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Self-Locking and Self-Sealing Fasteners

This page describes the details of our self-locking fastener technology.

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What They Are and How They Work

Threaded parts have a great...

  • Advantage: Parts which have been joined by threaded fasteners can be disassembled and reassembled when they must be serviced or replaced.
  •  Disadvantage: Threaded fasteners unintentionally back off when the assembly is in use, and the consequences are usually inconvenient, costly and may present serious legal implications for product warranties.

How They Work

The self-locking fastener was developed to retain the advantage of reusability while preventing the problems of accidental disassembly when pre-load is lost. Self-locking fasteners virtually eliminate the possibility of a bolted assembly coming apart during operation. To achieve this benefit, the self-locking fastener has to to be properly designed, engineered and installed.

When pre-load is lost, as shown in the chart below, a standard fastener quickly vibrates out, causing the assembly to loosen. The self-locking fastener’s primary function is to prevent backing off when the initial axial tension is lost.

vibration chart

Time to vibrate out after pre-load loss.

By doing so, self-locking helps maintain a tight joint and also helps prevent fatigue failure in the joint. Self-locking fasteners resist rotation on the first installation and on subsequent installations and removals. In addition, self-locking fasteners also decrease the tendency of the fastener to fatigue by reducing the vibration transferred to the fastener.

The Common Alternatives

Jam nuts, cotter pins, lock nuts, lock washers and similar devices also prevent the loss of the bolt or nut by back off but they result in added weight, inconvenience and cost. They do not reduce the tendency to fatigue when loose.

alternatives to self-locking fasteners diagram

Further, the insecurity of conventional mechanical locks are reason enough for most designers to reject them. Such insecurity arises from the frequency of split-type washers breaking, damage to surface areas caused by external locking devices, and the ineffectiveness of such devices when adjustments are needed.


Basic Types of Self-Locking Fasteners

Chemical Additive Type


Dri-Loc® is Long-Lok's chemical additive type self-locking fastener product.

This type involves two alternative methods of application, pre-applied and applied-in-place. These fasteners have a lower assembly torque than that achievable from prevailing torque type fasteners. Additionally, high breakaway torques are achieved due to the solid fill from the adhesive coating after full cure.

Both pre-applied and applied-in-place chemical adhesives offer the locking reliability characteristics of most other self-locking fastener types.

In-place chemical application of chemical additives, however, offer just one-time use and additional assembly time is required to coat the fastener with the additive. Downtime, application errors and contamination of other assembly parts are all inherent risks with in-place application of chemical additive fasteners.

Pre-applied chemical additive fasteners are manufactured with the adhesive coating pre-applied via a sophisticated micro-encapsulation process. The adhesives are usually an anaerobic type, or a two part epoxy, with the material positioned in a band around the threads and dry to the touch. When the fastener is rotated against its mating part the capsules of adhesive burst, releasing the material in and around the thread flanks, which then cures and forms the locking bond.

Since the adhesive coating is pre-applied, the assembly time for these fasteners is substantially less than for applied-in-place coatings. The bond also is more consistent.

Multiple reuse of a pre-applied chemical locking fastener is unlikely, since the relocking action depends upon the amount of reusable adhesive left on the bolt after the first installation. Most engineers design for a maximum of one installation before the bolt must be recoated.

Prevailing Torque Type

prevailing torque

The prevailing torque-type fastener is the only fastener that retains its locking torque independent of an axial load. When pre-load is lost, the prevailing torque fastener can prevent loosening of an assembly subjected to continued shock or vibration.

Prevailing torque fasteners are of two primary kinds - the deflected or distorted thread type and the material additive type. Both require additional installation torque to assemble the fastener before engaging the bearing surface.

Deflected or Distorted Thread Type (All-Metal)

Dyna-Thred II® is Long-Lok's all-metal type self-locking fastener product.

The deflected or distorted thread type fastener provides some reuse. However, most do not share the high reuse characteristics of the material additive types. Additionally, since distorted thread locking fasteners are designed to lock via an irregularity of thread conformity, they are highly sensitive to hole size.

The design engineer must carefully specify the mating hole and the class of thread fit as defined in ANSI-B 18.2.2. Further, sizes above .312 frequently experience galling because of the masses of metal involved as the distorted thread meets the mating thread flanks.

