Two types of coupling are available: Class T and Class SM, full size and slim size. It conform to API spec ... Two types of coupling are available: Class T and Class SM(spray metal rod coupling), full size and slim size. Dimensions ... : 5/8-3/4 T(SM), 3/4-7/8T(SM), 7/8-1T(SM), 1-1 1/8 T(SM). sucker rod coupling