Fasteners for Electric Vehicles: Non-Magnetic & HV-Rated.

Electric vehicle mechanical components are the precision fasteners, washers and fixings engineered for four constraints that scarcely mattered in traditional ICE car design: high-voltage (HV) isolation, low magnetic permeability, thermal-cycling resilience and gram-level weight targets.

Every fixing inside a modern EV touches at least one of those constraints, which is why fasteners for EV applications have diverged sharply from their combustion-engine equivalents over the last decade. A misspecified bolt at the battery, busbar or inverter level can degrade efficiency, compromise safety or void certification.

This guide maps the fastener components for electric vehicles that design engineers specify most often, the material decisions behind them and the Accu ranges and products that supply them, with a production-facing lens informed by motorsport development work.

Contents:

• Differences of Fasteners in Combustion Engines vs Electric Vehicles

• Battery Pack Assembly: Components Under Load, Voltage and Heat

• Busbar Assembly: Where Mechanical Meets Electrical

• Charging Infrastructure: Built for Outdoors, Built to Last

• Motor, Inverter and Drivetrain Fixings

• From Formula Student to Production: What EV Racing Teaches Us

• Accu's EV Mechanical Components Range at a Glance

Differences of Fasteners in Electric Vehicles vs Combustion Engines.  

Combustion fastener specification is dominated by heat, vibration and torque repeatability, mature engineering challenges with well-understood solutions. The four EV constraints of high-voltage (HV) isolation, low magnetic permeability, thermal-cycling resilience and gram-level weight targets change which materials and grades engineers reach for first, even when the joint geometry looks identical to a combustion equivalent.

The practical difference in fastener selection shows up in material choice.

• A combustion engine rarely needs a non-magnetic bolt; an EV battery module specifies them by default near the Battery Management System.

• An ICE exhaust manifold prioritises heat resistance; an EV busbar prioritises electrical continuity during operation, plus corrosion resistance against salt and humidity. 

• Where a combustion engine might run generic 8.8 bolts across half its assembly, an EV specifies 12.9 for clamping-critical joints, BUMAX stainless for non-magnetic structural positions and Grade 5 titanium where rotating mass or range-critical weight dominates.

Each subsystem is governed by a different dominant constraint: isolation at the battery, current density at the busbar, weather resistance at the charger and weight at the drivetrain. The sections that follow tackle each engineering constraint in turn in relation to a specific example area on an EV.

 

Battery Pack Assembly: Components Under Load, Voltage and Heat.

Battery pack assemblies combine three mechanical demands that rarely coincide elsewhere: precise clamping force on cells that swell with state of charge, electrical isolation between hundreds of live connections and magnetic neutrality near the battery management system (BMS) and current sensors. Every EV battery fastener has to satisfy all three before earning its place in the assembly.

Electric car battery components are specified against the full constraint set, not just the dominant one, for example, a 12.9 Steel Bolt with perfect preload is still the wrong part next to a Hall-Effect sensor because its ferromagnetism distorts the current-sensing magnetic field.
The discipline of fastening for EV batteries is ensuring the selected fixings work to meet all mechanical demands, not just the primary one.

Low-Magnetic Permeability Stainless Belongs Near the BMS.

A4 (316 marine) stainless steel is the default choice for non-magnetic fasteners near the BMS, Hall-effect current sensors and shunt resistors.

In its solution-annealed state, A4 stainless steel sits well below the 1.01 µr (relative permeability) threshold engineers typically specify for sensor proximity, lending it exceptional resistance to becoming magnetic over time.

This resistance matters as austenitic stainless is not automatically non-magnetic. Cold work during rolling or installation raises permeability measurably and A2 grade shifts more than A4 under the same stresses. Where magnetic interference near sensors is non-negotiable, A4 retains lower permeability than cold-worked A2. BUMAX stainless steel variants cover applications that also need 12.9-class preload.

Clamping Force and Torque Control in Battery Modules.

Clamping force keeps the battery module stack compressed so cells, cooling plates and busbars cannot shift under vibration, shock, or thermal cycling. That force comes from bolt preload. In M6–M10 sizes, 12.9 high-tensile Socket Cap Head Screws to DIN 912 deliver the highest preload per diameter. Where corrosion resistance or low magnetic signature is needed, BUMAX delivers a comparable preload class in an austenitic stainless.