For these reasons the design engineer has to provide a rigorous and tight specification for the mating hole and factory quality control has to ensure meeting that specification whenever using deflected or distorted thread type fasteners.

Material Additive Types

Poly-Lok®, Long-Lok®, Tek-Lok™, Omni-Lok® and Dual-Lok® are Long-Lok's material-additive type self-locking fastener products.

Material additive fasteners work by wedging the fastener into close proximity with the mating thread, causing a metal-to-metal drag in the circumferential direction directly opposite to the material locking device. The material used to produce the wedge customarily is an engineered plastic. A metal wedge can be used for highly demanding applications.

plastic wedge comparison

The upper image shows an ordinary threaded fastener with a Class 2A or 2B thread. Note the space between the male threaded fastener and its mating part.

The lower image shows what happens when a plastic wedge is added making the fastener a prevailing-torque type. Note how the plastic forces the opposite side of the male threaded fastener to wedge tightly against the previously open thread spaces.

The material additive, usually in the form of a patch, pellet, or strip on the fastener itself, tends to fill the open thread spaces between two coaxially oriented parts. The longer the locking feature, the greater the fastener assembly locking torque due to engaging more threads of the locking device. Also, the overall length of the locking element can assist in situations where various degrees of engagements are required for adjustment purposes.

Once the design engineer specifies the fastener material (tensile strength), the acceptable limits of the installation torque and the required prevailing “off” torque, the locking fastener engineer determines what type of additive and how much must be applied to achieve the desired results. So that Long-Lok engineers can do this, the design engineer must answer the following questions:

  • What material is the bolt? From this, the Long-Lok engineers determine its tensile strength.
  • What is the acceptable on-torque (installation torque)? The answer should always be lower than the maximum torque of an automatic tool if used to install the fastener.
  •  What torque is required to keep the assembly together? This must be calculated based on the particular application.

To assist in establishing these requirements, commercial and military standards are available. Refer to the latest revisions of IFI 124 if a commercial item is used, or to MIL-DTL-18240 if a military fastener is required.

In axially loaded applications, the externally threaded fastener must contain a self-locking feature having a minimum of two complete threads beyond the hole, slot, or groove used for the locking feature engaged with the mating part. This rule does not apply to patch-type self-locking fasteners.

Applications – Problems and Solutions

This section deals with some specific fastener applications. When explaining a solution to an application problem we have suggested certain types of Long-Lok fasteners. This is not to suggest that the fastener mentioned is recommended only for that specific application or that, in some cases, other fastener types could not be used.

The following examples incorporate the use of Long-Lok products as suggested solutions to commonly encountered fastening problems. They are offered as suggestions only since your own experience, unique set of application conditions, and use of Long-Lok’s extensive experience will solve your specific problem. Other companies may offer similar products, but only Long-Lok provides custom solutions to special problems in fastener engineering.

Problem: An outside truck mirror vibrates loose after a limited amount of rough driving.

Solution: Either Poly-Lok® or Long-Lok®, can be used to solve this type of problem.

Reliability of assembly in a high vibration environment is required. The resilient insert or patch type fastener is ideally suited since it does not rely on axial tension in the assembly to insure staying in place.

In general, the resilience of the plastic holds the fastener in place without adhesives or thread distortion, and provides locking action in the thread instead of at the bearing surface. The resilient nature of the locking plastic maintains fastener lock due to its vibration dampening characteristics.

Problem: Carburetor adjustment screws that must be able to operate in a high temperature environment vibrate out of adjustment.

Solution: The Poly-Lok® plastic additive fastener is best suited to solve this problem.

Here the requirement is for the plastic insert to be able to operate at 380° F. Nylon inserts cannot be used because nylon is limited to 250° F. Only Poly-Lok®, made of polyethylene terephthalate (a Long-Lok patented exclusive), meets the requirement of unusually high temperature, multiple reuse characteristics in accordance with MIL-DTL-18240 and IFI 124, and economical purchase characteristics.

In general, when optimum torque is necessary plus the ability to withstand an unusually high temperature range, plus a relatively low cost of purchase, self-locking prevailing torque Poly-Lok® is best suited for the job. The Poly-Lok® fastener maintains torque performance through temperatures as high as 400° F and as low as -100° F, and is not affected by hydrocarbons.

