Types of Inserts

There are two styles of Heli-Coil inserts.

The standard or FREE RUNNING insert

which provides a smooth free-running

thread; and SCREW-LOCKING which

provides self-locking torque on the male

member by a series of “chords” on one

or more of the insert coils. They are both

available in Tanged and Tangless; Inch and

Metric series.

Heli-Coil inserts are precision formed coils of

extremely hard, diamond shaped 304 stainless steel.

When installed into an STI tapped hole, they provide

permanent conventional 60° internal screw threads.

This assembled insert accommodates any standard bolt

or screw as per MIL-S-7742 and AS8879 (UNJ controlled

root radius). Inserts can be custom made in all materials

listed on p. 9.

Tanged Size Range:

» UNC #1 through 1-1/2 » UNF #2 through 1-1/2

» Metric Coarse M2 through M39 » Metric Fine M8 through M39

Inserts are also available in UNEF, UNS, 8UN, 12UN, 16UN, Spark Plug and Pipe Thread.

Types of Inserts

There are two styles of Heli-Coil inserts.

The standard or FREE RUNNING insert

which provides a smooth free-running

thread; and SCREW-LOCKING which

provides self-locking torque on the male

member by a series of “chords” on one

or more of the insert coils. They are both

available in Tanged and Tangless; Inch and

Metric series.

(Note: Inch series Screw-Locking inserts are dyed red for identification.)

Illustration of the Retention Principle

Free Running Insert Screw-Locking Insert

Heli-Coil tanged inserts are larger in diameter

before installation than the tapped hole. During installation,

the inserting tool applies torque to the tang reducing

the diameter of the leading coil permitting it to enter

the tapped thread. After installation, each high tensile

coil of the insert expands outward with a spring-like

action permanently anchoring the insert.

Tangless inserts eliminate the need for tang break-off and retrievals. Since no

loose tangs are left behind, they become the solution when Foreign Object Debris (FOD)

cannot be present in critical applications. Loose tangs can potentially cause damage and

result in the need for costly repairs.

Tangless inserts are manufactured using the same materials as standard inserts and

provide the same superior performance characteristics. They are available in Free

Running and Screw-Locking and in inch and metric series, coarse and fine.

Heli-Coil inserts provide a positive means for protecting and strengthening

tapped threads in any material. The unique design features of the insert offer

many benefits.

Stronger Assemblies

Tapped threads are strengthened because the inherent flexibility of the insert provides

a more balanced distribution of dynamic and static loads throughout the length of thread

engagement. This flexibility also compensates for variation in lead and angle error allowing

each coil to carry its share of the load.

No Thread Wear

Thread life is dramatically increased even after repeated assembly and disassembly

because the insert hardness and surface finish practically eliminate erosion of the

thread form due to frictional wear.

Corrosion Resistance

Under normal environmental conditions, Heli-Coil inserts minimize galvanic action within

the threaded assembly because of their superior corrosion resistance.

Design Flexibility

Bolt tensile strength can be balanced against parent material shear strength, assuring

bolt failure rather than parent material damage. Five insert lengths are available in each

thread size.

Eliminate Stress

Virtually no stress is introduced into the parent material because there is no staking,

locking, swaging or keying in place – the outward spring-like action of the insert

holds it in place.

Minimize Space & Weight

Heli-Coil inserts allow the use of smaller bosses and flanges than any other insert.

Heli-Coil inserts can generally be incorporated into existing designs, where no other

provision has been made for an insert, without increasing boss size.

Minimize Total Cost

Cost savings abound. Lower insert cost, lower installation cost and Heli-Coil inserts provide

design flexibility by allowing a wide choice of parent materials while maintaining maximum

threaded assembly strength.

True Clamping Torque

Maximum clamping action and bolt tension are assured with minimum wrench torque

because of the mirror-smooth surface finish of Heli-Coil inserts.

Wide Temperature Range

Heli-Coil stainless steel inserts can be used in temperatures ranging from -320°F to +800°F.

Quality & Reliability

Stringent Quality Assurance and Engineering Standards are rigidly enforced in all phases

of the manufacturing process. This assures integrity of your product design.

High Volume Production

Heli-Coil inserts are available mounted

on plastic strips and wound onto reels

(500 or 1000 inserts per reel). With power

installation tooling, use of strip feed inserts

will substantially increase installation rates

by minimizing handling.

Universal Acceptance

Heli-Coil Standard and Screw-Locking Inserts

are the original – and have an extensive

background of tension, torque, shear,

vibration and fatigue tests conducted by

American industry’s leading companies

as well as the U.S. Military. Successful

applications in the fields of aviation,

electronics, industrial, automotive and

military equipment provide a wealth

of experience and confidence in the

performance and reliability of Heli-Coil


Custom Design Services

In addition to the benefits listed above,

Heli-Coil provides a wide range of support

to solve fastening problems. This design

catalog is one of them. The following pages

are presented in a manner to make it easy

to “design-in” Heli-Coil inserts to take

advantage of the extraordinary benefits

they provide.

