If you're replacing the steering wheel on a classic car, one component you will almost certainly come across is the steering wheel boss.
Sometimes called a steering wheel hub or steering wheel adapter, the boss is the component that connects the steering wheel to the vehicle's steering column. It may not be the most visible part of a steering wheel installation, but it plays an essential role in ensuring the wheel is correctly positioned, securely mounted and able to control the steering as intended.
For classic and historic cars in particular, choosing the right boss is just as important as choosing the steering wheel itself.
In this guide, we'll explain what a steering wheel boss is, how it works, the different features you may encounter and what you need to consider when fitting a new steering wheel.
What is a steering wheel boss?
A steering wheel boss is the mechanical connection between the steering wheel and the steering column.
At one end, the boss attaches securely to the vehicle's steering column. At the other, it provides a mounting face onto which the steering wheel is bolted.
Modern production cars often incorporate this interface into the complete steering wheel assembly. On many classic cars, however, the steering wheel and boss are separate components.
This is particularly useful when fitting an aftermarket or period-style steering wheel because the boss effectively acts as the interface between a wheel and a particular vehicle.
The steering wheel may therefore be suitable for a wide variety of cars, while the boss itself needs to match the steering column of the vehicle.
Why does the correct steering wheel boss matter?
A steering wheel is the driver's primary control, so its connection to the steering column needs to be secure.
A correctly specified boss ensures that the steering wheel fits the column properly and transfers the driver's steering input without unwanted movement.
There is also another consideration: driving position.
The design and depth of a boss affects where the steering wheel sits in relation to the dashboard and driver. Changing the boss, wheel dish or a combination of the two can move the steering wheel closer to or further away from the driver.
For classic cars, the appearance of the boss matters too. A well-designed installation should complement the steering column and dashboard rather than looking like an obvious adapter has been added.
The anatomy of a steering wheel boss
Although designs vary considerably between vehicles, most steering wheel bosses share several basic features.
1. The boss body
The main body forms the structural centre of the boss.
Aluminium and aluminium alloys are commonly used because they provide a useful combination of strength, relatively low weight and excellent machinability.
Depending on the design, a boss may be machined from solid material or manufactured using other processes before being finished to the required dimensions.
The external shape can vary significantly.
Some bosses are relatively simple cylindrical components, while others are shaped specifically to meet the steering column shroud or dashboard of a particular vehicle.
This is particularly important on classic cars. Removing an original steering wheel can expose parts of the steering column that were previously hidden, so a correctly designed boss can help create a much more complete, factory-style appearance.
2. Steering wheel mounting face
At the driver-facing end of the boss is the steering wheel mounting face.
This contains a pattern of holes that allows the steering wheel to be bolted securely to the boss. One of the most important measurements when determining whether a steering wheel will fit an existing boss is the PCD, or Pitch Circle Diameter.
What does PCD mean on a steering wheel?
PCD is the diameter of the imaginary circle that passes through the centre of all the steering wheel mounting holes.
For example, a 6-hole 70mm PCD boss has six mounting holes positioned evenly around an imaginary circle measuring 70mm in diameter.
PCD is important because the steering wheel and boss need compatible mounting patterns. Two bosses may look almost identical, but if the number of mounting holes or their PCD is different, the same steering wheel will not simply bolt onto both.
Common steering wheel PCD sizes
There are several steering wheel mounting standards, particularly when dealing with classic cars and aftermarket steering wheels.
A common aftermarket configuration is:
6-hole / 70mm PCD – This uses six mounting bolts arranged around a 70mm diameter circle and is a widely used aftermarket steering wheel pattern.
Another pattern frequently encountered on classic cars is:
9-hole / 101.6mm PCD – This uses nine mounting holes arranged around a 101.6mm (4-inch) diameter circle. This is the mounting standard used by Moto-Lita steering wheels and bosses, so you may encounter it if your classic car already has a Moto-Lita boss installed.
These two patterns are not directly interchangeable. A conventional 6-hole 70mm PCD steering wheel will not simply bolt onto a 9-hole 101.6mm PCD boss, and vice versa.
