Spiralock® vs. Locking Inserts... FAQ’s

The KFS Tech Group occasionally gets questions concerning the use of Spiralock as a replacement for threaded inserts. Spiralock is a unique, self-locking, internal thread form that has specific application benefits. However, Spiralock is not a means of replacing all of the design features and benefits of helical coil locking inserts.

Spiral lock

Spiralock provides a locking action on the male fastener as long as the assembly clamp load is maintained (i.e. the bolt remains tight). If the assembly loosens, there is no longer any locking action or prevailing torque on the male fastener. However, locking helical coil inserts provide a locking action (prevailing torque) even if the assembly were to lose its clamp load, thus minimizing the possibility of the male fastener coming completely out of the assembly (see diagram below).

The following are additional features of helical coil locking inserts that are not provided by Spiralock:

  1. Stainless steel inserts provide hard (Rc 43-50), reusable threads in lightweight metals like aluminum, that are resistant to wear and stripping. With Spiralock, the thread would be tapped directly into the softer parent material.
  2. Inserts provide an abrupt starting thread that speeds the assembly time of the male fastener, and eliminates cross threading.
  3. Inserts provide stronger assemblies in parent materials than tapped threads alone. This is because inserts have a larger major diameter, which increases the shear area in the parent material.
  4. Maximum clamping action and bolt tension are assured with minimum wrench torque because inserts have a smooth (8-16 micro inch) finish.

Other factors to consider before using Spiralock to replace locking inserts:

  • Spiralock has special internal thread geometry. Special taps and gages are required. Three (3) gages are required to check thread geometry (2 gages for threads with pitches larger than 32 TPI). These taps and gages are more costly than standard STI taps and gages used with inserts, and they are not easily obtained. In-house inspection and recalibration of Spiralock gages requires signing a confidentiality agreement before specifications are supplied.
  • Testing: The manufacturer recommends that Spiralock be vibration tested whenever possible. Locking inserts have already met this requirement per earlier revisions of MIL-I-8846. (Vibration testing has been eliminated in current revisions, ref. NASM8846).
  • Spiralock provides locking action in one direction only. The male fastener can only be assembled from one end of a through hole. Inserts are bi-directional, and locking torque is provided when the male fastener is assembled from either end of a through hole.
  • Identification: It is quite difficult to look into a tapped hole and discern the presence of the Spiralock thread form. The manufacturer recommends bore scopes, special plug gages, or silicone impressions. Locking inserts are easily distinguished, as they are dyed red for identification.
  • Indication of Locking Torque: There is no visible means of inspecting a Spiralock assembly to determine if it is in a locked torque condition. If the assembly were to lose its clamp load, there would be no visible indication of this condition. The same is true of an insert assembly, however, there would still be prevailing torque applied to the male fastener to minimize the possibility of the bolt backing out of the assembly.
  • Spiralock guideline assembly torque values are provided by the manufacturer. With locking inserts, the prevailing torque applied by the insert’s locking coils can be added to the normal assembly torque for that particular male fastener.
  • Gasket Materials: With Spiralock, the manufacturer recommends that soft gasket materials be avoided. Gasket materials do not affect the locking torque action provided by locking inserts.
  • Soft Parent Materials: The manufacturer says that Spiralock can be used in soft materials, and adds, “inserts may be eliminated.” However, they also say that it should be recognized that soft materials can be deformed under local contact stresses, and that reusability can be affected. Inserts, however, induce virtually no stress into the parent material, because their outward spring-like action holds them in place.
  • Repairing a damaged or oversize Spiralock thread from: The manufacturer recommends contacting them to review the problem. Damaged or oversize insert assemblies are easily repaired by either of two readily available systems utilizing either inserts with larger wire cross-sections, or one insert inside another.
  • Adjusting Screw Applications: Spiralock is not suitable for these applications. However, the manufacturer notes that if the screw is kept in constant tension (i.e. lock nut or strong spring), then Spiralock may be applicable. Locking inserts, however, do not carry this limitation, because the locking action on the screw is applied by the locking coils, and is not dependent on the bolt being kept in constant tension.
  • Rethreading parts tapped with standard threads: This is not recommended or easily accomplished with Spiralock. However, with inserts, this is easily done by drilling and tapping the existing standard thread and installing an insert – without having to go to a larger thread size.


It is our opinion that locking inserts cannot be replaced by Spiralock for all of the previously discussed reasons. Moreover, if an insert loses its locking action because it has exceeded its design cycle life, it can easily be removed and replaced with another insert.

If you have any questions, please contact us.

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