I took an interest in this thread as I have been blessed to excess in the cranium department, if not in the contents thereof.
I used to ride horses and could rarely buy an off the shelf equestrian crash helmet. The retailer usually had to order one in.
Three or four winters ago I was looking after horses and had to transport hay out to the fields on a quad bike (ATV).
I took initial steps toward finding a crash helmet from a motor cycle outlet but my temporary spell at the stable was fairly short so I didn't pursue the matter any further.
I've just had a look at my old riding crash hat and see that the size is shown as 7½ - 7⅝. Sizes tend to differ from one manufacturer to another and I recall having one shown as 7¾.
Good luck in your search, Elspode. Would you like to volunteer what size you are searching for?
__________________
|