This vest has a hardness more than double that of 64 titanium. It has been domestically hardened and has a hardness of HRC 58. This is a specification close to the hardness of tools used for metalworking. The following hardness comparison is provided. You will feel that it is considerably harder than 64 titanium when you compare them. ＜Hardness Comparison＞ General stainless steel SUS304, etc. ... HRC equivalent 10 64 titanium ... HRC 28 Super-hard vest ... HRC 58 (HV650) ＜Size＞ Diameter: 7mm Hole diameter: Approximately 5.05mm Total length: Approximately 8.5cm Weight: 14g The size is matched with our stainless steel push rods and titanium push rods. The size will always fit either one. We have designed it so that those who have purchased our push rods do not have to worry when purchasing additional vests. We inspect everything before shipping. We cannot guarantee the size with other sellers. ＜Maintenance＞ The super-hard vest is a hardened product, so it will rust slightly. It can be removed by polishing with a commercially available file.