Problem: Engine cycling requires a self-locking part that is capable of withstanding cycling from ambient to 500° F and still meet multiple reuse requirements.

Solution: The Dual-Lok® plastic additive fastener is used to meet this requirement. In this application high-temperature Vespel® engineered plastic is captured in a unique, patented manner. With Dual-Lok®, two locking surfaces are exposed to better maintain required torques under severe conditions.

The Dual-Lok® method can be used with a wide variety of insert materials depending upon the application requirements. This dual locking element means of capturing the insert material eliminates strip fall out and is just one more of the unique products developed by Long-Lok engineers for special requirements.

Problem: A steel fastener has to be used to attach an electronic assembly to a cast aluminum housing.

Solution: A T-Sert® thin wall self-locking insert supplies a strong locking support in the soft metal. The T-Sert® is inserted into the soft metal hole and locked in place by its prevailing torque value. When the fastener is installed, it too is locked in place by the insert plastic so that the entire assembly becomes a prevailing torque assembly. The one-piece, solid wall construction of T-Sert® provides the ultimate in both torque and tensile strength, exceeding the requirements of MIL-N-25027 and MIL-I-45932.

The design of the T-Sert® self-locking insert reduces its cost to substantially below other types of inserts. Further, because of the ease of installation, the T-Sert® has lower installed cost than all other types of insert devices. T-Sert® installation can be made from either end resulting in additional savings in time. No special drills, taps or gauges are required for installation.

Because it does not require swaging or driving of rings the T-Sert® causes no undue stresses due to installation and edge distances are not critical.

Problem: Small lock washers used in the assembly of an electronic part work loose after the product has been sold. It is identified that the normal expansion and contraction of the assembly resulting from component heating is causing the problem.

Solution: Prevailing torque type fasteners are a popular solution to this problem. All lock washers require maintenance of the pre-load to maintain positive locking performance. Unfortunately, even normal temperature changes, in addition to shock or vibration, can cause pre-load loss which, in turn, eliminates any locking action.

Self-locking fasteners are designed to put the locking action in the thread, where it is needed. Since the locking action is in the thread, prevailing torque remains even as the pre-load changes with temperature.

Problem: A mechanical assembly must be fastened on the hot side of a gas turbine.

Solution: For applications up to 1200° F Dyna-Thred® II, an all metal lock is called for. The Dyna-Thred® II lock is achieved by forming a close tolerance hole axially into the bolt end of a standard thread form. Then a controlled area of the cavity is dilated to expand the periphery of the controlled thread area. The pitch diameter of the first thread is left unchanged for starting ease.

The result is an easily installed, highly reliable, self-locking fastener that meets all performance requirements of MIL-F-8961.

Problem: A mass produced toy that will take very hard use must be fastened at the lowest possible cost. The relatively light weight components, and the speed of application, require low driving torque type fasteners. High breakaway torque is needed to stand the rough use.

Solution: Dri-Loc®, a chemical additive self-locking fastener, is called for. Dri-Loc® fasteners are coated with a micro-encapsulated anaerobic adhesive. When the fastener is rotated in the mating part, the inert micro-encapsulated adhesive is released and forms a bond which, when it hardens, both locks and seals the assembly. Additionally, since Dri-Loc® fasteners require very low on-torque, assembly is faster and more economical. See specification IFI 125 for torque performance requirements.

Problem: In place fastener cost reduction is required to relieve rising inventory and assembly costs.

Solution: Replace bolts and lock washers with one piece self-locking fasteners. A self-locking fastener is, in effect, a lock washer and bolt all in one piece. That means you inventory fewer items.

A self-locking fastener is easier and faster to install. The result is less assembly time and lower assembly costs, and no margin of error. It adds up to savings in-place.

Problem: A borescope plug cap has to be fastened on the hot side of a jet engine with multiple reuse and multiple rework capabilities.

Solution: Omni-Lok® solves this problem.

Omni-Lok® is a patented process in which a longitudinal hole is drilled and a metal pin is installed in such a fashion that the pin extends above the root diameter of externally threaded parts but still is contained wholly within the major diameter. The pin provides an effective wedge which provides the prevailing torque feature.

