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Most washers simply sit in a joint and take up space. A few are asked to do something harder: stop a fluid from escaping. A sealing washer, sometimes called a bonded seal or a crush washer, is used at drain plugs, hydraulic fittings, brake unions, gas charged damper fill points, and any flange connection where the joint must be both mechanically tight and hydraulically dry. The mechanical and sealing requirements pull in different directions, which is why a sealing joint that relies on the wrong washer type will either leak or work loose, and sometimes both.
This guide covers the sealing side of the washer family: how a sealing washer actually closes a joint, which material suits which fluid and temperature, how the dimensions and the tightening torque interact, and the failure modes to expect when a sealing joint is specified without considering the seal function at all.
A washer becomes a sealing element when it is placed at a joint where fluid could otherwise travel along the mating faces and out past the fastener. The geometry of that joint is almost always imperfect: a machined face has tool marks, a cast surface is rough, and a thread has a helical path that fluid can follow. The sealing washer closes that path by deforming into the surface, by being compressed against a raised face, or, in the case of a bonded washer, by combining a metal backing with an elastomer bead that presses against the mating surface.
The distinction matters because a sealing washer is normally a single use part. Once it has been crushed into the surface, it has taken a set, and reusing it means reusing a part that no longer has the free height needed to deform and seal again. This is the most common cause of a joint that leaks after a routine service.
Soft metal and elastomer washers seal by conforming. Copper, aluminium, and fibre washers are softer than the faces they are clamped between, so the clamping force forces the material into the surface irregularities and closes the leak path. The amount of deformation is controlled by the torque applied and by the initial thickness, which is why the thickness tolerance on a crush washer is a functional dimension rather than a nominal one.
A bonded washer works differently. The metal backing is stiff and carries the clamp load, while a raised elastomer lip is compressed against the mating face to form the seal. Because the elastomer is confined in a narrow band, the local contact pressure is much higher than the average pressure across the washer, so a good seal is achieved with a modest bolt load. This is the reason bonded seals tolerate lower and less precise torque values than crush washers.
| Type | Sealing mechanism | Best suited to | Reuse |
|---|---|---|---|
| Copper crush washer | Plastic deformation into the faces | Sump plugs, brake unions, high temperature joints | Single use |
| Aluminium crush washer | Softer deformation, lower torque | Light alloy housings, low torque joints | Single use |
| Bonded seal (metal plus elastomer) | Compressed elastomer lip on a rigid backing | Hydraulic fittings, dampers, high pressure joints | Single use |
| Fibre washer | Fibre compresses and swells slightly | Water and low pressure fuel joints, older equipment | Single use |
| PTFE washer | Soft conforming, chemically inert | Aggressive fluids, chemical lines, low pressure | Single use |
| Nylon washer | Conforming, insulating | Water fittings, insulation, light duty sealing | Single use |
The choice begins with the fluid and the temperature, then moves to the pressure and the bolt load available, and finally to the space envelope. A copper washer will seal a brake union at high pressure but is a poor choice where the fluid attacks copper. A bonded seal with a nitrile elastomer is excellent for mineral oil at moderate temperature but will harden and fail in a high temperature application where a fluoroelastomer is required.
Copper is the traditional choice for high pressure and high temperature sealing on threaded joints, particularly brake unions and sump plugs. It is ductile even when relatively hard, it does not take a permanent set until it is actually crushed, and it tolerates the small surface imperfections found on a used joint. Copper also has good thermal conductivity, so it does not create a hot spot at the joint.
Two practical rules apply. First, annealed copper seals at a lower torque than work hardened copper, so a washer that has been sitting in stock for years in a hardened condition may need more torque than the drawing assumes. Second, copper and aluminium are far apart in the galvanic series, so a copper washer on an aluminium housing creates a corrosion couple. Where an aluminium housing is involved, an aluminium or bonded washer is usually the better choice.
The elastomer in a bonded washer determines the fluid and temperature compatibility, and it is the part that actually seals. The most common materials are nitrile, EPDM, and fluoroelastomer. Selecting between them is not a matter of preference; each has fluids it tolerates well and fluids that will destroy it.
| Elastomer | Temperature range | Good with | Avoid with |
|---|---|---|---|
| Nitrile (NBR) | Roughly -30 to +100 C | Mineral oil, hydraulic oil, water, air | Ketones, esters, strong ozone exposure |
| EPDM | Roughly -40 to +120 C | Water, steam, brake fluid, glycol | Mineral oil and hydrocarbon fuels |
| Fluoroelastomer (FKM) | Roughly -20 to +200 C | Fuels, oils, aggressive chemicals, high temperature | Some ester based and phosphate ester fluids |
The temperature range is a guideline for continuous service. Intermittent excursions above the upper figure shorten the life of the elastomer, and cold below the lower figure makes it stiff, so the seal may not form at first pressurisation in a cold climate.
The metal backing carries the clamp load and prevents the elastomer from being extruded out of the joint under pressure. A thicker backing supports higher pressure; a very thin backing can be dished by the bolt load and lose the preload on the elastomer bead. Where the joint sees pressure pulses rather than a steady pressure, the backing stiffness is the determining factor in fatigue performance.
