Replacement liners for One MIPS Junior 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. JR One K Size Helmet Size Head Circumference (in) X-Small 5 7/8 - 6 17½ - 18 Small 6 1/8 - 6 1/4 18½ - 19 Medium 6 3/8 - 6 1/2 19½ - 20 One K MIPS Junior Helmet Liner