Additionally, our Sales Engineers,

Applications Engineers and Design

Engineers are available for consultation

for specific designs. When the product gets

to the manufacturing phase, our extensive

experience in production tooling and

installation techniques ensures that you

can indeed make your product better

with Heli-Coil inserts.

Screw-Locking Inserts

» A resilient locking mechanism that grips the bolt and prevents it from

loosening under vibration or impact.

» Repeated assembly and disassembly without appreciable loss

of positive self-locking torque.

» Savings in space, weight and money, through the elimination

of lock wiring, lock nuts, lock washers, chemical compounds,

plastic pellets/patches and/or other locking mechanisms.

» Inch inserts dyed red for easy identifi cation.

» Meets NASM8846, MA1565, NASM21209, MA3329,

MA3330, MA3331.

Hi-Torque Inserts

» Similar to Screw-Locking but with higher prevailing torque which compensates

for reduced friction in highly lubricated applications.

» Ideal for higher vibration applications.

» Approximate 40% increase in prevailing torque levels.

» Available in #10 through 3/8" UNF only.

» Meets AS1394, AS3094, AS3095, AS3096, AS3097.

Stud-Lock Inserts

» Highest prevailing torque insert available.

» Enables use of threaded rod for space-saving stud applications.

» Allows for any class fi t of threaded rod.

» Eliminates inconsistencies caused by interference-fi t studs.

» Available for both straight and step studs, #10 through 1/2" UNC and UNF.

» Meets AS1229, AS3080, AS3081, AS3082, AS3083.

» RoHS compliant; contains no chromates

» Eliminates need for zinc primers and messy epoxies

» Provides uniform top to bottom coverage

» Provides enhanced visibility in the assembly

Primer-Free® II » Prevents galvanic corrosion between insert and parent material

» Eliminates locking torque issues associated with primers

» Eliminates clogging/fouling of installation tools

» Improves installation productivity

» Provides additional lubrication facilitating insert installation

Material Spec: None

Color: Matte black

» Provides additional lubrication in high friction applications

» High temperature resistance (400°F)

Dry Film Lubricant » Highly recommended with Heli-Coil Screw-Locking inserts

» Mildly corrosion resist

Heli-Coil offers maximum design fl exibility while adhering to conservative engineering practice

allowing use of Heli-Coil inserts in virtually any application or material. Five lengths of inserts

are available. In this design manual, the lengths are listed as multiples of the nominal thread

diameter of the screw; 1, 1-½, 2, 2-½, and 3. This choice of insert length balances the bolt

tensile strength against the shear strength of the parent material. This allows for the design of

assemblies where the bolt will fail before the parent material. Tables III and IV below show the

length of insert to be used with different combinations of bolts and parent materials.

The effect of corrosion on threaded

assemblies is dependent on many factors -

environment, types of metals used, sealing

mechanisms and length of service. The

following recommendations apply for

minimizing the effects of corrosion on

Heli-Coil stainless steel insert assemblies

at operating temperatures less than 800°F,

using carbon steel or alloy steel bolts.

The following defi nitions apply…

Normal Service. Natural atmosphere

environment with the screw always assembled

in the insert.

Severe Service. Mildly contaminated

atmospheric conditions involving moisture,

occasional exposure to salty air or sea spray

and the screw may be left out of the insert for

extended periods of time.

Extremely Severe Service. Assembly is

exposed to salt water, corrosive atmosphere

and/or the screw is out of the assembly

frequently allowing a blind hole to trap water.

Tangless threaded inserts provide many advantages over

Tanged inserts.

They eliminate the need for tang break off and tang retrievals

and allow for non-destructive insert removal. Tangless inserts

are easily adjusted and easily removed after installation.

Inch and metric size inserts are available in stainless steel and

gall resistant (or Nitronic 60) materials. Finish options include

Primer Free® II, Dry Film Lube (and Cadmium when required by Mil Spec.)

Standard boss configurations may be used with Heli-Coil inserts.

A boss diameter of twice the nominal bolt size is adequate for most

load conditions. For critical applications, the boss diameter should be

twice the Heli-Coil tap major diameter (Tables VII & VIII, p. 20-21).

Boss thickness is a function of the size and length of the insert chosen

and the particular requirements of the component being designed.

The use of Heli-Coil inserts generally minimizes the size of the boss

because their high strength characteristics allow for smaller or fewer


Class of Fit

Since Heli-Coil inserts are flexible, the class of fit of the final

assembly is a function of the tapped hole. Heli-Coil STI (Screw

Thread Insert) taps are available in inch series for both Class 2B and

3B. Metric Classes include 5H and 4H5H. Class 2B or 5H tapped

holes provide widest production tolerances while Class 3B or 4H5H

holes provide slightly tighter tolerances.

