Anodizing Aluminum Fasteners: Type II, Type III and What Happens to Thread Fit
Anodizing is electrochemically grown oxide: half the layer builds outward, half converts base metal. On a threaded part, that dual movement is the whole story.
The dimensional arithmetic
A 10 µm anodic layer raises external dimensions by 10 µm but consumes 10 µm of base metal — so a thread's pitch diameter grows by only the outward half, roughly half the coating spec. Still, on precision threads even 5 µm matters: M3 tolerances leave less than 50 µm total play.
Type II vs Type III
- Type II (decorative, 8–15 µm): standard for consumer products; threads usually need no masking at common class limits.
- Type III (hard anodizing, 25–50+ µm): abrasion resistance for aerospace and industrial tools, but dimensional growth demands masked threads or post-coating machining.
Practical specification advice
Call out masking for threads on Type III, specify dye color explicitly (blacks vary widely), and re-verify friction: anodized surfaces are abrasive and change torque-preload conversion — K-factor rises noticeably. Topprecision anodizes its aluminum fastener line in-house-controlled partner processes with thread-masking standard on precision sizes, and supplies friction data per finish class.
