Two Piece PCD Hub Adapters Explained: Why Some Setups Require Them (And Others Don’t)

Two Piece Hub Adapters Explained: Why Some Setups Require Them (And Others Don’t)

If you are researching two piece hub adapters, you are usually dealing with a fitment problem that cannot be solved with standard parts.

In most cases, this comes down to one of two issues:

• The PCD (bolt pattern) does not match
• The physical design of the adapter becomes impossible in a single piece

We manufacture custom hub adapters in-house, and this is where most confusion comes from. People assume all wheel adapters do the same job. In reality, some setups cannot physically be made as a one-piece adapter.

This is exactly where two piece hub adapters come in.

Two Piece Hub Adapters Explained (What They Actually Are)

What is a two piece hub adapter

A two piece hub adapter is a type of PCD conversion adapter that allows you to fit wheels with a different bolt pattern to your vehicle.

Unlike a standard adapter, it is split into two components:

• An inner mounting piece (hub side)
• An outer mounting piece (wheel side)

Each piece handles a different interface. This is not done for convenience. It is done because, in some cases, it is the only way to make the geometry work safely.

How two piece hub adapters work

With a standard setup, you have:

Hub → Wheel

With an adapter, you have:

Hub → Adapter → Wheel

With a two-piece design, that becomes:

Hub → Inner Adapter → Outer Adapter → Wheel

Each face is machined with:

• Correct PCD (Pitch Circle Diameter)
• Correct centre bore
• Correct fastener sizing

This allows two completely different bolt patterns to exist without interfering with each other.

Why Two Piece Hub Adapters Exist

When bolt patterns overlap

The above images shows a 5x120 PCD trying to be fit on the same spacer as a 4x100. You can see that there is no way to fit these two patterns on the same face without there being an overlap in the holes

 

This is the key point most articles miss. Every bolt pattern has fixed hole positions based on its PCD.

When you try to combine two patterns into one adapter, the hole positions can overlap.

For example:

• Converting between certain 5-stud PCDs
• The bolt holes sit too close together
• In some cases, they intersect completely

If you try to machine both patterns into a single piece of billet aluminium, you run into problems:

• There is not enough material between holes
• The drilling pattern weakens the mounting face
• Fastener seating becomes unreliable

This is not a theoretical issue. We see this regularly when designing custom PCD adapters.

Why a single piece adapter won’t work

A one-piece adapter relies on having enough material around each bolt hole.

If the holes overlap:

• You lose structural integrity
• The load path becomes uneven
• Clamping force is compromised

At that point, it is no longer a safe design. This is why some wheel adapters for PCD conversion simply cannot be made as a single part.

How two piece adapters solve the problem

This image shows the two pieces in a two piece adapter. The rear section bolts to the cars hub and has one PCD, the second piece bolts to the first piece and carries the PCD of the wheel

A two piece wheel adapter separates the two bolt patterns completely.

• The inner piece carries the vehicle PCD
• The outer piece carries the wheel PCD

Because of this:

• No hole overlap occurs
• Full material thickness is maintained
• Each mounting interface remains structurally sound

From a manufacturing point of view, this is the correct solution when patterns clash.

When You Need Two Piece Hub Adapters Instead of Wheel Spacers

When wheel spacers are no longer enough

A lot of people start with spacers. That makes sense. Hubcentric wheel spacers are designed to adjust offset and improve stance.

But they do not change:

• PCD
• Centre bore mismatch
• Stud configuration

If the wheel does not match the hub, a spacer will not solve the problem.

Changing bolt pattern (PCD conversion explained)

If you are trying to fit wheels from a different manufacturer, you will often run into a PCD mismatch.

For example:

• BMW: 5x120
• Audi: 5x112

This is where a bolt-on wheel adapter is required.

If that conversion results in overlapping bolt hole positions, you now need a two piece hub adapter.

Common real-world use cases

We typically manufacture these for:

• Track builds using mixed components
• Custom projects with non-standard wheels
• Cross-brand wheel swaps
• Vehicles with upgraded stud conversions

In each case, the adapter is solving a physical limitation, not just spacing the wheel out.

Stud Length and Clearance Issues (Often Overlooked)

Stud length and why it matters

This is another issue that comes up regularly in real-world builds. Some vehicles use wheel studs instead of bolts, and those studs can extend a significant distance past the hub face.

It is common to see 25 mm to 45 mm of stud protrusion

This directly affects adapter design.

