Helicoil Tap Sizes Chart according to ASME B18.29.1 standard (Helical Coil Screw Thread Inserts – Free Running and Screw Locking (Inch Series)).  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

Helicoil Tap Size Chart and Calculator:

Nominal Size in Inches
and Threads per Inch
Tap Style
Helicoil 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 Value
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.

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)