This is a tinned copper ring terminal. It is very oversized and works great on 1/0 and 2/0 awg size cables. Because these are oversized, you will need a special DIE-LESS hydraulic crimper. The Temco brand is the best one. I have found that using the hydraulic crimpers that use a die, they are usually not sized properly for this particular ring terminal and will leave casting marks.
Bare copper is fairly resistant to corrosion, but when used in humid or wet environments, it can quickly become damaged. By coating bare copper with a metal alloy, like tin, the copper's natural properties are strengthened giving it a longer life.
These are available in 1/4", 5/16",and 1/2" rings. The 5/16" fits my grounding bolts perfectly.
#FAM 1/0 and 2/0 AWG Tinned Copper Ring Terminal
These are very large and do not work well in all crimpers, so I recommend buying the Temco TH1818 Dieless Hydraulic crimper. It is easy to use and works perfectly on these parts.