actual fitFit, size & tolerance
The fit a particular pair of parts really has, worked out from their measured sizes rather than from the limits on the drawing.
An interactive guide to screw thread vocabulary: 215 terms, 82 letter symbols, the figure index and the thread series designations, with a thread you can reshape and click. Explanations are written for this site; the structure follows ASME B1.7. They are being reviewed, and entries still marked “being written” are not finished.
Axial section, drawn to scale from the values on the left. Click a label in the drawing, or a feature below, to read its definition.
Select a label in the drawing or one of the feature buttons to read what it means.
The fit a particular pair of parts really has, worked out from their measured sizes rather than from the limits on the drawing.
The size of a feature measured on a finished part. It is never exactly the size drawn, only somewhere inside the permitted range if the part is good.
The radial distance from the pitch line out to the crest of a thread.
The planned gap, or planned interference when negative, between two mating parts when both are made at the limits that leave the most material. On inch threads it is the distance the external thread's tolerance zone is moved below basic size, so that a bolt and nut of the same class can always be assembled.
A gaging result that only says whether a feature is acceptable, pass or fail, without giving a measured number. Go and no-go gages work this way.
The straight centre line around which the helix of the thread winds.
A way of building fits in which the hole always has the same limit at basic size and the shaft is made larger or smaller to give the fit needed.
Also written as:
A way of building fits in which the shaft always has the same limit at basic size and the hole is made larger or smaller to give the fit needed.
The ideal size, taken from the drawing or the standard, from which allowances and tolerances are measured. No real part is made exactly to it.
The diameter of measuring wire that touches the flanks right at the pitch diameter. It is used in the three-wire method, where the pitch diameter is worked out from the distance measured over the wires.
A tolerance that lets the size vary on both sides of the stated size, for example plus and minus the same amount.
A system in which the permitted variation of a size is placed on both sides of the basic size.
A thread whose crest was not machined, so the crest is still the original surface of the stock, for example the skin of a rolled or hot-finished bar.
A thread whose first turn is cut back at the end, so the start is blunt and not a thin sharp edge. It is less likely to be damaged and less likely to cross-thread.
The edge where the chamfer ends on the inner side, at the circle where the bevel meets the thread or the bore.
A bevel cut on an edge. At the end of a thread it removes the sharp corner and helps the mating part to start.
Any property of a part that can be measured or checked, such as a diameter, the lead or the flank angle.
The short code, such as 2A or 3B, that names a combination of allowance and tolerance. It tells the maker how accurately to produce the thread and the user how closely it will fit its mate.
A fit in which there is always a gap between the two parts, so the external part goes into the internal part freely whatever the actual sizes within the limits.
The explanation of this term is being written.
A thread that has a plating or other coating on it. The coating adds thickness, which has to be allowed for in the limits.
The explanation of this term is being written.
The part of a threaded length where the thread has its full form from crest to root.
How well two circles or cylinders share the same centre or axis.
Also written as:
A conical recess at the mouth of a hole, used to seat a screw head or to guide a tool into the hole.
The top surface of a tooth, joining its two flanks. It is the part of the thread farthest from the axis on an external thread and nearest to the axis on an internal thread.
Also written as:
The diameter taken over the thread tips. For an external thread it is the largest diameter; for an internal thread it is the smallest.
The radial distance by which the sharp tip of the basic form is cut away at the crest, leaving a flat or a rounded top.
The width of the flat on top of a crest, measured along the axis.
The explanation of this term is being written.
The pitch error built up over a stated length or number of pitches, found by comparing the actual distance between two threads with the nominal distance.
How close a surface is to a perfect cylinder: it takes in roundness, straightness and taper together.
The radial distance from the pitch line down to the root of a thread.
How far the thread of one part reaches into the groove of its mate, measured at right angles to the axis. It means the same as the height of thread engagement.
The size a part is designed to, taken as the basic size after any allowance has been applied. The tolerance is then worked from it.
Also written as:
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How far a size or another characteristic is from its intended value. In thread tolerance systems the word also names the distance from basic size to one edge of a tolerance zone.
The one deviation that fixes where the whole tolerance zone sits relative to basic size. It is the edge of the zone that lies closest to basic size. On inch threads the same job is done by the allowance.
The distance from basic size down to the smaller limit of the tolerance zone. It is written ei for an external thread and EI for an internal thread.
The distance from basic size up to the larger limit of the tolerance zone. It is written es for an external thread and ES for an internal thread.
The change in a thread's effective pitch diameter that is caused by a form error, such as an error in flank angle or in lead. It is added to the measured pitch diameter to give the functional size.
A stated size or distance, with its unit, for a feature on a drawing.
A thread whose lead varies as it winds round, so its path wobbles about a true helix instead of advancing at an even rate.
The distance between the axes or centres of two features that should share one.
Also written as:
The part of a threaded length that does the work in an assembly: the complete thread plus any partial thread that still takes load.
One single geometric part of a thread, such as a flank, the pitch diameter or the lead, that can be checked on its own.
Also written as:
The amount by which a measured result misses the true or intended value.
