Heli-Coil R1191-4 is a free running stainless steel helical thread insert designed for repairing or reinforcing 1/4-28 UNF internal threads. Thread Size: 1/4-28 UNF. The Heli-Coil Free Running Thread Insert can also be referred to as a helical thread insert, a thread repair insert, a 1/4-28 thread insert, a stainless steel wire thread insert, and a free running screw thread insert.