### Repairing Damaged Threads

Several methods of repairing damaged threads are available. Heli-Coil® (shown here), Keenserts® and Microdot® are among the most widely used. All involve the same principle — drilling out stripped threads, tapping the hole and installing a pre-wound insert — making welding, plugging and oversize fasteners unnecessary.

Two types of thread repair inserts are usually supplied — a standard type for most inch-coarse, inch-fine, metric-coarse and metric-fine thread sizes and a spark plug type to fit most spark plug port sizes. Consult the individual manufacturer's catalog to determine exact applications. Typical thread repair kits will contain a selection of pre-wound threaded inserts, a tap (corresponding to the outside diameter threads of the insert) and an installation tool. Spark plug inserts usually differ because they require a tap equipped with pilot threads and a combined reamer/tap section. Most manufacturers also supply blister-packed thread repair inserts separately plus a master kit containing a variety of taps and inserts plus installation tools.

Before effecting a repair to a threaded hole, remove any snapped, broken or damaged bolts or studs. Penetrating oil can be used to free frozen threads; the offending item can be removed with locking pliers or with a screw or stud extractor. After the hole is clear, the thread can be repaired.

Fig. 17: Damaged bolt holes can be repaired with thread with thread repair inserts

Fig. 18: Drill out the damaged threads with the specified bit. Drill completely through the hole or to the bottom of the blind hole.

Fig. 19: With the tap supplied, rethread the hole to receive the threaded insert. Keep the tap well oiled and back the tap out frequently to avoid clogging the threads.

Fig. 20: Screw the thread insert onto the thread installation tool until the tang engages the slot. Screw the insert into the tapped hole until it is 1⁄4 – 1⁄2 turn below the top surface. After installation break the tang off with a hammer and punch.

Fig. 21: In some cases threads can be restored by running a tap in the hole, or a die on the bolt