Omni-Lok® is especially suited for those threaded components which require a prevailing torque locking element which has one or more of the following characteristics:

It has high prevailing torque and can be used in unusual or non-conventional configurations. It withstands (1) very high (1200° F or more) or very low (cryogenic) temperatures, (2) greater than normal stress and vibration conditions, and (3) severe corrosion conditions. It conforms to MIL-F-8961 requirements.

Problem: An aircraft panel requires over 300 reuses of a self-locking prevailing torque type fastener.

Solution: Poly-Lok® meets this requirement. Poly-Lok® not only provides high temperature capability but can achieve more than 300 reuses and still meet the MIL-DTL-18240 and the IFI 124 minimum torque requirements.

Engineers should be particularly interested in specifying Poly-Lok® where the application calls for multiple reuses above the minimum of 15 called out by MIL-DTL-18240. A number of military applications can now be handled by a simple fastener that previously required complex mechanical locks or locking assemblies.

Application Considerations

Consideration of the following factors will eliminate the majority of application problems encountered when using self-locking fasteners. Review them carefully and consult the factory for any unanswered questions.

  1. Use only self-locking devices which are MIL-Spec and/or Q.P.L. approved and manufactured by a qualified Long-Lok facility.
  2. Control quality and design of both self-locking and mating parts within the limits of applicable standard thread specifications.
  3. Countersink the female mating part or chamfer the male mating part end point as described in the installation data.
  4. Mating parts must be clean and free of all foreign material.
  5. Use of self-locking elements in mating parts with keyway, slots, cross holes, or other thread form interruptions are not recommended and can adversely affect locking torque performance and reusability characteristics.
  6. Manufacture of some commercial strip type fasteners may produce a very slight metal burr at the engagement end of the longitudinal slot. These parts are usually acceptable for 90% of self-locking applications. If you specify that the parts be burr free (this term is defined in the Glossary), Long-Lok will remove end burrs.
  7. Extensive, independent test data demonstrates that when used properly, the tensile strength of Long-Lok strip and pellet type fasteners are not adversely affected by the milled slots or drilled holes used to accommodate their respective locking elements. Sizes of 1/4” diameter and smaller may exhibit a very slight reduction directly in the slotted or drilled hole area. However, similar test data has shown that a high tensile strength margin remains; all parts still meet minimum specification requirements. Torsional strength for these parts is only affected in sizes smaller than #8, and then only when tightened beyond yield strength. Shear strength is not lost in any size; high shear bolt grips are not altered. For strip type parts the ends of the insert slot are radiused to prevent “notch effect” or stress risers.
  8. Torsional resistance to turning (prevailing “on” torque) is greatest at the initial installation of the self-locking component with the mating thread form. This results from the thread form being worked into the locking feature, along with burnishing of the surface roughness of the threads. The second and third installations continue these actions to a lesser degree. The resulting torque value through subsequent repeated cycling will be reduced much more gradually, the degree of which will depend on the wear factor of the mating materials and resilience of the locking feature used.
  9. For externally threaded parts, tap drills should be sized to develop a 65% to 75% theoretical thread height in the mating hole. All holes should be tapped with a cut thread and not a rolled thread tap to insure quality of the mating thread and optimum prevailing torque performance.

Installation Recommendations

Applicable to material additive type locking screws (Poly-Lok®, Long-Lok®, Tek-Lok™, Omni-Lok® and Dual-Lok® fasteners) pre-applied adhesive type (Dri-Loc® fasteners) and thread sealing type (Vibra-Seal® fasteners).

Countersink. To maximize prevailing torque performance and prevent potential shearing of the locking material at installation in the mating thread form, the tapped hole should be countersunk.

installation data 1 drawing

Clearance Hole. Locking elements on externally threaded components typically protrude to .003 over the maximum major diameter; they pass through normal clearance holes with finger pressure. Consult factory if flush installation is required.

installation diagram 2

Chamfer. For locking features applied to internal thread forms use of a mating fastener or component with a chamfered end point is recommended.

installation diagram 3

Torque Calculations

The concept of “Seating Torque” is typically a primary design consideration in any given fastener application. Any designated value for seating torque will have a direct affect on the resulting amount of fastener pre-load (tension) and associated desired clamping force in the assembly.

preload diagram

Below are commonly applied formulae used to derive specific seating torque values associated with design pre-load requirements. Please note that these are offered for example purposes only. As such, they should not be used as the sole basis for specific design criteria of a given application or assembly.

