Helicoil Insert Tapped Hole Dimensions Calculator has been developed by using ASME B19.29.1 – 1993 standard (Helical Coil Screw Thread Inserts – Free Running and Screw Locking (Inch Series)) and calculates helicoil tapped hole dimensions.  The tapped hole which the helicoil insert is to be installed shall be in accordance with ASME B1.1, except that the diameter is larger to accommodate the wire cross section of insert. Acceptance of the threaded hole is determined by gauging with STI GO, NOT GO (HI), and plain cylindrical gages according to System 21 of ANSI/ASME B1.3M.

Helicoil inserts are screw thread bushings coiled from wire of diamond shape cross-section. They are fit into thread tapped holes to form internal threads with nominal size. Helicoil inserts are diametrically larger than the tapped hole. During the assembly operation, the diameter of the leading coil is reduced due to applied torque and this permits it to enter the tapped thread.

Helicoil tapping hole according to ASME B18.29.1-1993
Helicoil tapping hole according to ASME B18.29.1-1993


Nominal Size in Inches
and Threads per Inch
Tap Style
Insert Length
Class of Fit *

Note : * Class 2B tapped holes provide maximum production tolerances, but result in lower locking torques when screw locking inserts are used. The higher and more consistent torques are met by the screw locking inserts when assembled and tested in Class 3B tapped holes.


Parameter Symbol Value Unit
Nominal Size -- --- ---
Threads per inch -- ---
Tap Style -- ---
Class of Fit -- ---
Insert Length -- ---
Minimum Drilling Depth G --- inch
Minimum Countersink Diameter*  Mmin ---
Maximum Countersink Diameter* Mmax ---
Minimum Tapping Depth Tmin ---
Minimum Major Diameter Dmin ---
Maximum Pitch Diameter D2max ---
Minimum Pitch Diameter D2min ---
Maximum Minor Diameter D1max ---
Minimum Minor Diameter D1min ---

Note *: Countersink angle 120° +/- 5° .

Note: The minimum drilling depths allow for countersinking the drilled hole to have a good starting thread. The drilling depths also allow 3/4 to 1 1/2 pitches insert set-down with countersink below the top surface of the tapped hole.


STI Plug Style Tap: This type of tap has over 4 pitch thread chamfer. Plug style tap is widely used in through holes and where there is sufficient room at the bottom in blind holes.

STI Bottoming Style Tap: This type of tap has only 1 to 2 pitch thread chamfer. Bottoming style tap is ideal for blind holes that require threads to be close to the bottom of the hole.

Unified Screw Thread: A thread form used by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. It has 60° thread angle and dimensions are stated in inch units.

UNC (Unified National Coarse): Most commonly used type for general engineering applications. This thread form is used in materials with low tensile strength which makes threads more resistance against stripping (internal threads) .UNC give possibility for quick assembly.

UNF (Unified National Fine): External threads of this Fine Series have greater tensile stress area than comparable sizes of the coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.

Threads per inch: Number of full thread per an inch length.

  • ASME B18.29.1-1993, Helical Coil Screw Thread Inserts - Free Running and Screw Locking (Inch Series)