Class 3B or 4H5H holes are recommended for Screw-Locking


Standard bolts and screws that need no special

hardware are used with Heli-Coil inserts. The bolt

must engage the entire insert to insure maximum

assembly strength. It is strongly recommended that the

tang always be removed and bolt projection be equal

to the full tapped thread depth (Dimension C,

Tables VII & VIII, p. 20-21). If design limitations

prohibit this, contact us to obtain minimum bolt

projection data.

The suggested drill sizes listed for aluminum in Tables V & VI,

p. 18-19, are within the minor diameter limits specified in

NASM33537 or MA1567. Drill sizes listed for steel, magnesium

and plastic are larger (in most cases) allowing for parent material

“close-in” in soft materials and increased tap wear life in hard


The drill depths listed in this table allow for tap end clearance,

maximum insert “set-down”, countersink, and the chamfer on the

tap. These drill depths are minimum and should be increased where

possible, especially when using Spiral Pointed Taps, to allow for chip

clearance. The formula for the drill depth is given on p.18-19.

2. Countersinking

Countersinking the drilled hole is recommended to prevent a feather

edge at the top of the tapped hole and to help guide the insert into

the tapped threads. A 120° included angle countersink is necessary

to insure that the angle of the tapped thread and the countersink

are the same (120° ÷ 2 = 60° tapped thread).

3. Tapping

The dimensions for the depth of the full tapped thread (Dimension C,

Tables VII & VIII, p.20-21) are MINIMUM for blind holes with countersinks.

For through holes without a countersink the minimum full

tapped thread depth must be equal to the insert nominal length

(Dimension Q, p.12-13).

Is the tapped hole correct?

Accuracy of the finished thread when the insert is installed

is dependent upon the accuracy of the tapped hole. If the

finished tapped hole gages satisfactorily, the installed insert

will be within the thread tolerance.

It is not necessary to gage the installed insert.

After the insert is installed, the GO thread plug gage may

not enter freely; however, the insert will always seat itself

when the bolt or screw is installed and tightened. (Reference


Gage handles and all gage nibs are marked with the

extreme product limits for the particular size and class of fit.

(See p. 20-21, Tables VII & VIII, Pitch Diameter Limits.)

When gaging tapped holes which have been thoroughly

cleaned or which have a protective finish applied, the

gage should always be lubricated with light oil.

HI nib may enter provided a definite drag results on or

before 3rd turn from entry – Ref. FED-STD-H28, Screw

thread Standards for Federal Services.

Heli-Coil STI Thread Plug Gages for checking the tapped hole

are listed in the table at right.

Working gages provide a guaranteed minimum wear

allowance on the pitch diameter of the GO members

of two ten thousandths of an inch (.0002). These gages

are recommended for production in sizes 1/2 inch

and smaller.

Reference gages have pitch diameters on or close to minimum

(basic size). They are essentially laboratory or master gages

and should be used in case of conflict between two working

gages. Conflict can occur when one of the gages has

experienced more use and wear.

Both hand and power tools are available to install tanged inserts.

The various tools to install Heli-Coil inserts are presented on the

following pages.

For production runs, prototype work, salvage, and repair,

hand inserting tools are available. For high volume production,

power inserting tools are also available. Both types of tools are

dimensioned (p.29 & 31) to aid determination of accessibility to

the tapped hole.

Both hand and power inserting tools feature a threaded mandrel

which engages the insert and provides a positive lead to guide the

insert into the tapped hole easily and quickly.

Power inserting tools consist of an air motor, adapter and front

end assembly. The front end assembly consists of a prewinder,

mandrel and 3 spacers (1 for each length of insert to be installed).

The versatility and adaptability of Heli-Coil power inserting tools is

shown on p. 32. The tool can be hand held, vertically or horizontally

mounted, and adapted to both semi-automatic and fully automatic

installation stations. Heli-Coil power inserting tools can be adapted

to assembly stations, rotary tables and transfer lines.

All Heli-Coil tooling is backed by our extensive expertise and

experience in virtually any application. All tools are fully warranteed.

In addition, our Application Engineering Department is always

available to assist in installation techniques and special tooling

(longer or shorter length tools, etc.). For very high production,

Heli-Coil will provide for the successful development of automated

installation systems.

Heli-Coil manufactures various designs of hand

inserting tools.

Generally, finer pitch inserts are proportionately

larger in the free state than coarse pitch inserts

and thus have to be “pre-wound” to a smaller

diameter for installation.

Large coarse pitch inserts need only a

threaded mandrel tool for installation.