Head stud confusion
 

Have a guy telling me arp 625s are 12mm studs.. aren't 426s 12mm and 625s are 14mm.

Nope. They are both the same. Standard 12mm size. 625 plus made 14mm studs obsolete

So just strength is the differents between the 426s/ 425s( whichever is the last correct digit) and the 625 12mm??

Yup, like an extra 100,000 psi tensial strength. As well as 625 are like a stainless material and they don't stretch apparently.