Premium A4 marine-grade stainless steel, fully-threaded fin bolt (machine screw) with the latest Torx 30 head. These are for power box, tuttle box, deep tuttle box and foil box fins and windfoils. It's a really good idea to carry spares when you go windsurfing in case one gets lost or damaged.
The measurement is not the overall length. It is taken from the shoulder to the end of the bolt (i.e. the entire length of the threaded part). Sold as single units. Available in 10 sizes:
