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My metal airbox locates the filter using the external lip which is about 210mm. This means the external diameter (210mm) and the internal diameter (160mm) are both important. You can also see the shiny metal surface where the filter has been rubbing. The E-2601 has an OD of 187mm
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The later plastic airbox locates the filter using the internal lip, which means that the internal diameter is more critical, and the external diameter can be smaller than the factory size without any issue
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After a bit of research not the K&N website I ordered in a new KN E-2600 (210mm OD) and it fits perfectly and doesn't move about at all.
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