The following definitions are based on American National Standard ANSI/ASME
B1.7M-1984 (R2001) “Nomenclature, Definitions, and Letter Symbols for Screw
Threads,” and refer to both straight and taper threads.
Actual Size: An actual size is a measured size.
Allowance: An allowance is the prescribed difference between the design (maximum
material) size and the basic size. It is numerically equal to the absolute value of the ISO
term fundamental deviation.
Axis of Thread: Thread axis is coincident with the axis of its pitch cylinder or cone.
Basic Profile of Thread: The basic profile of a thread is the cyclical outline, in an axial
plane, of the permanently established boundary between the provinces of the external and
internal threads. All deviations are with respect to this boundary.
Basic Size: The basic size is that size from which the limits of size are derived by the
application of allowances and tolerances.
Bilateral Tolerance: This is a tolerance in which variation is permitted in both directions
from the specified dimension.
Black Crest Thread: This is a thread whose crest displays an unfinished cast, rolled, or
Blunt Start Thread: “Blunt start” designates the removal of the incomplete thread at the
starting end of the thread. This is a feature of threaded parts that are repeatedly assembled
by hand, such as hose couplings and thread plug gages, to prevent cutting of hands and
crossing of threads. It was formerly known as a Higbee cut.
Chamfer: This is a conical surface at the starting end of a thread.
Class of Thread: The class of a thread is an alphanumerical designation to indicate the
standard grade of tolerance and allowance specified for a thread.
Clearance Fit: This is a fit having limits of size so prescribed that a clearance always
results when mating parts are assembled at their maximum material condition.
Complete Thread: The complete thread is that thread whose profile lies within the size
limits. (See also Effective Thread and Length of Complete Thread.) Note: Formerly in pipe
thread terminology this was referred to as “the perfect thread” but that term is no longer
Crest: This is that surface of a thread which joins the flanks of the thread and is farthest
from the cylinder or cone from which the thread projects.
Crest Truncation: This is the radial distance between the sharp crest (crest apex) and the
cylinder or cone that would bound the crest.
Depth of Thread Engagement: The depth (or height) of thread engagement between two
coaxially assembled mating threads is the radial distance by which their thread forms overlap
Design Size: This is the basic size with allowance applied, from which the limits of size
are derived by the application of a tolerance. If there is no allowance, the design size is the
same as the basic size.
Deviation: Deviation is a variation from an established dimension, position, standard, or
value. In ISO usage, it is the algebraic difference between a size (actual, maximum, or minimum)
and the corresponding basic size. The term deviation does not necessarily indicate
an error. (See also Error.)
Deviation, Fundamental (ISO term): For standard threads, the fundamental deviation is
the upper or lower deviation closer to the basic size. It is the upper deviation es for an external
thread and the lower deviation EI for an internal thread. (See also Allowance and Tolerance
Deviation, Lower (ISO term): The algebraic difference between the minimum limit of
size and the basic size. It is designated EI for internal and ei for external thread diameters.
Deviation, Upper (ISO term): The algebraic difference between the maximum limit of
size and the basic size. It is designated ES for internal and es for external thread diameters.
Dimension: A numerical value expressed in appropriate units of measure and indicated
on drawings along with lines, symbols, and notes to define the geometrical characteristic
of an object.
Effective Size: See Pitch Diameter, Functional Diameter.
Effective Thread: The effective (or useful) thread includes the complete thread, and
those portions of the incomplete thread which are fully formed at the root but not at the
crest (in taper pipe threads it includes the so-called black crest threads); thus excluding the
Error: The algebraic difference between an observed or measured value beyond tolerance
limits, and the specified value.
External Thread: A thread on a cylindrical or conical external surface.
Fit: Fit is the relationship resulting from the designed difference, before assembly,
between the sizes of two mating parts which are to be assembled.
Flank: The flank of a thread is either surface connecting the crest with the root. The flank
surface intersection with an axial plane is theoretically a straight line.
Flank Angle: The flank angles are the angles between the individual flanks and the perpendicular
to the axis of the thread, measured in an axial plane. A flank angle of a symmetrical
thread is commonly termed the half-angle of thread.
Flank Diametral Displacement: In a boundary profile defined system, flank diametral
displacement is twice the radial distance between the straight thread flank segments of the
maximum and minimum boundary profiles. The value of flank diametral displacement is
equal to pitch diameter tolerance in a pitch line reference thread system.
Height of Thread: The height (or depth) of thread is the distance, measured radially,
between the major and minor cylinders or cones, respectively.
Helix Angle: On a straight thread, the helix angle is the angle made by the helix of the
thread and its relation to the thread axis. On a taper thread, the helix angle at a given axial
position is the angle made by the conical spiral of the thread with the axis of the thread. The
helix angle is the complement of the lead angle. (See also page 1967 for diagram.)
