... : 0.01in ~ 0.02in 6)Smoothness: 3.1×10-3 7)Basic Hardness: HRC 16-23 8)Surface Hardness: HRC 55 ... )Coupling diameter: 5/8"to3/4 ",3/4"to7/8". 7/8"to1", 1"to1-1/8" 12)Main specification of ... them are available in sizes of 5/8",3/4",7/8",1",1-1/8" If you are interested in ... of our best service all the time. API 3/4" *7/8" Sub Sucker Rod Couplings,Class SM