Replacement liners for One K Defender helmets. If the helmet isn't fitting quite right the liner that comes with it can easily be swapped out for a different size for a perfect fit. See sizing for restrictions on which liner size can go in which shell size. Note: These liners fit in the long oval shaped helmets as well. One K Size Helmet Size Head Circumference (in) Compatible Shell X-Small 6 3/8 - 6 1/2 19½ - 20 XS, SM, or MD Small 6 5/8 - 6 3/4 20½ - 21 XS, SM, or MD Medium 6 7/8 - 7 21½ - 22 XS, SM, or MD Large 7 1/8 - 7 1/4 22½ - 23 LG, XL, or XXL X-Large 7 3/8 - 7 1/2 23½ - 24 LG, XL, or XXL XX-Large 7 5/8 - 7 3/4 24½ - 25 LG, XL, or XXL One K Defender Helmet Liner