A thread cut on the outside of a part, as on a bolt or a screw.
Also written as:
A physical part of a workpiece, such as a thread, a hole or a surface.
How loosely or tightly two mating parts go together, as decided by the difference between their sizes before assembly.
The explanation of this term is being written.
The straight side of a thread that connects a crest to the neighbouring root.
The angle between one flank and a line at right angles to the axis, measured in a section along the axis. When both flanks are alike it is half the thread angle.
A crest or root that is cut off flat, with a surface parallel to the axis, in place of a rounded or pointed shape.
The flank on the trailing side as the screw is turned into its mate. It is the opposite of the leading flank.
The diameter at which the flat or straight part of a flank ends and the rounded root begins.
Also written as:
Also written as:
Also written as:
Checking a thread with gages that act like its mating thread, so the check covers the combined effect of pitch diameter, lead, flank angle and form instead of one element at a time.
The size of the perfect thread that the actual thread behaves like when it is assembled. It is the pitch diameter after the effect of errors in lead, flank angle and form has been taken into account.
The sharp-cornered V obtained by extending the two flanks until they meet. The basic thread profile is built on it.
A tool used to check whether a size or a feature is within its limits, or to measure it.
A reference piece used to set an adjustable gage or a measuring instrument to the correct size before it is used on parts.
A gage fitted with a dial or an electronic indicator that shows how far the thread is from a set size, so it gives a reading instead of a pass or fail.
A fixed gage that checks a thread against its limits, normally with a go member and a not-go member.
A gage with an external thread that is screwed into an internal thread to check it.
A gage with an internal thread that is screwed onto an external thread to check it.
A gage with two or more jaws or rolls that is passed over an external thread, or opened and closed on it, to check its size.
Also written as:
The reference plane on a taper thread at which the gage is set and at which the size of the thread is defined.
Also written as:
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The distance from the point of the sharp V to its base, measured at right angles to the axis. It is written H and is the starting value for the other height dimensions of the thread.
The radial distance between the crest and the root of one thread. It is smaller than the height of the fundamental triangle because the tips are truncated.
The radial overlap between the thread of one part and the thread of its mate in an assembled pair.
Also written as:
The curve that winds round a cylinder or a cone and advances at a constant rate, like the path of a spring. The ridge of a screw thread follows a helix.
The angle between the helix and a plane at right angles to the axis. In this standard it means the same as the lead angle.
The difference between the actual path of a thread and a true helix. It covers drunken lead and other lead errors.
Also written as:
Also written as:
The explanation of this term is being written.
The part at the end of a threaded length where the cutting tool ran out, so the thread fades away and has less than full height.
A thread whose crest has been cut away or whose root is not fully formed, so its height is less than the full thread height.
A fit in which the external part is larger than the internal part before assembly, so the parts have to be forced together and then grip each other.
A thread formed on the wall of a hole, such as the thread of a nut or of a tapped hole.
The distance a thread advances along the axis in one full turn.
The angle between the helix at the pitch diameter and a plane at right angles to the axis. Its tangent is the lead divided by the circumference at the pitch diameter.
The explanation of this term is being written.
The difference between the actual lead and the nominal lead, measured over a stated length.
The flank on the front side as the screw is turned into its mate. It is the opposite of the following flank.
The state in which a feature holds the least material the limits allow: the smallest external thread or the largest internal thread.
Screwed in by turning counter-clockwise, and marked LH. The usual thread is right-hand.
The length of thread over which a gage takes hold and checks the thread.
The length over which the two threaded parts are assembled together.
The length along the axis over which the thread has its full form.
The length along the axis over which an external thread and an internal thread are in contact. It governs how much load the joint can carry before the threads strip.
One of the two extremes between which a size must lie, the largest permitted or the smallest permitted.
The flank that carries the working load when the thread is in service. On an asymmetric thread it is the flank on the side the load pushes against.
The radial gap between the crest of one thread and the root of the thread it is screwed into, seen at the major diameter of an assembled pair.
The tightest cone that encloses an external taper thread, or the largest cone that fits into the grooves of an internal taper thread, with the same taper as the thread.
For a bolt-type straight thread, the cylinder that wraps the crests; for a nut-type straight thread, the cylinder that sits in the grooves. The major diameter is its diameter.
The largest diameter of a thread: the diameter of the major cylinder, or of the major cone at a stated position on a taper thread.
The state in which a feature holds the most material the limits allow: the largest external thread or the smallest internal thread. It is the tightest case for assembly.
The doubt that remains about a measured value, stated as a range around the result in which the true value is expected to lie.
Also written as:
In an assembled pair, the radial space left between the inner tip of the nut thread and the bottom of the bolt groove it faces.
The cone that lies at the bottom of the grooves of an external taper thread, or the one that just fits through the opening of an internal taper thread, with the same taper as the thread.
For a bolt-type straight thread, the cylinder that sits in the grooves; for a nut-type straight thread, the cylinder that touches the inner tips. The minor diameter is its diameter.
The smallest diameter of a thread: the diameter of the minor cylinder, or of the minor cone at a stated position on a taper thread.