Torque alone can be unreliable. Thread friction varies with plating and lubricant, meaning some of the applied torque is consumed by that friction rather than converted to bolt tension. Torque-to-yield (TTY) tightening solves this by stretching the bolt past its elastic limit, producing a repeatable preload independent of friction coefficient.

Where TTY is not specified for a battery module, a controlled-friction thread coating such as AccuLock, Precote 80, or Anu-Lok 180 paired with controlled torque-based tightening narrows the calibration window.

Nylon 66 Shoulder Washers for High-Voltage Isolation.

What the EV industry calls HV insulated washers, Accu lists as Nylon 66 Shoulder Washers. The part has two insulating surfaces: a flange that sits under the bolt head and isolates it from the conductor face and a sleeve through the clearance hole that isolates the bolt shank from the conductor bore. Together, they break both short-circuit paths at the joint. Nylon 66 resists acids, oils, greases and industrial chemicals typical of battery enclosures.

They are usually specified with a paired standard flat washer on the back face for two-sided isolation, or a full Nylon, Polycarbonate, or PEEK Socket Cap Head Screw, where the entire fastener must be non-conductive. Typical positions include busbar terminations, BMS mounts and PCB fixings inside the high-voltage accumulator.

Cell Compression with Belleville Washer Stacks.

Battery pouch-cells swell during charge and shrink during discharge. The battery module has to absorb that dimensional change without losing contact pressure or crushing the cell.

A plain washer cannot adapt in this way: once torqued, it has no travel left. A Belleville Washer features a spring, which stores preload when deflected and releases it as the cell breathes, working to hold the module between roughly 100–300 kPa over the life of the battery pack.

Accu stocks Belleville Washers to DIN 6796 in A2 stainless and acetal, metric M3–M24, to ±0.13mm thickness tolerance. Acetal variants suit positions needing non-metallic compression near HV terminals.

Belleville Washer Stacks

Belleville washers are unique due to how they behave when stacked; the same washer can deliver softer travel or stiffer clamping depending on how it is oriented against its neighbours.

 

 

 

When In series: With the washers cupped in alternating directions, each washer deflects independently under the same applied load. Deflection roughly doubles per washer added to the stack while load stays constant, which gives the joint softer travel and a longer working range. This is the right arrangement where the bolted joint needs to absorb more dimensional change than a single washer could accommodate, such as larger temperature swings or cells with greater swell over their charge cycle.

 

 

 

 

 

 

 

When In parallel: With the washers nested the same way, the stack acts as a single thicker washer with a higher spring rate. Load roughly doubles per washer added while deflection stays constant, giving stiffer clamping and a higher holding force. This suits joints where preload retention is the priority and the dimensional change being absorbed is small, such as busbar joints under thermal cycling.

 

Busbar Assembly: Where Mechanical Meets Electrical.

Busbar joints are structural fasteners that must also carry current, which means material, finish and clamping choices all interact with resistance and heat generation across the life of the pack. A busbar bolt tightened correctly on day one can still fail the pack thousands of cycles later if contact force has relaxed or corrosion has intruded.

Bi-Metallic Corrosion Between Aluminium and Copper Busbars.

Aluminium and copper sit roughly 0.5V apart on the galvanic scale, an electrochemical ranking that lists metals by how readily they give up electrons in the presence of an electrolyte. The wider the voltage gap between two metals on the scale, the more aggressively the less noble metal corrodes when they touch in a wet environment. When moisture or salt bridges aluminium and copper, that 0.5V gap drives a small current through the joint. The aluminium becomes the anode and dissolves; the copper sits protected as the cathode. The result is oxidation at the interface, rising contact resistance, localised heating and eventually an open circuit. Pack humidity, road-spray salt and condensation from thermal cycling all accelerate the reaction.

Two mitigation steps stack well to help prevent this.
First, break the material couple at the contact face; tin or nickel plating on the busbar or more easily on the bolt head means the aluminium and copper never meet directly.
Secondly, isolate the fastener itself. A Nylon 66 Shoulder Washer plus a nylon sleeve keeps the bolt shank out of contact with both conductors and an EPDM-bonded sealing washer under the head blocks moisture ingress at the joint face.

