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Center Cap Rattles at Speed — Diagnosis in the Right Order
A center cap that rattles is not an annoyance — it's a departure announcement. Caps that move in their seat work looser, and the end of the story is a cap on the shoulder somewhere behind you. Diagnose in this order; the first three checks need one removal and one caliper.
1. The 1 mm miss
Center cap sizing is metric and tight: a cap 1 mm small won't lock; that's the whole rule of this site. If the cap is a recent purchase, measure it — OD across the face, ID across the clip tips — and compare against the documented family for your wheel in the Cap Finder. "Close" is the rattle.
2. Worn clip tips
On an original cap that used to be silent, suspect the clips. Tips shave down a few tenths at a time against the wheel opening across years of removals, until the interference fit is gone. Measure across several opposing clip pairs and read the largest consistent figure; pairs that disagree wildly mean the cap is done — replace it by part number.
3. Wrong depth
A cap whose clips are too shallow for the wheel's recess never reaches the retention groove; it sits in the opening held by friction alone and buzzes at speed. Depth is measured base-to-clip-tip — see Clip Depth Explained.
4. The tired spring ring
On spring-ring caps, a ring that has lost tension (or a substitute ring of the wrong gauge) gives the same symptom. Replace the ring with the correct one — and make sure there isn't a second, forgotten ring in the wheel doubling the problem in the other direction.
Next steps
Run your numbers through the Cap Finder, or re-check them with the How to Measure walkthrough before you order.
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Related guides
Sources: the 1 mm rule and measurement method via EngineerFix guides and our fitment research; worn-clip measurement practice per our Subaru fitment notes. Verify by measuring.