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A washer controls the interface beneath a fastener
Bolt tension becomes compressive force at the bearing face. A washer can spread that force across a larger area, provide a known friction surface or bridge an oversized clearance hole.
Its hardness and geometry must resist the load without cupping or embedding. Adding an arbitrary washer changes grip length, thread engagement and tightening behaviour.
Washer families
| Type | Main function | Typical application | Service concern |
|---|---|---|---|
| Plain flat washer | Spreads load and protects face. | General bolted joints. | Diameter, thickness and hardness. |
| Hardened structural washer | Supports high-strength bolt preload. | Suspension, chassis and driveline where specified. | Chamfer and grade must match. |
| Crush/sealing washer | Deforms to close a fluid joint. | Drain plugs, banjos and sensors. | Usually single-use and material-specific. |
| Bonded sealing washer | Metal ring controls elastomer compression. | Hydraulic/fluid fittings. | Fluid and temperature compatibility. |
| Belleville/disc spring | Provides axial spring force. | Clutches, bearings and thermal joints. | Cone direction and stack arrangement. |
| Wave washer | Takes up light axial clearance. | Motors, trim and bearing preloads. | Limited load and installed height. |
| Tab washer | Mechanically locks by bent tab. | Hub, gearbox or shaft nuts. | New part and exact tab location. |
Load spreading and bearing stress
Soft surfaces
Aluminium, plastic and slotted brackets can crush beneath a small head. A specified large washer reduces local pressure, but its edge must not interfere with a radius or moving part.
High-strength joints
A high-preload bolt needs a washer hard enough to avoid permanent indentation. Embedding relaxes bolt tension after tightening.
Selection checklist
| Check | Variation | Risk if wrong |
|---|---|---|
| Inside diameter | Close, normal or oversize clearance. | No fit or poor support around hole. |
| Outside diameter | Standard, large-series or application shape. | Insufficient spread or interference. |
| Thickness | Grip, stiffness and spacing function. | Reduced engagement or geometry change. |
| Hardness | Soft sealing metal to hardened steel. | Dish, crack or fail to seal. |
| Surface/coating | Zinc, phosphate, plain or low-friction. | Corrosion and torque relationship change. |
| Geometry | Flat, chamfered, conical, wave or tabbed. | Wrong orientation/function. |
| Material compatibility | Fluid, heat and galvanic pairing. | Swelling, corrosion or leakage. |
Sealing washer mechanisms
A soft-metal washer conforms to microscopic surface variation under controlled clamp. A folded crush ring collapses through a designed range. A bonded washer uses a metal outer ring to prevent its elastomer squeezing out.
Each needs clean undamaged seats. Extra torque cannot compensate for a groove, crack or tilted fitting and may strip the parent thread.
Copper, aluminium and bonded seals
| Seal | Useful property | Limitation |
|---|---|---|
| Annealed copper | Ductile and heat tolerant in specified fittings. | Work-hardens and can promote galvanic reaction. |
| Soft aluminium | Conforms well against aluminium/steel faces. | Exact alloy and thickness matter. |
| Folded metal ring | Predictable collapse over larger movement. | Orientation and single-use rule. |
| Metal-elastomer bonded seal | Low clamp sealing with controlled extrusion. | Rubber must suit fluid and heat. |
| Fibre washer | Conforms in selected low-pressure joints. | Not a general modern fuel/hydraulic seal. |
| PTFE/special polymer | Chemical resistance for designed applications. | Creep and installation procedure vary. |
Banjo-bolt washers
A banjo fitting normally uses one seal on each side of the banjo. Ports in the bolt must align with its internal annulus; adding extra washers can move them and reduce thread engagement.
For brake and fuel systems, fit exact new seals and torque once. Bleed or prime the system and inspect under controlled pressure. Never reuse a washer to avoid opening a safety-critical hydraulic circuit later.
Spring and Belleville washers
A Belleville washer has a load-deflection curve determined by cone height, thickness and material. It can be stacked in series for travel or parallel for force. Reversing one washer changes the stack.
A split spring washer flattens under relatively low load and may not prevent loosening in a correctly preloaded joint. Do not add it where not specified.
Tab and keyed washers
A tab washer locates on a key, flat or housing and bends against a nut after tightening. The locking tab must reach the specified flat without reducing nut torque.
Always fit a new washer when the tab has been bent. Repeated bending work-hardens and cracks it. Do not substitute a chisel stake into an unrelated plain washer.
Chamfer and orientation
Hardened washers can have a chamfer to clear the radius under a bolt head. Placing the sharp side against that radius holds the washer off the surface and concentrates load.