These two measures are additive, not alternatives and production packs typically use both, coupled with other design considerations.

Belleville Washers for Contact Force Retention.

Busbars relax for a different reason than battery cells do.
Cells swell as previously discussed, whereas busbars cycle thermally under current load, expanding as they heat and loosening as they cool. The failure loop is well documented and self-reinforcing: fastener clamp load decays over time, resulting in contact resistance rising, heating increasing, the joint softening further and the busbar assembly deteriorating.

A Belleville Washer stack helps break the loop by holding axial force across its working deflection face, helping the joint keep preload as the metal breathes. Product choice at the busbar depends on bolt size and environment. Belleville in A2 Stainless Steel can handle enclosed packs. 

For dry, enclosed battery packs, A2 stainless Belleville Washers cover the standard structural range of metric fasteners. For smaller terminations, where the bolt size sits below the Belleville range, Accu's Disc Spring Washers in A4 (316) stainless take over. The A4 grade adds corrosion resistance at terminations exposed to humidity or salt  the same conditions that drive bi-metallic corrosion elsewhere on the busbar.

Material and Plating Choice for Busbar Bolts.

A busbar bolt carries two loads at once: mechanical preload holding the joint together and electrical current through the bolt body whenever the contact faces relax. Material choice sets the balance between the two.

A4 (316) stainless resists corrosion in humid or salt-exposed packs but has roughly 40 times the resistivity of copper, so it contributes meaningfully to joint heating at high current.
12.9 high-tensile steel delivers the highest preload per diameter for structural busbar joints, but bare steel oxidises at the contact face and needs nickel or tin plating to carry current cleanly. 

Plating choice then sets the torque figure. Different platings change the friction at the thread interface, which changes how much of the applied torque converts to actual bolt preload rather than being lost to friction. Published torque tables assume a specific plating and K-factor, so lubricated or dry-assembly K-factors are normally called out directly on the joint drawing.

 Charging Infrastructure: Built for Outdoors, Built to Last.

EV charging infrastructure hardware is specified for brutal operating environments: UV exposure, rain, salt spray, freeze-thaw cycling and mechanical impact, often across a projected 20-year service life. EV fixings here are selected as much for enclosure-sealing behaviour as for tensile strength and insulating properties.

UV-Stable Polymer Fixings.

Standard nylon embrittles under prolonged ultraviolet (UV) exposure, weakening the polymer and leading to surface micro-cracks. This results in the fastener losing tensile strength long before it begins to look degraded.

Correct material grade selection fixes this. 

• PA66 (Nylon 66) outperforms PA6 in UV stability for exterior fittings, including charging infrastructure. 

• PEEK and PPS handle continuous high-temperature cycling and aggressive chemical contact, such as electrolyte splash or coolant lines.

• Polycarbonate holds its impact strength at low temperatures better than nylon, which matters for cold-climate charging units. 

• RENY (glass-filled PA) sits where metallic strength is needed without conductivity.

Flammability matters equally. UL 94 is the Underwriters Laboratories standard that classifies how plastics respond to a small flame. A rating of V-0 is their highest vertical-burn rating, meaning the polymer self-extinguishes within 10 seconds and does not drip flaming material. V-0 is the baseline for charging hardware adjacent to live conductors. PEEK meets V-0 by default; Nylon and Polycarbonate need to be specifically formulated for the rating, depending on use so always check the UL rating of your fasteners or speak with our engineers.

Sealing Washers for Enclosure Integrity.

An IP rating of IP67 requires that an assembly survive temporary immersion in water. This rating not only applies to the general enclosure seal but also to that of any penetrating fasteners as well. Sealing Washers in 18-8/304 stainless with a bonded EPDM rubber ring deform under torque, flowing into surface irregularities on both the bolt head and the mating face to create a watertight seal.