There are also other mounting patterns in use, so it's always worth identifying what is already fitted to your car before ordering a replacement steering wheel.
Already have a Moto-Lita or other aftermarket boss?
You don't necessarily need to change it.
This is particularly relevant if your classic car already has a Moto-Lita 9-hole 101.6mm PCD boss, a 6-hole 70mm PCD boss, or another suitable aftermarket boss installed.
Les Leston can produce steering wheels to suit a wide range of existing boss mounting patterns.
So, rather than removing a perfectly good boss simply because you're changing the steering wheel, we can make your Les Leston wheel to suit the existing installation where appropriate.
If you're unsure which boss or PCD you have, photographs and a few simple measurements can usually help identify the mounting pattern.
And if your car doesn't currently have an appropriate boss, we can also help supply the correct boss for your vehicle.
3. The splined centre
Turn a steering wheel boss over and one of its most important features is found in the centre.
Most steering columns use a series of fine ridges known as splines.
The centre of the boss contains a corresponding splined opening designed to fit closely over the steering column. Once installed, these interlocking splines allow the rotational movement of the steering wheel to be transferred to the steering column.
The number and dimensions of these splines vary between vehicles.
For example, one car may use a different spline count, diameter or profile from another — even when the steering wheels themselves look similar.
This is why choosing a boss simply by measuring its outside diameter isn't sufficient.
4. Spline count and diameter
When identifying the correct steering wheel boss, spline count is one of the specifications that may need to be checked.
Depending on the vehicle, you might encounter steering columns with 36, 40 or another number of splines.
But spline count alone doesn't determine compatibility.
The diameter and profile of those splines must also match. Two steering columns can have the same number of splines while still requiring completely different bosses.
The correct specification therefore needs to take the complete steering column design into account.
5. Tapers, keyways and locating features
Not every steering column relies solely on splines.
Depending on the vehicle manufacturer and period, the column may also incorporate a taper, keyway or other locating feature.
A taper helps the boss seat firmly onto the steering column as it is tightened.
A keyway uses a corresponding locating feature to control the orientation or security of the connection.
Some older vehicles may use additional fastening arrangements such as pinch bolts, while many steering wheel bosses are retained by a central nut on the end of the steering column.
These differences are another reason why steering wheel bosses are usually vehicle-specific components.
6. Horn connections
The boss may also play a part in operating the vehicle's horn.
On cars where the horn button is positioned in the steering wheel, an electrical connection needs to be maintained between the rotating steering wheel and the vehicle's wiring.
Depending on the car and boss design, this can be achieved through wiring passing through the centre of the boss, electrical contacts or a slip-ring arrangement.
The correct solution depends heavily on the original vehicle.
Some classic cars don't have a horn button in the steering wheel at all, instead using a stalk, dashboard or other control. In these applications, the centre of the steering wheel can often be finished with a decorative centre cap or badge.
7. Boss depth
One specification that is easily overlooked is boss depth.
The depth of the boss contributes to the overall distance between the steering wheel and the dashboard.
A deeper boss moves the wheel towards the driver, while a shallower boss positions it closer to the steering column.
This can have a noticeable effect on the driving position, particularly when combined with the dish or offset of the steering wheel itself.
On a classic or competition car, relatively small changes in steering wheel position can make a surprising difference to arm position, dashboard clearance and overall comfort.
When choosing a steering wheel setup, it is therefore worth considering the boss and steering wheel together, rather than treating them as completely separate components.
8. Quick-release steering wheel bosses
Some steering wheel installations use a quick-release boss or hub.
Instead of permanently bolting the steering wheel to a conventional fixed boss, a quick-release mechanism allows the wheel to be detached from the steering column.
These systems are most commonly associated with competition and motorsport applications, where removing the steering wheel can make it easier for a driver to enter or exit a vehicle with a tight cockpit.
Depending on the system, part of the hub remains permanently attached to the steering column while the wheel and mating section can be released.
For a road car, it's important to consider the suitability of any quick-release system for the vehicle and its intended use before installation.