Why stud length limits adapter thickness

A hub adapter must sit flat against the hub.

If the studs distance from the hub is greater than the thickness of the adapter , the studs would poke in to the wheel. This obviously can't work.

An example would be a stud length of 45mm and an adapter thickness of 20mm. The studs would poke through the adapter and still protrude another 25mm beyond. This 25mm is where the wheel is i.e. the studs would not allow the wheel to sit flush.

This is one of the most common fitment problems we see.

Your two options

If you are dealing with long studs, there are only two solutions.

Option 1: Increase adapter thickness

The adapter must be at least as thick as the stud protrusion.

For example:

• 30 mm stud protrusion → minimum 30 mm adapter

This ensures proper fastener clearance and mounting depth.

Option 2: Change or shorten the studs

Alternatively, you can:

• Install shorter studs
• Convert to a bolt-through setup

This gives more flexibility when designing the adapter.

Why this matters in custom adapters

This is why off-the-shelf parts often do not work.

They are not designed around:

• Stud protrusion
• Internal clearance
• Real mounting conditions

When we design custom hub adapters, this is one of the first things we check.

Two Piece Hub Adapters vs Wheel Spacers

Key differences in function

• Wheel spacers adjust position
• Hub adapters change fitment

If your wheel already matches the hub, spacers are the correct solution.

If it does not, you need an adapter.

Safety differences and considerations

Both rely on:

• Flat mounting surfaces
• Correct clamping force
• Proper thread engagement

However, adapters introduce an additional interface.

That means tolerances, machining accuracy, and material quality become even more important.

Which one should you choose

Use spacers if:

• PCD matches
• You only need to adjust offset

Use adapters if:

• PCD is different
• The wheel cannot be mounted directly

If bolt patterns overlap, a two piece hub adapter is required.

Two Piece Hub Adapters vs One Piece Adapters

What is a one piece adapter

A one piece adapter combines both bolt patterns into a single component.

This works well when:

• Hole spacing allows it
• There is sufficient material between holes

When a one piece adapter is not possible

If the drilling patterns overlap or sit too close together:

• The adapter becomes structurally weak
• Fastener seating is compromised

In these cases, a one-piece design is not suitable.

Why two piece designs are used instead

A two-piece design allows each pattern to exist independently.

This maintains:

• Material strength
• Proper hole spacing
• Reliable load transfer

From a manufacturing perspective, this is the correct approach for complex conversions.

Custom Two Piece Hub Adapters (What You Need to Know)

Why most adapters are custom made

There is no universal solution.

Each setup has unique requirements:

• PCD combinations
• Centre bore sizes
• Stud or bolt configuration
• Required thickness

This is why most custom hub adapters UK are made to order.

What information is required

To manufacture a correct adapter, we need:

• Vehicle PCD
• Wheel PCD
• Centre bore dimensions
• Stud or bolt details
• Required thickness (including stud clearance)

Without this, the part cannot be designed correctly.

How we manufacture two piece hub adapters

Our process is straightforward and controlled:

1. Confirm all fitment data
2. Design in CAD
3. Machine from billet aluminium using CNC
4. Check tolerances and mounting faces
5. Apply finishing where required

Every step is focused on ensuring proper load distribution and fitment.

Ordering Custom PCD Adapters in the UK

Off-the-shelf vs custom adapters

Off-the-shelf adapters can work for simple conversions.

However, they often do not account for:

• Overlapping bolt patterns
• Stud protrusion
• Precise centre bore requirements

This is where custom solutions become necessary.

Lead times and pricing expectations

Custom adapters are low-volume, precision components.

Lead time depends on:

• Design complexity
• Machining time
• Finishing

Pricing reflects the level of engineering involved.

Where to get custom hub adapters

If your setup requires a proper solution, you can view our custom PCD adapters here:

👉 Custom PCD Adapters UK – Brightstone Engineering

We design and manufacture all adapters in-house, with full control over tolerances, materials, and fitment.

Final Thoughts on Two Piece Hub Adapters

Two piece hub adapters exist because some fitment problems cannot be solved any other way.

If bolt patterns overlap, a single-piece adapter is not possible.

If stud length creates clearance issues, the design must be adjusted accordingly.

These are not edge cases. They are common in real-world builds.

If your wheels already match your car, spacers are enough.

If they do not, and especially if the geometry becomes complex, a properly designed adapter is the only correct solution.

And when that happens, it needs to be engineered and machined correctly from the start.