EPDM resists ozone, UV exposure and water-based fluids, including coolant, screenwash, brake cleaner and salt-laden road spray, which makes it the default compound for charging inlets, junction-box lids and on-board charger enclosures. EPDM does not perform well against fuels, mineral oils or hydrocarbon solvents.
A suitable pairing in EV’s for sealing washers are A4 (316) stainless bolts, where salt spray is a concern or with PEEK/Polycarbonate variants, where the bolt itself must be electrically isolated.
When working with sealing washers its critical to torque the fastener to the washer manufacturer's spec, not the bolt spec, since over-compression permanently sets the EPDM and may result in leaks during the next service cycle.

 

Motor, Inverter and Drivetrain Fixings.

Motor, inverter and drivetrain assemblies place a different constraint set on fasteners than battery or busbar hardware. Here, the dominant problem is mass and specifically where the mass sits, because rotating mass affects acceleration, braking response and steering precision in a way static mass does not. Thermal cycling and vibration sit alongside weight as secondary constraints and the right fastener has to balance all three.

Grade 5 Titanium for Rotating and Structural Mass

Grade 5 Titanium (Ti-6Al-4V) is the default specification where weight saving matters most, providing the perfect solution for fasteners for motors in electric vehicles. At tensile strengths above 895 MPa and roughly 40% the density of steel, a titanium screw delivers comparable preload to stainless steel at a fraction of the rotational inertia. Suspension uprights, wheel hubs, brake caliper mounts and motor structural ties are the typical application zones. The cost premium over stainless rules titanium out as a general-purpose drivetrain fastener, but at any joint where rotating or unsprung mass directly affects performance, the weight saved can quickly earn the premium back.

Aluminium 7075 T6 for Enclosures and Static Structural Mounts

Aerospace-grade Aluminium 7075 T6 covers the second tier of weight-critical applications, where the joint is structural but not rotating. Inverter housings, motor covers, gearbox case fixings and accessory mounts are typical positions. 
The T6 designation refers to a heat treatment process that brings the alloy to peak strength, followed by a solution-treatment, which roughly doubles the aluminium's strength. The result is a fastener that holds threads cleanly under repeated assembly and disassembly, which matters at access panels and serviceable enclosures where a softer aluminium grade would gall or strip.

Vibration and Thermal-Cycling Protection

Drivetrain joints sit closer to the source of vibration than almost any other fastener position on the vehicle. Motor, gearbox, regenerative braking and road input feed continuous high-frequency oscillation into the surrounding bolted assemblies, alongside thermal cycling from inverter switching losses and motor heat.
Where battery clamps loosen through cell swell and busbar bolts through conductor thermal expansion, drivetrain bolts loosen through pure micro-rotation: each oscillation backs the bolt fractionally and the cumulative effect unwinds the joint over thousands of cycles.

Pre-applied threadlocking compounds are the standard counter, with selection driven by peak sustained joint temperature: AccuLock for ambient and warm running, Anu-Lok 180 for the middle band, and Precote 80 up to roughly 200°C at inverter-adjacent and motor-adjacent positions.

Nyloc nuts handle the geometric cases that pre-applied compounds cannot: through-bolted joints where the bolt is fixed by the assembly geometry and only the nut is accessible at final torque. A typical example is a motor mount where the bolt passes through a chassis bracket and clamps the motor housing on the far side; the bolt head sits captured against the bracket and cannot be patched, so the locking function has to live on the nut side. 

 

From Formula Student to Production: What EV Racing Teaches Us.

Formula Student EV programmes compress decades of production EV learnings into a one-year build cycle, making them a useful proving ground for fastener specification.
The University of Strathclyde Motorsport team's USM23 electric race car is a case in point, documented in the Strathclyde USM23 case study. Their accumulator design uses Accu's Nylon fasteners specifically for the HV isolation role, a specification pattern that has become standard across electric vehicle fasteners at cell, module and pack level. 

“The electrically insulating nylon fasteners from Accu are imperative for mounting safety-critical components in our high-voltage battery. As they're insulating, they allow us to secure everything safely without the risk of short circuits, protecting the driver and everyone involved.”

Taylor Phillips
Head of Accumulator, University of Strathclyde Motorsport.

The takeaway is that production EV battery design shares the same core specification problems as the USM23 accumulator, only at a different scale and cycle life. Weight budgets, HV isolation, magnetic neutrality and preload discipline translate directly from racing into production.

 

Accu's EV Mechanical Components Range at a Glance.

