These bolts are coarse threaded and made of quality "Marine Grade" type 316 stainless steel and are suited for resisting rust & corrosion in most environments. (For chemical, salt water or coastal environments type 316 stainless is a must! Standard 304/18-8 will not hold up!) These are standard six sided external hex heads and depending on length can be fully or partially threaded. Length is measured from underneath the head.