Eccentric bolt: device, purpose and rules of use in a car

When it comes to fasteners in the automotive industry, most people think of standard bolts, nuts or screws. But there is one variety that often remains in the shadows, despite its versatility - eccentric bolt. This unusual fastener with an offset thread center solves problems that a regular bolt cannot handle: from adjusting gaps to fixing parts at non-standard angles.

In this article, we will look at what an eccentric bolt is, how it works, and why you can’t do without it in a number of automotive jobs, from installing wheels to repairing suspension. You will learn where exactly this fastener is used, how to choose it correctly, and what installation errors lead to breakdowns. We'll also compare eccentric bolts with alternative solutions so you can choose the best option for your application.

What is an eccentric bolt and how does it work?

Eccentric bolt is a fastening element in which the axis of the threaded part is offset relative to the axis of the head. It is this displacement (eccentricity) that allows you to adjust the position of the parts when tightening. Outwardly, it resembles a regular bolt, but with one key feature: its geometry is asymmetrical.

The operating principle is based on the rotation of the bolt around its axis. When the head is rotated, the threaded part describes a circle, the radius of which is equal to the eccentricity value. For example, if the eccentricity is 2 mm, then with a full turn of the bolt, its threaded part will move by 4 mm (the diameter of the circle). This property is actively used for precise fitting of parts without the need to sharpen or modify them.

It is important to understand that eccentric bolts come in two types:

• 🔧 With fixed eccentricity — the offset is set at the production stage and does not change. Such bolts are used when a strictly defined displacement is required (for example, for wheel rims).
• 🔄 With adjustable eccentricity — allow you to smoothly change the displacement value during installation. More often used in suspensions or mechanisms where dynamic adjustment is needed.

In the automotive industry, the most common type of bolt is the first type - fixed offset. They are often confused with eccentric bushings or eccentric washers, but these are different details. The bolt performs the fastening function, and the bushings and washers serve for alignment or compensation of gaps.

Design and materials: what are eccentric bolts made of?

Structurally, an eccentric bolt consists of three main parts:

1. Head - can be hexagonal, turnkey or with an internal polyhedron (for example, Torx or Allen). The shape depends on the application: for cars, hex heads with an open-end wrench are more often used.
2. Eccentric part - the area between the head and the thread, where the axis shifts. There may be a thickening or a special shape to increase strength.
3. Threaded part - standard metric or inch thread corresponding to the mounting holes of the parts.

Materials for manufacturing are selected based on the loads:

• 🛠️ Carbon steel (brands St3, St45) - the most common option for general purpose bolts. Strength grade 8.8 or 10.9 sufficient for most tasks.
• 🔩 Alloy steel (for example, 40X) - used for bolts operating under conditions of high loads or vibrations (suspension, transmission).
• 🛡️ Stainless steel (brands A2, A4) - relevant for parts in contact with moisture or aggressive environments (for example, in the engine compartment).
• 🔥 Titanium alloys — used in racing cars or tuning, where minimum weight with high strength is important.

Coatings also play an important role:

• ⚫ Oxidation — black matte layer that protects against corrosion (often used for class bolts 8.8).
• ✨ Galvanizing (galvanic or hot) - silver shine, rust resistance.
• 🟡 Yellow or white zinc — additional protection for working in aggressive environments.

Critical nuance: wheel bolts are often coated with a layer diamond-like carbon (DLC), which reduces friction when tightening and prevents “sticking” to aluminum discs.

Where are eccentric bolts used in a car?

The scope of application of eccentric bolts in cars is wider than it seems at first glance. They are used wherever required:

• 🔧 Hole misalignment compensation - when the fastening points of the parts do not match perfectly.
• 📏 Adjusting the gaps - for example, between a brake disc and a caliper.
• 🔄 Fixation at an angle - when the part needs to be fixed not perpendicular to the surface.

Let's look at the key areas of application:

Scope of application	Specific example	Typical Bolt Size	Eccentricity
Wheels	Mounting discs with non-standard PCD (bolt pattern)	M12×1.5, M14×1.25	1–3 mm
Suspension	Wheel alignment adjustment (in levers or struts)	M10×1.25, M12×1.75	2–5 mm
Transmission	Mounting the gearbox to the engine with displacement compensation	M8×1.25, M10×1.5	0.5–2 mm
Body work	Installing bumpers or body kits with mismatched mounts	M6×1.0, M8×1.25	1–4 mm
Tuning	Installation of non-standard parts (spoilers, guardrails)	Individually	Up to 10 mm

Eccentric bolts are especially widely used during installation non-original wheels. For example, if your car has a bolt pattern 5×112, and you want to install disks with PCD 5×114.3, then you can’t do without eccentric bolts with an offset of 2.15 mm. They allow you to “adjust” the mount so that the disk sits evenly, without distortions.

In suspension, eccentric bolts are often built into silent blocks or levers for adjusting wheel alignment angles. This is cheaper and more reliable than using shims or spacers.

Advantages and disadvantages of eccentric bolts

Like any technical element, eccentric bolts have their pros and cons. Their correct use provides significant benefits, but errors in selection or installation can lead to serious problems.

Benefits:

• ✅ Versatility — one bolt can replace several standard fasteners by adjusting the offset.
• ✅ Installation accuracy — allow you to achieve the ideal position of parts without mechanical modification.
• ✅ Save time — no need to drill new holes or sharpen parts.
• ✅ Strength — when properly tightened, they can withstand loads no worse than standard bolts.
• ✅ Compactness — do not require additional elements (for example, spacers or adapters).

Disadvantages:

• ❌ Difficulty of selection — you need to know exactly the required eccentricity and thread size.
• ❌ Risk of self-unscrewing - vibrations can cause the bolt to rotate, changing the displacement. Solved by using lock nuts or thread lockers.
• ❌ Limited adjustment — a fixed eccentricity does not always allow for a perfect fit.
• ❌ Price — high-quality eccentric bolts are more expensive than standard ones.

⚠️ Attention: Never use eccentric bolts to fasten parts that experience shock loads (e.g. wheel bearings or steering components). Axle misalignment can lead to uneven force distribution and premature wear.

In some cases, alternatives to eccentric bolts include:

• 🔧 Eccentric bushings — allow you to adjust the position of the part, but require additional fasteners.
• 📐 Adjusting washers - compensate for gaps, but do not provide such accuracy as bolts.
• 🔩 Adapters - used to change PCD disks, but increase wheel offset (ET).

How to choose an eccentric bolt: step-by-step instructions

Selecting an eccentric bolt is a task that requires attention to detail. An error in size or eccentricity may result in the part not fitting into place or the fastening being unreliable. Here is an algorithm that will help you avoid mistakes:

Step 1: Determine Bolt Type

• For rims bolts with a fixed eccentricity corresponding to the difference in PCD.
• For pendants or bodywork Adjustable offset bolts may be needed.

Step 2. Measure key parameters

• 📏 Thread diameter — measure the standard mounting hole with a caliper or use the vehicle documentation.
• 🔄 Thread pitch - for example, M12×1.5 (12 mm diameter, 1.5 mm pitch).
• 🔧 Bolt length - must be sufficient to pass through the parts being connected and ensure reliable tightening.
• ↔️ Eccentricity — the amount of axis displacement. For disks it is calculated as the difference between original and new PCD, divided by 2.

Step 3: Select Material and Finish

• For wheels alloy steel with zinc coating is optimal or DLC.
• For pendants suitable grade steel 10.9 with oxidation.
• For bodywork you can use stainless steel A2.

Step 4: Check compatibility

• For discs: the bolt must fit PCD, diameter of the central hole (DIA) and departure (ET).
• For suspension: make sure the bolt does not interfere with other components (such as brake hoses).

⚠️ Attention: When purchasing disc bolts, avoid cheap, unmarked analogues. Low quality steel may burst when tightened, causing the wheel to lose track. The best choice is bolts from trusted brands: Bimecc, Febi, TRW or Gunther.

How to properly install an eccentric bolt

Installing an eccentric bolt requires care, especially when it comes to critical components such as wheels or suspension. Here are step-by-step instructions taking into account common mistakes:

1. Surface preparation

• Clean the threaded holes from dirt and rust using brush or compressed air.
• Check that the bolt is not damaged or deformed.

2. Pre-installation

• Insert the bolt into the hole and make sure it rotates freely (no binding).
• For fixed eccentricity bolts, rotate the head until the threaded part is in the desired position.