Desired Preload

Preload is calculated using the following formula:

preload formula

Seating Torque (without locking element)

Seating Torque is calculated using the following formula:

seating torque formula

Seating Torque (With locking element present)

Seating Torque for fasteners incorporating a locking element is calculated using the following formula:

seating torque with lock formula

The average prevailing torque value produced by the presence of a self-locking element must be added to the non-locking seating torque value to achieve the desired pre-load value.

*K Factor

The pre-load value results from the amount of applied seating torque. The torque coefficient of friction factor (K) is measure of the lubricity of a fastener. As such, it must be considered for compensation of numerous design variables such as configuration, size, surface texture, etc. For example, since cadmium plating adds lubricity, it has a smaller “K Factor” than plain steel. Therefore, to develop the same desired pre-load value, a cadmium plated part would require less applied seating torque than the same part in plain finish.

Note: The following “K Factors” are averages offered for example purposes only. The accuracy of any selected “K Factor” is subject to many application variables not discussed here.

Finish K Factor
Plain Steel .20
Zinc .21 to .33
Cadmium .15 to .20
Blakc Oxide .16 to .19
Dry Film Lube, Grease, Oil or Wax .12
Empirical Torque Adders

The Empirical Torque Values shown below approximate the additional prevailing torque developed by Material Additive type fasteners when compared to non-locking fasteners. Add these values to standard non-locking fastener seating torque values to calculate the torque required to develop a given tension (pre-load) in the assembly. Values do not apply to all-metal types (Dyna-Thred II®). Consult factory for appropriate values or alternative design considerations for all-metal types.

Example: If a non-locking No.10 screw requires 30 inch pounds of torque to develop desired tension: a Long-Lok No.10 screw requires the same torque plus 6 inch pounds or 36 inch pounds to develop the same tension.

Size Empirical Value Size Empirical Value
#0 2.0  (inch-oz.) 1/4" 12.0  (inch-lbs.)
#1 4.0  (inch-oz.) 5/16" 24.0  (inch-lbs.)
#2 9.0  (inch-oz.) 3/8" 32.0  (inch-lbs.)
#3 1.0  (inch-lbs.) 7/16" 40.0  (inch-lbs.)
#4 1.5  (inch-lbs.) 1/2" 60.0  (inch-lbs.)
#5 2.0  (inch-lbs.) 9/16" 80.0  (inch-lbs.)
#6 3.0  (inch-lbs.) 5/8" 120.0  (inch-lbs.)
#8 4.0  (inch-lbs.) 3/4" 160.0  (inch-lbs.)
#10 6.0  (inch-lbs.) 7/8" 240.0  (inch-lbs.)
#12 8.0  (inch-lbs.) 1" 320.0  (inch-lbs.)

Fastener Torque Ranges

Assumes no lubrication on threads. Torque values are based on friction coefficients of 0.12 between threads and 0.14 between nut and washer or head and washer, as manufactured (no special cleaning).

Size Root Area (sq. in.) Torque Range
(Class 8, 150 KSI, bolts*)
10-24 0.0145 23 to 34 in.-lb.
10-32 .0175 29 to 43 in.-lb.
1/4-20 .0269 54 to 81 in.-lb.
1/4-4-28 .0326 68 to 102 in.-lb.
5/16-18 .0454 117 to 176 in.-lb.
5/16-24 .0524 139 to 208 in.-lb.
3/8-16 .0678 205 to 308 in.-lb.
3/8-24 .0809 230 to 345 in.-lb.
7/16-14 .0903 28 to 42 ft.-lb.
7/16-20 .1090 33 to 50 ft.-lb.
1/2-13 .1257 42 to 64 ft.-lb.
1/2-20 .1486 52 to 77 ft.-lb.
9/16-12 .1620 61 to 91 ft.-lb.
9/16-18 .1888 73 to 109 ft.-lb.
5/8-11 .2018 84 to 126 ft.-lb.
5/8-18 .2400 104 to 156 ft.-lb.
3/4-10 .3020 117 to 176 ft.-lb.**
3/4-16 .3513 139 to 208ft.-lb. **
7/8-9 .4193 184 to 276 ft.-lb. **
7/8-14 .4805 213 to 320ft.-lb. **
1-8 .5510 276 to 414 ft.-lb. **
1-14 .6464 323 to 485 ft.-lb. **
1 1/8-7 .6931 390 to 585ft.-lb. **
1 1/8-12 .8118 465 to 698ft.-lb. **
1 1/4-7 .8898 559 to 838ft.-lb. **
1 1/4-12 1.0238 655 to 982ft.-lb. **

*The values given are 50 and 75 percent of theoretical yield strength of a bolt material with a yield of 120 ksi. Corresponding values for materials with different yield strengths can be obtained by multiplying these table values by the ratio of the respective material yield strengths.

