... produced according to API Spec 11B.Its basic material is the bar of AISI 5140(40Cr) API SUB Sucker Rod Coupling, ... : 2% 10)Min. Hardness of the layer: 595HV0.2 11)Coupling diameter: 5/8"to3/4 ",3/4"to7/8". 7/8"to1 ... ", 1"to1-1/8" 12)Main specification of 3/4"to7/8"sub sucker ... , slim hole sucker rod coupling,all of them are available in sizes of 5/8",3/4",7/8",1",1-1/8" ...