3. Tightening

• Tighten the bolt criss-cross (if there are several of them) to avoid distortions.
• Use torque wrench to maintain the tightening torque (see table below).
• For wheel bolts the torque is usually 90–120 Nm, for suspension - 50–80 Nm (see the car manual for exact values).

4. Check

• After tightening, check whether the position of the part has changed (for example, the disc should sit smoothly, without runout).
• When installing the discs, spin the wheel - it should not catch on the arch or brake caliper.

Bolt type	Tightening torque (Nm)	Recommendations
Wheels (steel)	90–120	Tighten in 2 stages: first 50 Nm, then finally.
Wheels (aluminum)	70–90	Use thread lubricant (eg Copaslip).
Suspension (levers, silent blocks)	50–80	Tighten on a loaded suspension (vehicle on wheels).
Transmission (box, engine)	40–60	Pre-lubricate the threads Loctite 243 to prevent self-unscrewing.

⚠️ Attention: When installing eccentric bolts into the suspension, never tighten them with the wheel hanging! This will lead to incorrect geometry and accelerated tire wear. Tightening should only be done after the machine is lowered to the ground or jacked under the lever.

What happens if you overtighten the eccentric bolt?

Overtightening can lead to thread deformation or even breakage of the bolt head. In the case of wheel bolts, this is fraught with the loss of the wheel while moving. In the suspension, over-stretching will cause premature wear of the silent blocks or bearings.

Common mistakes and how to avoid them

Even experienced auto mechanics sometimes make mistakes when working with eccentric bolts. Here are the most common ones and ways to prevent them:

1. Incorrect calculation of eccentricity

• 🔢 Problem: The bolt was purchased “by eye”, and the offset does not correspond to the required one. For example, for disks with PCD 5×114.3 to a car with PCD 5×112 you need an eccentricity of 2.15 mm, not 2 mm.
• ✅ Solution: Use online calculators or formula: (New PCD - Native PCD) / 2.

2. Ignoring the tightening torque

• 🔧 Problem: The bolts are tightened “by hand” or with excessive force, which leads to stripping of the threads or deformation of the disk.
• ✅ Solution: Always use a torque wrench and follow the manufacturer's recommendations.

3. Lack of thread fixation

• 🔄 Problem: The bolt unscrews itself due to vibrations, especially in the suspension or on the wheels.
• ✅ Solution: Apply a thread locker (for example, Loctite 243) or use lock nuts.

4. Use of bolts for purposes other than their intended purpose

• ❌ Problem: The disc bolts are installed into the suspension or vice versa.
• ✅ Solution: Check markings and certificates - wheel bolts are marked Wheel Bolt, for suspension - Suspension.

5. Neglecting verification after installation

• 🚗 Problem: After installation, the disc runout or suspension geometry is not checked.
• ✅ Solution: Be sure to test the car at low speed and check for vibrations.

FAQ: Frequently asked questions about eccentric bolts

Can eccentric bolts be used for all types of rims?

No, not for everyone. Eccentric bolts are only suitable for wheels that have a bolt pattern (PCD) differs from the standard one by no more than 5–7 mm. If the difference is greater, adapters or new drives will be required. Also, bolts are not recommended for use on wheels with a very large offset (ET), as this may upset the balance.

How often should I check the tightness of the eccentric wheel bolts?

After installation, check the tightening after 100–200 km. Then - every 5-10 thousand km or before a long trip. It is especially important to check the bolts after off-road driving or during sudden temperature changes (in winter).

What is the difference between eccentric bolts and eccentric bushings?

A bolt is a threaded fastener that secures a part. A bushing is a hollow cylinder with an offset hole that fits over a bolt or stud for alignment. Bushings are often used in the furniture industry or for mounting shelves, while bolts are often used in the automotive and mechanical engineering industries.

Is it possible to make an eccentric bolt yourself?

Theoretically yes, but in practice it is not recommended. Manufacturing requires a lathe, high-quality steel and precise dimensions. Homemade bolts will not withstand dynamic loads and may burst. An exception is temporary bolts for stationary equipment (not for cars!).

Why are eccentric suspension bolts sometimes sold with control arms?

This is due to the fact that in some car models (for example, BMW E46 or VW Golf IV) the suspension arms have adjusting eccentric bolts for adjusting the camber. They are included as a set, as they require precise fitting for a specific model. Replacing such a bolt with a standard one will make it impossible to adjust the wheel alignment angles.