** Bolts of 0.75 in. diameter and larger have reduced allowables (75 percent of normal strength) owing to inability to heat treat this large a cross section to an even hardness.

Table reprinted with permission from MACHINE DESIGN, Nov.19, 1987. A Penton Publication.

Specifications and Standards


MIL-DTL-18240: Performance specification for prevailing torque type fasteners performing to +250°F — both external and internal threads. Defines patch, strip, and pellet type configurations and offers 5 cycle reusability and torque performance requirements.

IFI 124: Industrial Fastener Institute commercial performance standard and test procedure for prevailing torque type fasteners using non-metallic locking elements. Offers 5 cycle reusability and torque performance requirements.

IFI 524: Same as IFI 124, but for metric size thread forms.

NAS 1283: Design standard for male threaded self-locking fasteners. Incorporates MIL-DTL-18240 performance standards and test requirements by reference.

MS 15981: Design standard for design and usage limitations of externally threaded self-locking fasteners. Incorporates MIL-DTL-18240 performance standards and test requirements by reference.

MIL-F-8961: Design and performance standard for high temperature (450°F + 1200°F) externally threaded self-locking fasteners. Offers 15 cycle reusability and torque performance requirements.

MIL-I-45932: Design and performance standard for thin wall, self-locking screw thread inserts. Includes design, performance and torque requirements for integral locking device type, both metallic and non-metallic.

MIL-S-82496A: Procurement specification for integral O-ring, self-sealing machine screws. Offers design, performance and test requirements for self-sealing fasteners including reusability and minimum pressure sealing capability.

IFI 125: Industrial Fastener Institute commercial performance and test procedure for chemically coated (pre-applied and assembly-applied adhesive) lock screws. Offers prevailing and breakaway torque performance criteria

IFI 525: Same as IFI 125, but for metric size thread forms.


Below are industry accepted manufacturing standards for the commercial fastener types listed. In lieu of MS, NAS or customer specifications, these can be used for inspection/acceptance criteria:

Category (Head Styles) Long-Lok Style Code Standard
machine screws: includes round, pan, oval, flat, binder, fillister, truss and hex A, D, F, G, J, M, N, T, V ANSI B18.6.3
socket screws: includes button, socket cap and flat socket B, G, J, U, X ANSI B18.3
finished bolts: includes hex cap W ANSI B18.2.1

Long-Lok, over the years, has worked closely with the U.S. Military, the Industrial Fastener Institute and the National Aerospace Standards Committee to insure that our products meet, or exceed, the stringent requirements of these technical bodies. Not all manufacturers can meet these requirements.

If you wish to use a Long-Lok self-locking element in a fastener that, under the AN, MS or NAS part number is non-locking, simple substitute the desired Long-Lok insert prefix for the AN, MS or NAS prefix.

Custom Solutions to Special Problems in Fastener Engineering

Not every thread form is part of a fastener. And no one knows this better than Long-Lok!

Every year we process some very sophisticated components... large and small. What they all have in common is a thread form that must stay secured when assembled.

With our Poly-Lok® Patch, Long-Lok® Strip, Tek-Lok® Pellet and our Dri-Loc® Adhesive, we are confident we can live up to our motto, “Custom Solutions to Special Problems in Fastener Engineering.” Our strip and pellet inserts are available in a variety of materials including Kel-F® and Vespel®. For even more control over prevailing or locking torque loads, Long-Lok can custom formulate both Poly-Lok patch material and Dri-Loc adhesive resins. This broad range of locking options combined with our unmatched fastener engineering experience enable us to offer capabilites unavailable from any other company.

So don’t think of Long-Lok only when considering common locking fasteners! As you can see from the components pictured here, we can apply a thread locking method to an endless variety of thread forms. These include internal thread forms ranging from standard hex nuts to specially designed internally threaded components. In many cases we can manufacture the entire component, but we welcome you to send your components to us for self-locking processing only.

