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Screw Thread Insert (STI) taps are used for cutting oversized threads for helical coil screw thread inserts. The insert offers a stronger tapped thread for a balanced distribution of loads.

Diagram of a Screw Thread Insert (Sti) Tap

STI Taps Diagram
STI Chamfer Diagram
STI Taps Flute Diagram

Essential Tips:

  • STI taps typically have several flutes constructed in their design. They are the grooves or valleys cut into the body of the tap. Higher numbers of flutes increase the strength of the tap and reduce space or chip flow.

  • Thread size is the number of Threads Per Inch (TPI) which is measured along the length of the tap. Metric sizes are referred to as thread pitch.

  • Chamfers are the length of the tapering threads at the front of the tap. Both the chamfered portion of the tap and the first full thread beyond the chamfer produce the finished thread of the part.





Types of STI Taps:

Spiral Point Sti taps Image

Spiral Point STI taps have a flute geometry that shoots chips ahead of the cutting action to reduce loading and clogging within the flute. Not recommended for abrasive materials. For use in longer through and blind holes.

Sti Spiral Flute Taps Image

Spiral Flute STI taps have a flute geometry draws chips away from aluminum, copper and softer materials. They have a high helix angle. For use in blind holes.

Sti Pipe taps Image

Pipe STI taps are used for internal threading of pipes and pipe fittings to make a pressure-tight joint. Pipe threads require higher cutting forces than regular machine thread tapping because the threads require 100% thread depth. They are designed for cutting oversized threads.

Sti Hand Pipe taps Image

Hand STI taps have a flute geometry that shoots chips ahead of the cutting action to reduce loading and clogging within the flute. Commonly used by either hand and machine tapping operations in through and blind holes. Not recommended for abrasive materials.

Sti Thread Forming taps Image

Thread forming STI Taps are used to form internal threads by displacing the metal without producing chips in large holes. They are ideal for aluminum, brass, copper, steel and soft metals. For use in tapping holes according to screw thread standards.


Class of Fit is the standard identification system describing the tolerance and closeness of fit between the threaded hole and the tap. Unified threads are defined with an A (external) or B (internal). Metric H (internal) or G (external).

Example Applications:

  • Class 1A & 1B - For frequent quick assembly, loose assembly.
  • Class 2A & 2B - Fit is medium loose to eliminate seizure in assembly. Used for screws, bolts and nuts.
  • Class 3A & 3B - Accuracy of thread is required, gages are used to ensure a tight fit.

Thread Limit is a standard notation system indicating a level of tolerance for the thread outside the basic thread size of the tap. The limits are identified by a letter "H" for inch or a "D" for metric, followed by a number. Thread limits have been established to provide a choice in the selection of the tap size best suited to produce the class of thread desired.

Taps Image

The difference in size from one H limit to the next is 0.0005″ increments for taps through 1″ diameter. Sizes over 1″ diameter are separated by .001″ diameter increments. If the threads in the part are too loose, smaller numbers such as H1 or H2 are used. If the threads are too tight, the H limit number is increased. Proper selection of the H limit number ensures that the threads are within the tolerance required by the part print. Best rule of thumb: always select the largest "H" limit possible to achieve proper class of fit and maximum tool life.

Thread Limit (H & D) Cross Reference Guide

H1 /D1 Basic plus .0005" - .0010"
H2 /D2 Basic plus .0005" - .0010"
H3 /D3 Basic plus .0010" - .0015"
H4 /D4 Basic plus .0015" - .0020"
H5 /D5 Basic plus .0020" - .0025"
H6 /D6 Basic plus .0025" - .0030"
H7/D7 Basic plus .0030" - .0035"
H8/D8 Basic plus .0035" - .0040"
H9/D9 Basic plus .0040" - .0045"
H10/D10 Basic plus .0045" - .0050"
H11/D11 Basic plus .0050" - .0055"
H12/D12 Basic plus .0055" - .0060"

Chamfer refers to the length of the tapering threads at the front of the tap. Both the chamfered portion of the tap and the first full thread beyond the chamfer produce the finished thread of the part.


Sti Chamfer Image

Bottoming chamfers are used for threading blind holes to the bottom. They have 1 to 2 chamfer threads.

Modified Bottoming chamfers are similar to bottoming chamfers, but they are longer and have more teeth. They are used for threading to the bottom of blind holes and have 2 to 2-1/2 chamfer threads.

Taper chamfers, also known as starter taps, have a longer chamfer and require a less aggressive cutting action. They have 7 to 10 chamfer threads.

Plug chamfers are the most commonly used chamfer and are designed for efficiently threading through and blind holes. They have 3 to 5 chamfer threads.

Semi-Bottoming chamfers are used for blind holes. They have 3 to 3-1/2 chamfer threads.


Bright provides a smooth, polished finish on the tool. It increases chip flow in softer materials such as aluminum, wood and plastic.

Titanium Nitride (TiN) is a multi-purpose coating which increases chip flow in softer materials. The heat and hardness resistance allows the tool to run at higher speeds than uncoated tools.

Titanium Carbonitride (TiCN) is harder and more wear resistant than TiN. It is used on stainless steels, cast iron and aluminum alloys.

Oxide, also known as black oxide or steam oxide, is a surface treatment that prevents chip building, galling and welding on the workpiece. It is commonly used on low carbons, stainless steel and ferrous metals.

Nitride is a thin, hard-shell coating that supports surface hardness of the tool. It is used where abrasive or wearing conditions exist.

Taps Image


Inserts Used with STI Taps:

Free Running Inserts Image

Free Running Inserts are coils used to repair tapped holes that have been stripped due to wear, corrosion or over-torque.

Sti Screw locking inserts Image

Screw Locking Inserts are used to replace worn, damaged or stripped threads. They are designed with screw locking coils that grip the screws or bolts to prevent loosening under vibration. They do not require other thread locking mechanisms.

Sti thread locking inserts Image

Thread-Locking Inserts are used to replace worn, damaged or stripped threads. These inserts feature an adhesive or nylon locking element on the external thread that locks them in place and seals against liquid and gas.