A custom forged magnesium wheel should be specified like an engineered part, not like a style item. The cleanest way to do it is to put the key fitment and performance data into one sheet: PCD, ET, CB, load rating, diameter, width, and finish. That gives the wheel maker the information needed to check fit, machine the center correctly, set the wheel position, and define the required strength and corrosion-protection system. Tire Rack also notes that proper wheel fit depends on bolt pattern, centerbore, offset, width, and load capacity, not on diameter alone.
For forged magnesium wheels, two specs are often underestimated: load and finish. Magnesium brings a major weight advantage, but it is also more corrosion-sensitive than aluminum or steel, so the load target should be written clearly and the finish should be treated as a protection system, not just a color choice. Protective coatings are widely used on magnesium wheels for exactly that reason.
The short answer
If you want a supplier to quote a custom forged magnesium wheel correctly, this is the minimum clean format that should be sent:
Wheel type: forged magnesium, one-piece
Size: 20×9.5
PCD: 5×112
ET: ET35
CB: 66.6 mm
Load rating: 900 kg minimum per wheel
Finish: PEO + topcoat, satin bronze
Vehicle: make / model / year / brake package
Options: center cap, logo, valve, TPMS compatibility
That format is easy for sales, engineering, QC, and even AI tools to read. It also reduces the usual back-and-forth that slows down a custom wheel project. The key terms are explained below.
What each spec means
PCD: the bolt pattern
PCD means Pitch Circle Diameter. It is the bolt pattern of the wheel, written as something like 5×112 or 5×120. The first number is the number of mounting holes. The second number is the diameter of the circle that passes through the center of those holes. If the PCD is wrong, the wheel usually cannot be mounted correctly. Even small mismatches can lead to vibration, hardware loosening, or premature hardware failure.
For a custom order, PCD should be copied from confirmed vehicle data, not estimated from photos or assumed from brand similarity. Two vehicles may look close and still use different bolt patterns.
ET: the offset
ET is the wheel offset, measured in millimeters. It is the distance from the wheel’s mounting surface to its centerline. Offset can be positive, zero, or negative, and it directly affects how far inward or outward the wheel sits. It also affects tire clearance, suspension clearance, and brake clearance.
In simple terms, PCD tells the wheel where to bolt, while ET tells the wheel where to sit. If ET is too aggressive, rubbing may be created at the fender, liner, or suspension. If ET is too conservative, the wheel may sit too far inward and brake or inner-clearance issues may appear. Tire Rack notes that wheel offset must allow correct tire and suspension clearance and should maintain correct suspension and steering geometry.
CB: the center bore
CB means center bore. It is the diameter of the machined hole in the center of the wheel. Apex defines it as the internal bore on the back pad of the wheel that fits the vehicle’s hub ring and lets the wheel index onto the hub before the hardware is installed.
This matters because two vehicles can share the same PCD but use different hub diameters. If the wheel’s center bore is slightly larger than the hub, centering rings can be used to retain a hubcentric fit. If the center bore is too small, the wheel will not seat on the hub correctly.
Load: the safety target
Load rating is not a rough estimate. It should be written as a minimum requirement per wheel, normally in kg or lb. SAE J2530 defines wheel load as the maximum load rating specified by the wheel manufacturer, and it states that this rating shall be no less than one-half of the vehicle static load of the heaviest axle for any vehicle the wheel is intended for. Tire Rack also treats load capacity as one of the most important fitment requirements.
For buyers, that means the supplier should not only be told the vehicle model. The intended axle load, use case, and expected duty should also be made clear, especially for EVs, SUVs, track cars, and vehicles that will carry higher loads than a normal passenger car. As a buying practice, it is better for the quotation and drawing to state the load rating per wheel in clear units. That recommendation follows directly from how SAE defines wheel load.