Cupped, serrated and sealing washers also have direction. Record orientation during dismantling, then verify from service information rather than copying a possibly incorrect prior repair.
Washer condition
| Finding | Meaning | Action |
|---|---|---|
| Dished flat washer | Too soft/thin or overloaded. | Replace with specified grade; inspect joint. |
| Crushed sealing washer | Plastic deformation already consumed. | Renew. |
| Split bonded elastomer | Overcompression, heat or fluid attack. | Replace and confirm compatibility. |
| Bright fretting marks | Joint movement or lost preload. | Diagnose fastener and faces. |
| Cracked spring/tab washer | Fatigue or repeated bending. | Replace immediately. |
| Heavy corrosion | Section and friction altered. | Renew and restore protection. |
| Edge interference mark | Wrong OD or off-centre fit. | Use exact geometry. |
Spacers are not ordinary washers
Calibrated shims and spacers set bearing preload, sensor gaps and alignment. Replacing them with general washers changes thickness and flatness. Keep measured shims assigned to their locations.
Do not stack washers to correct a bolt that is too long or a bracket that does not fit. Correct the part and joint geometry.
Thermal joints and washer stacks
Exhaust, turbocharger, battery and electric-machine joints can expand at different rates. A disc spring or specially hardened washer may preserve clamp as parts heat and cool. Replacing it with a flat mild-steel washer removes that spring characteristic even when the dimensions look similar.
Stacked Belleville washers face the same direction when higher force is required and opposite directions when more travel is required, but the exact arrangement must come from the assembly drawing. Surface coating and edge condition influence how each disc slides during compression.
In a mixed stack, the order of plain washer, spring element, seal and nut can determine both friction and contact area. Photograph the untouched assembly, keep components grouped and renew any disc that has flattened, cracked or lost free height. Do not add a second spring washer to increase locking; it changes installed height and bolt stretch.
Friction and torque
The washer's surface finish affects under-head friction, which consumes much of tightening torque. A low-friction plated washer can raise bolt tension compared with a rough corroded one.
Use the exact dry/lubricated procedure and supplied washer. Do not coat a wheel, hub or structural washer with anti-seize unless specified.
Safe installation sequence
- Identify washer type, size, hardness, material and orientation.
- Inspect fastener, hole and both mating faces.
- Remove corrosion and old seal without scratching the seat.
- Fit the washer flat and centred, with chamfer or cone correctly directed.
- Start fastener fully by hand and confirm thread engagement.
- Apply only the specified lubricant or locking compound.
- Tighten in sequence using calibrated torque/angle method.
- Complete tab bending, split pin or other locking stage.
- Pressure-test fluid joints and clean residue.
- Inspect after the stated heat/load cycle where required.
Common mistakes
- Selecting by hole diameter alone.
- Using a soft washer under a high-strength fastener.
- Reusing drain or banjo sealing washers.
- Stacking washers to shorten a bolt.
- Adding a spring washer not specified by the joint.
- Installing a chamfered or Belleville washer backwards.
- Lubricating a dry-torque bearing face.
- Rebending a used tab washer.
Safety and roadworthiness
Washers used in brakes, steering, suspension, wheels, hubs, seats and driveline joints are safety-critical. Missing or wrong parts can release clamp or fluid unexpectedly.
Insecure components and fluid leakage can affect UK MOT inspection. Restore the exact fastener stack rather than improvising a visually similar ring.
Automotive washer FAQs
Q: What does a flat washer do?
A: It spreads load, protects the face and can provide controlled friction.
Q: Are all washers the same hardness?
A: No. Soft sealing washers and hardened structural washers serve opposite roles.
Q: Can a crush washer be reused?
A: Generally no; its designed deformation has already occurred.
Q: Why are two washers used on a banjo?
A: They seal both faces of the banjo fitting.
Q: Can washers be stacked?
A: Not unless the joint explicitly specifies a stack.
Q: Does a split washer prevent every nut loosening?
A: No. Use only the locking method engineered for the joint.
Q: Which way does a chamfer face?
A: Usually towards the bolt-head radius, but follow the part procedure.
Q: What is a Belleville washer?
A: A conical disc spring providing controlled axial force and travel.
Q: Can a tab washer be bent twice?
A: No. Repeated bending can crack the work-hardened tab.
Q: Can a general washer seal oil?
A: Only the specified sealing material and geometry should be used.
Q: Why does coating matter?
A: It changes corrosion protection and tightening friction.
Q: Can a washer fix a bottoming bolt?
A: Do not improvise; use the correct bolt and joint dimensions.
Q: Can a missing washer affect the MOT?
A: It can when clamp, security or fluid sealing of a required component is compromised.