Higbee Cut: See Blunt Start Thread.
Imperfect Thread: See Incomplete Thread.
Included Angle: This is the angle between the flanks of the thread measured in an axial
Incomplete Thread: A threaded profile having either crests or roots or both, not fully
formed, resulting from their intersection with the cylindrical or end surface of the work or
the vanish cone. It may occur at either end of the thread.
Interference Fit: A fit having limits of size so prescribed that an interference always
results when mating parts are assembled.
Internal Thread: A thread on a cylindrical or conical internal surface.
Lead: Lead is the axial distance between two consecutive points of intersection of a helix
by a line parallel to the axis of the cylinder on which it lies, i.e., the axial movement of a
threaded part rotated one turn in its mating thread.
Lead Angle: On a straight thread, the lead angle is the angle made by the helix of the
thread at the pitch line with a plane perpendicular to the axis. On a taper thread, the lead
angle at a given axial position is the angle made by the conical spiral of the thread with the
perpendicular to the axis at the pitch line.
Lead Thread: That portion of the incomplete thread that is fully formed at the root but
not fully formed at the crest that occurs at the entering end of either an external or internal
Left-hand Thread: A thread is a left-hand thread if, when viewed axially, it winds in a
counterclockwise and receding direction. Left-hand threads are designated LH.
Length of Complete Thread: The axial length of a thread section having full form at both
crest and root but also including a maximum of two pitches at the start of the thread which
may have a chamfer or incomplete crests.
Length of Thread Engagement: The length of thread engagement of two mating threads
is the axial distance over which the two threads, each having full form at both crest and
root, are designed to contact. (See also Length of Complete Thread.)
Limits of Size: The applicable maximum and minimum sizes.
Major Clearance: The radial distance between the root of the internal thread and the
crest of the external thread of the coaxially assembled designed forms of mating threads.
Major Cone: The imaginary cone that would bound the crests of an external taper thread
or the roots of an internal taper thread.
Major Cylinder: The imaginary cylinder that would bound the crests of an external
straight thread or the roots of an internal straight thread.
Major Diameter: On a straight thread the major diameter is that of the major cylinder.
On a taper thread the major diameter at a given position on the thread axis is that of the
major cone at that position. (See also Major Cylinder and Major Cone.)
Maximum Material Condition: (MMC): The condition where a feature of size contains
the maximum amount of material within the stated limits of size. For example, minimum
internal thread size or maximum external thread size.
Minimum Material Condition: (Least Material Condition (LMC)): The condition where
a feature of size contains the least amount of material within the stated limits of size. For
example, maximum internal thread size or minimum external thread size.
Minor Clearance: The radial distance between the crest of the internal thread and the
root of the external thread of the coaxially assembled design forms of mating threads.
Minor Cone: The imaginary cone that would bound the roots of an external taper thread
or the crests of an internal taper thread.
Minor Cylinder: The imaginary cylinder that would bound the roots of an external
straight thread or the crests of an internal straight thread.
Minor Diameter: On a straight thread the minor diameter is that of the minor cylinder.
On a taper thread the minor diameter at a given position on the thread axis is that of the
minor cone at that position. (See also Minor Cylinder and Minor Cone.)
Multiple-Start Thread: A thread in which the lead is an integral multiple, other than one,
of the pitch.
Nominal Size: Designation used for general identification.
Parallel Thread: See Screw Thread.
Partial Thread: See Vanish Thread.
Pitch: The pitch of a thread having uniform spacing is the distance measured parallel
with its axis between corresponding points on adjacent thread forms in the same axial
plane and on the same side of the axis. Pitch is equal to the lead divided by the number of
Pitch Cone: The pitch cone is an imaginary cone of such apex angle and location of its
vertex and axis that its surface would pass through a taper thread in such a manner as to
make the widths of the thread ridge and the thread groove equal. It is, therefore, located
equidistantly between the sharp major and minor cones of a given thread form. On a theoretically
perfect taper thread, these widths are equal to one-half the basic pitch. (See also
Axis of Thread and Pitch Diameter.)
Pitch Cylinder: The pitch cylinder is an imaginary cylinder of such diameter and location
of its axis that its surface would pass through a straight thread in such a manner as to
make the widths of the thread ridge and groove equal. It is, therefore, located equidistantly
between the sharp major and minor cylinders of a given thread form. On a theoretically
perfect thread these widths are equal to one-half the basic pitch. (See also Axis of Thread
and Pitch Diameter.)
Pitch Diameter: On a straight thread the pitch diameter is the diameter of the pitch cylinder.
