An intentional difference between the maximum material limits of mating parts. It is also the minimum clearance (positive allowance) or maximum interference (negative allowance) between such parts.
When a fastener has been heated and cooled to make it free of hardness caused by working or pervious heat treatment.
A fastener in some intermediate stage of manufacturing, yet not completed.
A small undesirable amount of material extending out from the edge of a hole, slot shoulder or end of a screw resulting from a machining operation.
Case Hardened
A fastener of ferrous material having a surface which has been hardened more than the core.
Class of Thread
An alphanumerical designation recognized by the fastener industry to indicate a standard grade of allowance and/or tolerance for a thread. Also known as "the fit" of the thread.
A bevel on a flat surface or a flare at the end of a hole; usually measured in degrees such as 82° for commercial applications and 100° for aerospace.
A fastener that has a lower carbon content on the surface than at its core.
Numerical values expressed in units of measure that are indicated on a drawing along with lines, symbols, and notes to define the characteristics of a fastener.
The ability of a fastener to deform before it fractures. The most meaningful test for ductility is passing the wedge test in conjunction with a Rockwell hardness reading within the specification. The lower the ratio between yield and tensile, the more ductile the fastener.
External Threads
Threads that are formed on the outside of a fastener such as a screw. Designated as "2A" or "3A" fit.
A mechanical device used to hold tow or more objects in a definite position with respect to each other until intentionally loosened.
The maximum stress a fastener can withstand for a specified number of repeated applications or cycles of load until the extreme is reached.
Commonly used to describe the condition of the surface of a fastener as a result of chemical or organic treatment following manufacturing.
The general term used to signify the range of tightness which may result from the application of a specific combination of allowances and tolerances in the design of mating parts. See Class of Thread.
A device used to check whether a thread is within the maximum or minimum tolerance limits
The unthreaded portion of the fastener. It can also be measured as the thickness of the material which the fastener was designed to secure.
A method of heat treating metals by heating to a temperature within or above the critical range, then holding at that temperature for a given length of time - cooling rapidly, usually by quenching in oil or water.
Treat An operation involving heating and cooling of a metal fastener to obtain desirable conditions or properties.
High Strength Fastener
A fastener having high tensile and shear strengths attained through combinations of materials, work-hardening and heat treatment.
Particles of non-metallic impurities contained in materials.
Internal Threads
Threads formed on the inside of a fastener such as the threads of a nut. Designated as "2B" or "3B" fit.
A process that produces an intentional roughened surface of hash marks or diamond patterns by means of a forming tool called a knurl.
Surface defects caused by folding over fins or sharp corners into the surface of the material during manufacturing.
The distance a screw will advance in one 360? rotation of the threads.
Left-Hand Threads
Threads that wind in a counter-clockwise direction. Designated by the size call-out followed by "LH".
Mechanical Properties
Those properties which involve a relationship between strain and stress.
Non-Ferrous Material
Any metal that does not contain an appreciable amount of iron such as copper, brass and aluminum.
Materials that have a maximum magnetic permeability of 2.0 (air=1.0) for a field strength of H=200 oersteds using a magnetic indicator per MIL-I-17214. Generally associated with the 18-8 stainless steel series.
A process of removing particles and surface impurities from stainless steel by chemicals (usually a nitric acid dip). Recommended process to improve corrosion resistance.
Physical Properties
The properties that define the characteristics of the material or the fastener itself.
Refers to the finish of a fastener. "Plain" indicates no supplementary surface treatments have been done to the fastener. Non-corrosion resistant parts may be oiled and still be referred to as plain.
The application of a metallic deposit or corrosion inhabiting coating on the surface of the fastener by electrolysis, impact or other suitable means.
Proof Load
The tension applied load that a fastener can withstand without deformation. It is expressed as an absolute value instead of a minimum/maximum value. To pass as acceptable, the after load length must be the same as the original length within a small tolerance.
The process of trimming or removing material with dies in a press.

` Denotes the suitability f a fastener for the purpose for which it is intended. It should not be confused with precision or workmanship as it is possible that precision parts of good workmanship and finish can be poor quality if they fail to perform as they were intended to. Also good quality parts do not always require precision or fine finish to serve as intended. Optimally a quality fastener should possess all of the beneficial characteristics above.

Right Hand Threads

Threads that wind in a clockwise direction. All threaded fasteners have right-hand threads unless noted otherwise.

Rockwell Hardness

A test measure using either a steel ball or a diamond shero-conical penetrator. The depth of the indentation is measured and a hardness number relating tot eh depth is assigned. The higher the number, the harder the material.

SAE Specifications

Standards developed by the Society of Automotive Engineers Incorporated from which many screw thread standards conforming to Unified and American Standards are derived.

Shear Strength

The maximum transverse load a fastener can withstand just before it fractures. There is a single shear test concerned with one transverse plane and a double shear which pertains to two planes. In fasteners it may be assumed to be approximately 60% of specified minimum tensile strengths.

Torsional Strength

A load measured in terms of torque requiring a fastener to be twisted off of it axis until it snaps apart. At the point the measurement is taken in torque pounds. This practice is generally reserved for tapping screws and some stainless steel metric screws.


Also called impact strength, it is the ability of fastener to accept punishment by absorbing energy in the form of impact or shock loading. This test is usually reserved for low temperature applications and aerospace fasteners.


Short length flat and oval head screws may have 30% of the head height reduced by shaving the countersink, thereby increasing thread length. Binder head machine screws may be undercut on special request, to allow wire to pass beneath the head.

Yield Strength

The tension applied load at which a fastener has been stressed beyond its elastic limit and becomes deformed. This test has been somewhat replaced by the Proof Load test. Yield strength is expressed in Ksi (thousand pounds per square inch), PSI (pounds per square inch) or Mpa (Mega Pascal) in metric.

| Home | Bolts | Hex Keys | Inserts | Locking Devices | Machine Screws | Nuts | Pins | Platings |
| Retaining Rings | Rivets | Security Fasteners | SEMS Screws | Sockets | Self-Tapping Screws | Washers |
20 Rancho Circle, Lake Forest, CA 92630 - Tel. 949.770.7711 Fax 949.770.0705 -