it's a 3/8" axle, my mate has one on the front of his bike. it is hollow, so you can use a normal skwer in them, but there is a 6mm thread cut into the inside of the axle so you can use these bolts. you may have stripped a bit of this thread out? r chewed the end of the hub or fork? you could try just using a qr skewer? or an allen key skewer? would do the job