Guides / Finishing

Type II or Type III Anodizing for a Bike Part

September 7, 2026

Nearly every machined aluminum bike part gets anodized, and the choice between the two common types is made far more often by habit than by engineering. The two finishes do different jobs, cost different money and move your dimensions by different amounts.

What the two processes actually are

Both grow an aluminum oxide layer out of the part itself rather than coating something onto it. The difference is bath temperature and current density.

Type II runs warmer at a lower current. The oxide it grows is porous, which is why it takes dye, and relatively thin. Type III, usually called hard anodize or hardcoat, runs near freezing at a much higher current. The layer is denser, harder and several times thicker.

	Type II	Type III
Typical thickness	5 to 25 microns	25 to 50 microns
Color options	Full dye range	Dark grey to black, alloy dependent
Surface hardness	Moderate	High, comparable to hardened steel at the surface
Effect on a diameter	Grows or shrinks 5 to 25 microns	Grows or shrinks 25 to 50 microns
Effect on fatigue life	Measurable	Larger
Relative cost	Baseline	Higher

The dimensional problem nobody plans for

This is where good drawings and bad drawings separate.

Take a stem with a 28.6 mm steerer bore, machined on size, then given a 20 micron Type II coating. The bore comes back about 0.02 mm smaller. On a clamp bore that is opened by a pinch bolt anyway, nobody notices. Now take a hub shell with a 6902 bearing seat at 28 mm, machined for a press fit with 0.01 mm interference, then hard anodized at 40 microns. The seat is now 0.04 mm tighter than designed and the bearing either will not go in or goes in crushed.

There are three ways to handle it and you have to pick one on the drawing:

1. Mask the feature. The anodizer plugs or caps the bore. Cheapest to design, costs money every batch.
2. Machine an allowance. Cut the bore oversize by the expected growth. Requires you and the shop to agree on the coating thickness, which means specifying it, not just saying “anodized”.
3. Machine after anodizing. Only for features you can reach. The coating is hard and it will not be kind to a cutter.

Whichever you choose, write the coating thickness on the drawing as a range. “Type II anodize, black, 15 to 20 microns, mask bore A” is a spec. “Anodized black” is a wish.

The fatigue penalty on structural parts

Anodizing reduces fatigue life on aluminum, and the thicker the coating the bigger the penalty. The oxide layer is hard and brittle, it cracks under strain long before the aluminum would, and those cracks act as starters for the metal underneath.

This matters on exactly the parts you care most about: cranks, stems, steerer clamps and e-bike motor mounts. Published testing on 7075-T6 cranks under EN test conditions found a clear relationship between anodized depth and cycles to failure.

Practical rules that follow from it:

• Do not hard anodize a fatigue-critical part just because it looks tougher. It is not tougher in the way that matters.
• If a part must be hard anodized for wear, mask the high-stress fillets and radii.
• If you plan to certify a part by test, test it in the finish you will ship, not bare.

Which parts get which

Type II for stems, faceplates, brake adapters, seat collars, spacers, bottle cages, chainrings and most of what a brand sells in colors. It is the standard aftermarket look and it is kinder to fatigue life.

Type III for surfaces that rub or take grit: pedal bodies, chain guides, chainring pins, pivot faces, bash plates. Accept that the color is dark and that batch to batch appearance varies more than Type II.

What to send the shop

A drawing that names the type, the color, the thickness range, which surfaces are masked and whether a sample needs approving before the batch runs. Say if the part will be fatigue tested. Say if the color has to match another part you already sell, because matching color across alloys and batches is its own problem.

Get that on the drawing and the finish stops being the thing that delays the order.

Frequently asked questions

Keep reading

• Finish Options for a Machined Bike Part
• Why Your Anodized Parts Do Not Match