread brass to stainless, but be cautious about galvanic corrosion where dissimilar metals meet in the presence of moisture — the less-noble metal corrodes faster. Keep such joints dry, use appropriate isolation where needed, and in wet or corrosive environments prefer a single compatible material throughout.
What are the temperature and pressure limits for brass vs stainless fittings?
Exact ratings depend on the specific fitting, size and wall thickness, so always work from the manufacturer's rating table. As a general guide, stainless steel tolerates higher pressures and temperatures than brass, which is why instrumentation and high-pressure hydraulic lines favour stainless. For any critical line, confirm the rated working pressure at your operating temperature. Need exact figures for your size and media? Ask our team for the rating table.
Which fittings resist corrosion best in marine or coastal environments?
SS316 is the standard choice for salt-laden marine, coastal and offshore air because its molybdenum content resists chloride pitting far better than SS304 or brass. Avoid plain brass in these settings due to dezincification and salt attack. Pair SS316 fittings with compatible SS316 tube for best results.
Sizing, Threads & Compatibility
How do I measure the right size compression fitting?
Compression fittings are sized to the outside diameter (OD) of the tube, not the bore — e.g. 6mm, 1/4", 8mm, 1/2". Measure the tube OD accurately with calipers and match the fitting to it. Getting this wrong (an olive too large or small for the tube) is a direct cause of leaks. Vikay offers sizes from 1/8" to 1" and 6mm to 25mm.
What is the difference between BSP and NPT threads?
NPT (National Pipe Taper, common in North America) and BSP (British Standard Pipe, common across much of the world) have different thread angles and pitches, so they are not interchangeable — forcing them together damages threads and leaks. BSP further splits into tapered BSPT (seals on the thread) and parallel BSPP (seals on a face/washer). Always confirm the thread standard before ordering.
Can I mix fitting brands, like one maker's nut with another's body?
It is not recommended. Different manufacturers hold different internal tolerances on ferrules, tapers and threads, so mixing brands (for example one brand's nut and ferrules with another's body) can prevent proper make-up and cause leaks or failures. Keep each connection all from one compatible system.
What is the difference between tube fittings and pipe fittings?
Tube fittings connect to thin-walled tube using a compression ferrule and are sized by tube OD — ideal for instrumentation and precise routing. Pipe fittings join threaded or welded pipe sized by nominal bore (NB) and are built for structural, higher-flow plumbing runs. Choose based on whether you're running tube or pipe.
Browse tube fittings and pipe fittings.
What sizes and materials does Vikay Fittings offer?
Vikay supplies compression, tube, flare and instrumentation fittings and valves in brass and stainless steel (SS304 and SS316), in sizes from 1/8" to 1" and 6mm to 25mm metric, across BSP and NPT thread standards. The range covers elbows, tees, unions, connectors, adapters, bulkheads, ferrule fittings and a full valve line. Request a quote for specifics.
Applications & Selection
What industries use compression and instrumentation fittings?
They're used across instrumentation and process control, hydraulics and pneumatics, oil & gas, chemical and petrochemical, power generation, water treatment, mining, and general industrial piping. Anywhere tubes must be connected reliably and often re-serviced, compression and double-ferrule fittings are the go-to. Vikay supplies these sectors in over 20 countries.
Are compression fittings suitable for high-pressure hydraulic systems?
Yes, when correctly specified. Stainless steel double-ferrule fittings are designed for high-pressure hydraulic and instrumentation service and grip the tube firmly against pressure surges and vibration. Always match the fitting's rated working pressure to your system's maximum, use the correct tube wall thickness, and make up to the specified turns.
Can compression fittings be used underground or buried?
They can, but buried joints are hard to access and monitor, so material and protection matter. Use corrosion-resistant material for the soil conditions, protect against galvanic corrosion and moisture, and where codes require it, use fittings rated and approved for buried service. Where possible, keep joints accessible in a chamber or sleeve.
Which fittings are best for instrumentation and analytical systems?
Stainless steel double-ferrule (twin-ferrule) tube fittings are the industry standard for instrumentation, sampling and analytical systems — they give leak-tight, gas-tight, vibration-resistant, remakeable connections on small-bore tube. SS316 is preferred where corrosion or high purity is a concern. See our stainless steel tube fittings.
How do I choose between compression, flare and welded fittings?
Pick compression/ferrule fittings for fast, tool-light, remakeable tube connections in instrumentation and hydraulics. Choose flare fittings for vibration-prone gas, refrigeration and automotive lines where a flared cone seal is preferred. Use welded fittings for permanent, highest-integrity pipe joints where the connection won't need servicing. The trade-off is serviceability versus permanence.
Compare flare fittings and weld fittings.
Still not sure which fitting you need?
Our engineers help specify the right brass or stainless steel fitting for your pressure, media and thread standard.
Disclaimer: The information in this Q&A is provided for general guidance only. Fitting selection, pressure/temperature ratings, material suitability and installation practices vary by application. Always check with your consultant or a qualified engineer before selecting or using any fitting in your system.