 

Every component has a free 3D CAD model on its product page and when purchased a Certificate of Conformity as standard, giving design teams the documentation trail needed for HV safety certification from prototype through to production.

For UK and US builds, AccuPro unlocks unlimited express delivery on every order, large or small. In-stock components dispatch the same working day on typical EV prototyping orders, which means a respecification from an A2 bolt to BUMAX, or from stainless to Grade 5 titanium, rarely costs more than a day on the build schedule. As one of the leading Electric Car component suppliers, Accu holds a comprehensive EV fastener range in stock across battery, busbar, charging and drivetrain applications.

Wrapping Up: Fasteners for Electric Vehicles.

To sum up our journey through EV fastener specification: the four constraints of high-voltage isolation, low magnetic permeability, thermal-cycling resilience and weight efficiency reshape material choice at every subsystem, from battery pack to busbar to charging enclosure. 

Getting the specification right at the fastener level is what keeps production EVs safe, efficient and certifiable across 20-year service lives. As a UK-based electric car fastener manufacturer and precision component supplier, Accu stocks the full EV range from a single catalogue with free 3D CAD downloads and a Certificate of Conformity as standard.

Further Reading.

• What Causes Galvanic Corrosion And How Can I Prevent It? 

• What Are IP Ratings?

• An Engineer's Guide To Titanium Grades

Frequently Asked Questions.

Q: What are the differences of fasteners in combustion engines vs electric vehicles?

A: EV fasteners layer four constraints on top of combustion specification: high-voltage isolation, low magnetic permeability, thermal cycling under current load and weight targets driven by driving range.
Combustion designs prioritise heat and vibration resistance; EVs add non-magnetic A4 or BUMAX stainless near sensors, Nylon 66 shoulder washers for HV joints and Grade 5 titanium for weight-critical rotating assemblies.

Q: Are all stainless steel fasteners non-magnetic?

A: No. Austenitic grades such as A2 and A4 (316) show low magnetic permeability when solution-annealed, but cold work from heading, rolling, or machining raises permeability measurably. A4 retains lower permeability under deformation than A2. Where proximity to Hall-effect sensors or a battery management system is critical, specify A4 or BUMAX and verify the as-supplied permeability on the material certificate.

Q: What material is best for busbar bolts in an EV battery pack?

A: It depends on the joint. A4 stainless suits corrosion-exposed positions where current density is moderate. 12.9 high-tensile steel with nickel or tin plating delivers the preload needed for high-current joints. Always specify plating, surface finish and the intended friction coefficient directly on the drawing.

Q: How do Belleville washers maintain contact force under thermal cycling?

A: Belleville Washers to DIN 6796 store elastic energy across their working deflection range. As the joint relaxes from thermal expansion or vibration, the washer releases that stored energy to maintain axial clamp load. Parallel stacks raise holding force; series stacks provide additional travel. Correct pre-load during assembly is critical to full-life contact-force performance on busbar joints.

Q: How do you prevent galvanic corrosion between aluminium and copper busbars?

A: Plate the contact surfaces with tin or nickel to break the galvanic couple. Use bi-metallic transition joints, typically friction-welded aluminium-to-copper strips, where a direct bolted interface is unavoidable. Isolate fasteners from dissimilar metals with Nylon 66 Shoulder Washers at the mechanical joint. Control moisture and salt ingress with sealed enclosures and EPDM-bonded sealing washers.

Q: Why are Nylon 66 shoulder washers used in high-voltage battery assemblies?

A: Nylon 66 Shoulder Washers electrically isolate a metal fastener from a live busbar, terminal, or PCB mount. The sleeve breaks the conductive path at the mechanical joint, preventing short circuits across HV terminals and reducing the risk of arc faults. 

Q: Are Grade 5 titanium fasteners suitable for EV battery modules?

A: Yes, for weight-critical structural applications. Grade 5 Titanium (Ti-6Al-4V) offers tensile strength above 895 MPa at roughly 40% the density of steel, with low magnetic signature and excellent corrosion resistance. Cost is higher than stainless, so titanium is normally specified for motorsport accumulators, structural tie-bars and weight-sensitive production EVs rather than every module bolt.