PCD and ET explained: how to specify a forged wheel so the order is right the first time

Most wheel orders that go wrong are not machining errors — they are specification errors.

The three numbers that matter

PCD (bolt circle diameter) is the number of bolt holes arranged in a circle, measured in millimetres across the centre of the holes. A 5x112 wheel has five holes on a 112 mm circle. It is the least negotiable number on the whole order: it is a physical interface with the hub, and no amount of clever machining will make a 5x112 fit a 5x130 hub.

Centre bore (CB) is the diameter of the hole that locates the wheel on the hub spigot, plus the pilot hole. This is where most mistakes happen, because centre bore is quoted in two ways and the two are not interchangeable. A wheel that is 5x112 with a 66.5 mm bore is not the same as one with a 66.6 mm bore, and the 0.1 mm difference decides whether the wheel centres itself or rattles. Where the original wheel is a two-piece, the barrel bore and the centre bore are separate values and both have to be right.

ET (offset, Einbautiefe) is the distance in millimetres between the wheel mounting face and the centreline of the wheel. It is signed: positive ET means the mounting face sits outboard of centre, negative means inboard. ET is what decides whether the wheel sits flush or whether the tyre clears the fender arch. The rule of thumb is that a higher ET pushes the wheel inward and inward is safer for clearance, while a lower ET pushes the wheel outward and can cause rubbing.

Reading them off an existing wheel

You do not need to measure anything. Take the OE wheel off the car and read the markings cast into the back of the hub or rim:

• The PCD is usually stamped as a number pair, e.g. 5x112, 5x120, 5x130, 4x100.
• The ET is usually marked with a prefix such as ET35, ET45, sometimes with a minus sign for negative offset.
• The centre bore may be marked as a plain number (66.5, 67.1, 72.5) or as an alphanumeric manufacturer code (for example BMW 5x112 commonly uses a 66.5 bore, Mercedes 5x112 uses 66.6).

The last point is where buyers lose money. Because the number is drilled and honed to a specific figure, the difference between 66.5 and 66.6 is not academic — it is the difference between the wheel seating against the hub and the wheel sitting with a millimetre of play. When the marking is missing, photograph the hub face and measure the bore directly, then confirm with us before production.

What to send us

1. Diameter and width, e.g. 20x9J
2. PCD, e.g. 5x112
3. Centre bore, e.g. 66.6
4. ET, e.g. 35
5. Construction — monoblock, 2-piece or 3-piece
6. Finish — paint, brush, high polish, chrome, PVD
7. Quantity per design, and whether it repeats across models
8. Destination port

Send a photograph of the OE wheel back face if you are unsure. We will confirm the fitment back to you in writing before tooling starts.

One more thing about width

Width is quoted in inches and in J (the standard width designation), and the two are not the same number. A wheel marked 9J is 9 inches of rim width, and it will accept a tyre from roughly 205 to 235 section width. If your intended tyre is at the wide end of that range, tell us — it affects both the offset we recommend and the machining of the inner lip on a 2-piece build.

Specifications confirmed before cutting. That is the whole difference between a wheel that fits and a pallet you cannot use.