With the aid of a sketch, drawing or, better yet, an actual sample part from you, a Long-Lok sales engineer will gladly assist you in determining the locking method best suited to your component’s application requirements.

See for Yourself

Send us a sample of your part or fastener, along with a brief description of its application and performance requirements, and we’ll process it using the most appropriate Long-Lok thread locking or sealing method. Or, to sample a standard Long-Lok fastener, simply supply us with a Part Number. Samples are provided free of charge.


Breakaway Torque: also known as “Off Torque” or “Removal Torque”.The torque necessary to start relative rotation between a locking fastener and its mating thread with no axial load on the screw. This will change with reuse and be referred to as the “1st Off”, “5th Off” torque and so forth. This is the torque due to the locking element only.

Breakloose Torque: amount of torque required to overcome the axial load of a seated fastener (pre-load), relieving the tension, initiating movement in the off direction. The torque due to clamp load only. A value unrelated to prevailing torque.

Burr Free: in the context of self-locking components, this reference applies to the milled area of strip and pellet installation areas. MIL-DTL-18240 allows for, “Burrs in the area of the locking feature not adversely affecting torque performance”. Long-Lok Fasteners offers a higher quality standard that provides an absence of significant burrs in the region of the milled slot or drilled hole feature as viewed under five power magnification. Additionally, the mechanical verification of this condition is the thread’s ability to freely accept a “Go” ring gage after removal of the locking strip or pellet insert. This higher standard is specified by adding the suffix code “HQ” to the Long-Lok part number when ordering.

Clamp Load: a measure of compressive force in a joint or assembly, resulting from the existing pre-load of a fastener. Typically measured in “pounds”, a value of clamping force imparted by the elastic tension of the fastener in the assembled mating parts.

Cold Flow: continued deformation of a material under stress.

Elastomer:  any synthetic or natural material with a resilience characteristic or “memory” sufficient to cause it to return to its original shape or form after major or minor distortion.

Installation Torque: also known as “On-Torque”. Highest value of prevailing or running torque exhibited by a part while installing or engaging to the mating thread form prior to start of any axial load (pre-load).

Locking Torque: a measure of the performance of a self-locking device, i.e. resistance to rotation. Typically in the “off” direction, not to include torque required to induce or relieve a parts axial load (pre-load). Compare Breakaway Torque.

Permeability: the rate at which a liquid or gas, under pressure, passes through a solid material by diffusion and solution. As related to elastomers, it is the rate of gas flow expressed in atmospheric cubic centimeters per second (ATM CC/CM2/CM/SEC).

Pre-Load: a measure of the axial load imparted on a fastener. A result of the amount of the applied seating torque, typically measured in pounds per square inch (psi) to create tension in the fastener. Commonly accepted to be 80-85% yield strength.

Prevailing “Off” Torque:

  •  Maximum: the highest reading indicated by a torque measuring device while the screw or nut is being backed off throughout the first 360 degrees of rotation after the axial load is reduced to zero after breakaway.
  •  Minimum: the lowest reading indicated by a torque measuring device while the screw or nut is being backed off throughout the first 360 degrees of rotation after the axial load is reduced to zero after breakaway.

(note: The use of a memory device on the torque wrench makes it easy to record the maximum prevailing torque, but the indicator must be watched carefully in order to record the minimum prevailing torque.)

Prevailing Torque Type: threaded fastener frictionally resistant to rotation due to a built-in wedge. This type of fastener retains its locking ability independent of axial tension or pre-load.

Prevailing Torque: also known as “Running Torque”. Unique to self-locking fasteners or components. A measure of the inherent ability of a part to produce frictional resistance to rotation in a mating thread form. Exists independent of clamp load, inclusive of both installation and removal torque. Typically an average measure of a parts prevailing torque characteristic in either the installation or removal cycle, independent of clamp load.

Seating Torque: Sometimes referred to as “Loading Torque”. Typically measured in “inch pounds”, a value of torque applied to a fastener to induce a compressive load under the head bearing surface. Results in creating an axial load (pre-load) that imparts tension or a stretching/elongation characteristic to a male threaded part.

Loctite®, Dri-Loc® and Vibra-Seal® are Henkel AG trademarks.