Are all steering wheel bosses the same?
No.
This is probably the most important thing to understand when buying a steering wheel boss.
Even cars from the same manufacturer can use different steering column designs depending on the model or year of production.
Compatibility can depend on:
- Steering column diameter
- Number of splines
- Spline profile and dimensions
- Column taper
- Keyway or locating features
- Retaining method
- Horn arrangement
- Boss depth
- Steering column shroud
- Steering wheel mounting pattern and PCD
As a result, a boss that fits one classic car shouldn't automatically be assumed to fit another.
Even within a single model range, manufacturers sometimes changed steering columns during production.
Can I keep my existing steering wheel boss?
In many cases, yes.
If your car already has an aftermarket steering wheel and boss fitted, there may be no reason to replace the boss when changing the steering wheel.
The first step is to identify the mounting pattern and dimensions of the existing boss. This includes checking the number of mounting holes and the PCD.
At Les Leston, we can make steering wheels to suit a wide variety of boss configurations. This means that if you already have a suitable boss fitted to your car, you may be able to simply remove your current steering wheel and replace it with a Les Leston wheel made to the appropriate mounting specification.
This includes many cars already fitted with common aftermarket boss patterns, including 6-hole 70mm PCD and Moto-Lita 9-hole 101.6mm PCD configurations.
It's a particularly useful option for classic cars where a suitable boss has already been sourced and correctly installed.
What if I don't have a boss?
If you're replacing an original steering wheel and don't already have an aftermarket boss, you'll usually need a boss suitable for your particular car.
Because the steering column specification can vary according to manufacturer, model and year, it's important to identify the vehicle accurately.
At Les Leston, we can help supply the correct boss for your car alongside your new steering wheel, giving you a complete setup rather than leaving you to work out compatibility between components yourself.
Choosing a boss for a classic car
When selecting a boss, start with the exact make, model and year of your vehicle.
If there were significant changes during the production run, additional information may be required to establish which steering column is fitted.
You should also consider the steering wheel you want to use.
Its diameter, dish, mounting pattern, PCD and centre arrangement can all influence the finished installation.
If you're restoring a period car, appearance may be particularly important. The ideal installation should look appropriate to the vehicle's interior rather than simply functioning as a mechanical adapter.
Can a Les Leston steering wheel fit my existing boss?
Yes.
A common misconception is that fitting a Les Leston steering wheel means replacing every part of an existing steering wheel installation.
That's not necessarily the case.
Les Leston steering wheels can be custom-made to suit many existing steering wheel bosses, making them an excellent option if your car already has a suitable aftermarket hub fitted.
For example, if your car already has a Moto-Lita 9-hole 101.6mm PCD boss, you don't necessarily need to remove the boss simply because you want to fit a Les Leston steering wheel. We can produce the wheel with a compatible mounting arrangement where appropriate.
The same principle applies to many other existing boss configurations, including the common 6-hole 70mm PCD pattern.
If you're unsure what boss you currently have, measurements and photographs of the existing installation can help establish the mounting arrangement.
Alternatively, if you're starting from the original steering column, we can help you find the appropriate boss and steering wheel combination for your car.
The boss might be hidden, but it's an important part of your steering wheel
The steering wheel naturally gets most of the attention, but the boss behind it is doing an essential job.
It connects the wheel to the steering column, determines how the wheel mounts, contributes to its position in the cockpit and, on many cars, helps accommodate the horn connection and steering column trim.
Most importantly, the boss needs to be correct for the vehicle.
Whether you're restoring a classic road car, changing an existing aftermarket steering wheel or creating a completely new steering wheel setup, understanding the boss makes it much easier to choose the right components.
Looking for a steering wheel for your classic car?
If you already have a boss fitted, Les Leston can create a steering wheel to suit many existing boss configurations, including common 6-hole and 9-hole mounting patterns.
If you don't have one, we can also help supply the correct boss for your vehicle.
Explore the Les Leston steering wheel range or get in touch with the make, model and year of your car and we'll help you work out the right setup.