On a taper thread the pitch diameter at a given position on the thread axis is the diameter
of the pitch cone at that position. Note: When the crest of a thread is truncated beyond
the pitch line, the pitch diameter and pitch cylinder or pitch cone would be based on a theoretical
extension of the thread flanks.
Pitch Diameter, Functional Diameter: The functional diameter is the pitch diameter of
an enveloping thread with perfect pitch, lead, and flank angles and having a specified
length of engagement. It includes the cumulative effect of variations in lead (pitch), flank
angle, taper, straightness, and roundness. Variations at the thread crest and root are
excluded. Other, nonpreferred terms are virtual diameter, effective size, virtual effective
diameter, and thread assembly diameter.
Pitch Line: The generator of the cylinder or cone specified in Pitch Cylinder and Pitch
Right-hand Thread: A thread is a fight-hand thread if, when viewed axially, it winds in a
clockwise and receding direction. A thread is considered to be right-hand unless specifically
Root: That surface of the thread which joins the flanks of adjacent thread forms and is
immediately adjacent to the cylinder or cone from which the thread projects.
Root Truncation: The radial distance between the sharp root (root apex) and the cylinder
or cone that would bound the root. See also Sharp Root (Root Apex).
Runout: As applied to screw threads, unless otherwise specified, runout refers to circular
runout of major and minor cylinders with respect to the pitch cylinder. Circular runout, in
accordance with ANSI Y14.5M, controls cumulative variations of circularity and coaxiality.
Runout includes variations due to eccentricity and out-of-roundness. The amount of
runout is usually expressed in terms of full indicator movement (FIM).
Screw Thread: A screw thread is a continuous and projecting helical ridge usually of
uniform section on a cylindrical or conical surface.
Sharp Crest (Crest Apex): The apex formed by the intersection of the flanks of a thread
when extended, if necessary, beyond the crest.
Sharp Root (Root Apex): The apex formed by the intersection of the adjacent flanks of
adjacent threads when extended, if necessary, beyond the root.
Standoff: The axial distance between specified reference points on external and internal
taper thread members or gages, when assembled with a specified torque or under other
Straight Thread: A straight thread is a screw thread projecting from a cylindrical surface.
Taper Thread: A taper thread is a screw thread projecting from a conical surface.
Tensile Stress Area: The tensile stress area is an arbitrarily selected area for computing
the tensile strength of an externally threaded fastener so that the fastener strength is consistent
with the basic material strength of the fastener. It is typically defined as a function of
pitch diameter and/or minor diameter to calculate a circular cross section of the fastener
correcting for the notch and helix effects of the threads.
Thread: A thread is a portion of a screw thread encompassed by one pitch. On a singlestart
thread it is equal to one turn. (See also Threads per Inch and Turns per Inch.)
Thread Angle: See Included Angle.
Thread Runout: See Vanish Thread.
Thread Series: Thread Series are groups of diameter/pitch combinations distinguished
from each other by the number of threads per inch applied to specific diameters.
Thread Shear Area: The thread shear area is the total ridge cross-sectional area intersected
by a specified cylinder with diameter and length equal to the mating thread engagement.
Usually the cylinder diameter for external thread shearing is the minor diameter of
the internal thread and for internal thread shearing it is the major diameter of the external
Threads per Inch: The number of threads per inch is the reciprocal of the axial pitch in
Tolerance: The total amount by which a specific dimension is permitted to vary. The tolerance
is the difference between the maximum and minimum limits.
Tolerance Class: (metric): The tolerance class (metric) is the combination of a tolerance
position with a tolerance grade. It specifies the allowance (fundamental deviation), pitch
diameter tolerance (flank diametral displacement), and the crest diameter tolerance.
Tolerance Grade: (metric): The tolerance grade (metric) is a numerical symbol that designates
the tolerances of crest diameters and pitch diameters applied to the design profiles.
Tolerance Limit: The variation, positive or negative, by which a size is permitted to
depart from the design size.
Tolerance Position: (metric): The tolerance position (metric) is a letter symbol that designates
the position of the tolerance zone in relation to the basic size. This position provides
the allowance (fundamental deviation).
Total Thread: Includes the complete and all the incomplete thread, thus including the
vanish thread and the lead thread.
Transition Fit: A fit having limits of size so prescribed that either a clearance or an interference
may result when mating parts are assembled.
Turns per Inch: The number of turns per inch is the reciprocal of the lead in inches.
Unilateral Tolerance: A tolerance in which variation is permitted in one direction from
the specified dimension.
Vanish Thread: (Partial Thread, Washout Thread, or Thread Runout): That portion of
the incomplete thread which is not fully formed at the root or at crest and root. It is produced
by the chamfer at the starting end of the thread forming tool.
Virtual Diameter: See Pitch Diameter, Functional Diameter.
Washout Thread: See Vanish Thread.
Provided by Frank Xu