Hi there All, I'm used to what I would call 'normal' UNF and UNC thread designations, such as 1/2"*20 UNF, but I've got a few things on a drawing that don't make sense to me - I wonder if anyone could shed some light on them. I have a stud marked as being threaded 1/2-20 UNF-2A I have another stud marked as 3/4-16 UNF-2A I have a hole marked as 1/2-13 UNC-2B * .75 Does anyone know what the stuf after the UNF or UNC refers to? Yours, Rich. MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.