The Charles Owen Riding Hat Liners are designed to fit a range of the Charles Owen Plus models and other select helmets, including the Pro II Plus, Ayr8 Plus, AyrBrush and Race II hats.They can be used to refresh your helmet and lengthen its lifespan. They are easily clipped on to the hat making it simple to remove and wash when needed.Some Charles Owen Helmets (including the Pro II Plus, Ayr8 Plus, AyrBrush and Race II hats) are bought in different shell sizes. You then purchase the hat liner in your "cm" hat size (measured around the widest circumference of the head), this creates the size of the helmet. On each shell you can interchange the liners to create a different size.We also recommend purchasing a spare liner. This means that you can use the spare whilst washing the other or take the two liners to a stay away shows for a fresh head each time you ride.For more info on sizing and which size liner to purchase for your hat, please see the Size Guide below. Charles Owen Ayr8 Plus/Pro II Plus/Race II Riding Hat Liner