This threaded Insert converts RIKON's 4-Jaw Chucks that require inserts to fit onto lathes with 1-1/4" x 8 TPI threaded spindles. The Insert's internal threads are precision machined to 1-1/4"x 8 TPI UNS RH specifications for perfect chuck lathe mounting and running-true use. Its external threads install into the back of RIKON insert-type chuck bodies only.