Where the fluid attacks metals or elastomers, PTFE is often the only practical option. It is chemically inert across a very wide range, it works from cryogenic temperatures up to a few hundred degrees, and its low friction helps when torque has to be kept low. Its limitation is the same one that affects it in any load bearing role: it creeps, so a PTFE washer under a sustained high load will thin over time and reduce the joint preload. Where that matters, a PTFE faced metal washer, or a PTFE envelope washer over an elastomer core, combines chemical resistance with dimensional stability.
Fibre washers are a lower cost option for water and low pressure fuel joints. They seal by compressing and by absorbing a small amount of fluid, which makes them forgiving of imperfect faces, but they degrade with temperature and they do not tolerate aggressive solvents.
The inner diameter must clear the thread or the stud and allow the washer to sit flat against the sealing face. Too small and the washer will ride up the thread instead of seating. The outer diameter sets the sealing face width, which combined with the bolt load determines the contact pressure. On a raised face or a spot faced joint, the washer outer diameter has to fit inside the face, so the seat diameter should be measured rather than assumed.
Thickness on a crush washer is a working dimension rather than a finished dimension, because the part is designed to be reduced when it is tightened. The starting thickness, the expected crush, and the finished thickness should be considered together, particularly where the fastener has limited thread engagement. A washer that is too thick can run the fastener out of thread engagement before the seal is formed, which is a common reason for a joint that will not stop weeping.
Every sealing joint has a torque window. Below the lower limit the washer does not deform enough to close the leak path. Above the upper limit the washer is crushed flat, the elastomer is extruded or split, or the housing thread is stripped. Bonded seals and crush washers have different windows, and a torque value taken from a bonded seal specification should not be applied to a copper washer, or the reverse.
Reuse is the other rule. A sealing washer that has been tightened once has already taken its set. Reusing it on the next service means the joint is sealed by a part with reduced free height and, in the case of a bonded seal, an elastomer bead that has already been compressed. The saving is negligible compared with the cost of a leak, and most leaks reported immediately after a service trace back to a reused sealing washer or a joint face that was not cleaned.
| Failure | Usual cause | How to confirm |
|---|---|---|
| Weeping at the thread | Washer not deformed enough, low torque, wrong inner diameter | Remove and inspect for an unmarked, undeformed face |
| Leak after a service | Washer reused, or the joint face not cleaned | Compare washer thickness with a new part |
| Elastomer split or extruded | Over torque, pressure above the backing rating | Inspect the bead for a split ring shape |
| Elastomer hardened and cracked | Temperature above the material limit, incompatible fluid | Check the elastomer grade against the duty |
| Corrosion at the joint | Galvanic couple between washer and housing material | Identify both metals and check the couple |
| Joint loosens over time | Washer creeping or crushing further under load | Check preload, consider a stable material or a locked fastener |
Suspension components use sealing washers in several places. Gas charged dampers have a fill point that must be sealed after charging, and a bonded seal is the usual answer because it tolerates the pressure and can be fitted and torqued quickly on a production line. Hydraulic lines, ride height sensors, and height control valves use sealing washers at every union. Where a damper is rebuilt, the fill point washer should always be replaced, because the original part has already been crushed and, in a nitrogen filled unit, the pressure will find any path that remains.
In these applications the washer is a pressure retaining component, not a spacing item, so the specification should name the elastomer, the metal backing, and the corrosion protection. For the wider set of damper components that work with these joints, see the shock absorber parts catalogue.
Before specifying a sealing washer, confirm the fluid and its compatibility with the chosen elastomer; the maximum and minimum service temperature, including cold start; the working pressure and whether it is steady or pulsating; the bolt or plug load that can be applied and the resulting torque window; the material of the mating housing, to avoid a galvanic couple; the space available for the washer at the sealing face; and the fastener thread engagement that remains after the washer thickness is accounted for. A specification that answers all seven points will almost always work the first time.
No, if the joint has already been tightened. Copper seals by deforming into the mating surfaces, so the original thickness and hardness no longer apply and the washer may not develop enough deformation to seal again. Reuse also risks under or over torquing the joint on the second assembly.
A crush washer is a soft metal part that seals by deforming across its whole face, and it needs a reasonably high and consistent torque. A bonded seal combines a stiff metal backing with a small elastomer bead, so the high contact pressure is concentrated on the bead and a good seal is achieved at a lower and less critical torque.
The three most likely causes are a reused sealing washer, a joint face that was not cleaned of old washer material, and a torque value taken from the wrong specification. Check these three before considering a design change, because they account for the large majority of immediate post repair leaks.
A fluoroelastomer is usually the right choice above roughly 100 C, with nitrile below that and EPDM reserved for water, steam, and glycol duties. Confirm compatibility with the actual fluid as well as the temperature, because an elastomer can be temperature capable and still be attacked by the fluid.
Send the fluid, the temperature and pressure duty, the mating materials, the thread size, and the space available, and our engineers will propose the washer type, the elastomer or metal, and the torque window. Any shape, size, and thickness can be produced to drawing or sample, and both single pieces and batch quantities are available. Review the washer and spacer range, look at the wider shock absorber parts catalogue, check the custom manufacturing service, or contact our sales engineers with your requirement.