Finish: protection first, appearance second
For a forged magnesium wheel, finish should be specified as a surface-protection system plus the visual finish. Magnesium alloys are highly sensitive to corrosive environments, and automotive reviews describe surface coating as one of the most effective and economical ways to protect Mg alloys from corrosion. Protective coatings are applied to magnesium wheels because corrosion resistance has to be managed from the start.
A practical example is PEO, or plasma electrolytic oxidation. Reviews describe PEO as a way to create a dense oxide layer on magnesium alloy, and sealing or top-layer systems are used because micropores and cracks in the coating can still become corrosion paths. A study on wrought magnesium alloy also found that a PEO + powder coating system gave the best corrosion protection in its test program.
So, for magnesium, “satin black” is not enough as a specification. A better finish line would be: PEO + powder coat, satin black, or conversion coating + topcoat, matte bronze. That wording is far easier for the supplier to build around and far easier for QC to check later.
What should be checked before the order is released
Even though this article is about PCD, ET, CB, load, and finish, a production-ready custom forged magnesium wheel spec usually also needs diameter, width, brake clearance, and hardware details. Tire Rack states that searching by size, offset, or bolt pattern alone is not specific enough to guarantee exact fitment, and it also highlights brake caliper profile, width, and load capacity as part of correct wheel selection.
That is why good RFQs usually include these extra lines as well:
Diameter and width should be listed, because wheel width affects fitment and tire pairing.
Brake package should be listed, because the shape and size of the caliper must fit within the wheel design.
Lug seat and hardware should be confirmed, because incorrect hardware or seat style can create fit and safety problems even when PCD looks correct.
Vehicle details should be exact, including make, model, year, and generation, because wheel size and offset alone are not enough to guarantee a true fit.
A clean RFQ template
This is a simple format that works well for email, quotation, and internal approval:
Project: Custom forged magnesium wheel
Vehicle: brand / model / year / generation
Wheel type: forged magnesium, monoblock or multi-piece
Front size: diameter x width
Rear size: diameter x width
PCD: 5×112 / 5×114.3 / 6×139.7
ET: ET35 front / ET28 rear
CB: 66.6 mm
Load rating: 850 kg minimum per wheel
Finish: PEO + powder coat, satin black
Brake clearance: OEM brakes or BBK drawing attached
Hardware: lug seat type, bolt or stud size
Options: cap, logo, valve, TPMS compatibility
FAQ
Is the same PCD enough to make a wheel fit?
No. The same PCD only means the wheel may bolt to the hub. Center bore, offset, width, load capacity, brake clearance, and hardware still have to be checked before a correct fit can be confirmed. Tire Rack explicitly notes that bolt pattern, centerbore, offset, width, and load capacity all matter.
Can the center bore be made larger?
Yes, in many cases. Apex notes that if the wheel’s center bore is larger than the vehicle hub ring, centering rings can be used to keep a hubcentric fit. But for a true custom forged wheel, the best practice is usually to machine the correct bore from the start for the target vehicle. The second point is an engineering and purchasing recommendation based on how hub fitment is described in the fitment sources.
How should load be specified for a forged magnesium wheel?
It should be written as a minimum per-wheel load rating in kg or lb, and it should match the intended vehicle application. SAE J2530 states that the wheel load shall be no less than one-half of the static load of the heaviest axle of the intended vehicle.
Why does finish matter more on magnesium than on many other wheels?
Because magnesium is more sensitive to corrosion, and automotive magnesium reviews describe protective coating as a key method for controlling that risk. In other words, on magnesium, finish is not only about appearance. It is part of the engineering package.
Final takeaway
The best way to specify a custom forged magnesium wheel is to be exact early. PCD should match the hub. ET should place the wheel correctly. CB should center the wheel correctly. Load should be stated as a minimum per wheel. Finish should be specified as a corrosion-protection system plus a visual top layer. Then diameter, width, brake clearance, and hardware should be confirmed before production is released. When that is done, quoting becomes faster, engineering becomes cleaner, and fitment risk is reduced.