A thread made of two or more helical ridges wound side by side. The lead is the pitch multiplied by the number of starts, so it advances faster than a single start thread of the same pitch.
The general size used to name a thread or part, such as 1/4 inch or M10. The real dimensions of the thread differ from it.
Also written as:
Also written as:
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An old way of describing how much of the full thread height was cut in a tapped hole, given as a percentage. The term is no longer used.
The distance from a point on one thread to the same point on the next thread, measured parallel to the axis.
The imaginary cone used to define the pitch diameter of a taper thread.
The imaginary cylinder used to define the pitch diameter of a straight thread.
The diameter of the imaginary cylinder, or cone on a taper thread, that cuts the thread where the ridge and the groove have equal width along the axis. It lies between the major and minor diameters and is the diameter that most controls how two threads fit.
A pitch diameter that includes the effect of errors in lead and flank angle, so it describes how the thread assembles and not only how big it is. It is checked with functional gages.
The explanation of this term is being written.
A way of fixing the pitch diameter from the groove: the diameter of the cylinder that cuts a groove exactly as wide as the basic groove width, which is half a pitch on the standard form.
A way of fixing the pitch diameter from the ridge: the diameter of the cylinder that cuts a thread ridge exactly as thick as the basic ridge thickness, which is half a pitch on the standard form.
The line, in a section along the axis, where the pitch cylinder or the pitch cone cuts the thread.
The plane at right angles to the axis that passes through the point where the thread ends in the vanish cone.
Also written as:
Also written as:
The ideal outline in a section along the axis from which all the sizes and tolerances of a thread series are worked out.
The outline a thread is designed to have when it is made at its largest-material limit. It is the basic profile with the crest and root shaped for the series.
The outline of a thread when it is made exactly at one of the limits of its tolerance zone. The finished thread must stay between the two limiting profiles.
The outline of a thread as it appears in a section that contains the axis.
A dimension shown for information only. It carries no tolerance and is not used to accept or reject a part.
A thread that tightens when turned clockwise. It is the normal thread, used when nothing else is said.
The bottom surface of a groove, joining the two flanks on each side of it.
Also written as:
The diameter at the base of the grooves: the smallest diameter of an external thread, the largest of an internal one.
The radius of the curve at the bottom of a groove, where the thread has a rounded root.
The radial distance by which the sharp point of the basic form is cut away at the root.
A crest or root shaped as a curve with a radius, instead of a flat or a point.
How close a cross-section is to a true circle.
A cross-section with several equal bulges, so it is not round, although a two-point measurement across it can read the same in every direction.
Any departure of a cross-section from a true circle.
A cross-section that is out of round in the form of an oval, with two high spots opposite each other.
The variation shown by an indicator held against a surface while the part is turned about its axis. It combines the effects of eccentricity and form errors.
The explanation of this term is being written.
A ridge of constant section that winds in a helix around a cylinder or a cone, outside or inside.
Also written as:
The point where the two flanks of a tooth would meet if they were carried up to the tip without any truncation. It is a reference point, not a real surface.
A construction cone drawn through the corner points the flanks would make if they were extended out to the outer tips of an external taper thread, or down to the groove bottoms of an internal one. It does not exist on the real part.
A construction cylinder drawn through the corner points the flanks would make if they were extended out to the outer tips of an external straight thread, or down to the groove bottoms of an internal one. It does not exist on the real part.
A construction cone drawn through the corner points the flanks would make at the groove bottoms of an external taper thread, or at the inner tips of an internal one. It does not exist on the real part.
A construction cylinder drawn through the corner points the flanks would make at the groove bottoms of an external straight thread, or at the inner tips of an internal one. It does not exist on the real part.
The point where the two flanks of a groove would meet if they were carried down to the bottom without any truncation. It is a reference point, not a real surface.
Also written as:
The area of metal that would have to tear for a thread to strip. It is a surface at about the pitch diameter, and its size depends on the length of engagement.
Also written as:
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Checking one element of a thread by itself, such as the pitch diameter alone, in place of the combined check made by a functional gage.
A thread made of one helical ridge only, so its lead equals its pitch.
The general measure of a feature. The standards separate several kinds: actual, basic, design, functional, limit and nominal size.
The explanation of this term is being written.
The axial distance from the end or face of a taper-threaded part to the reference position of the gage or mating part, which shows how far the threads have gone together.
A thread cut on a cylinder, so its diameter stays the same along its length. Parallel thread is another name for it.
How close a line or an axis is to a perfect straight line.
Flaws on a surface such as scratches, pits, tool marks or burrs.
The fine pattern of a surface: its roughness, its waviness and the direction in which the tool marks run.
The letters and codes used to name a thread, such as 1/4-20 UNC-2A, and the letters used on drawings for its dimensions.
A thread cut on a cone, so its diameter changes along its length. Pipe threads that seal by wedging are examples.
The rule for designing limit gages: the go gage checks all features of the thread together over the full length, and the not-go gage checks one feature at a time.
The area used to work out the tensile strength of a threaded part. It is an effective cross-section somewhere between the minor and the pitch diameter, not the area of the minor diameter.
A helical ridge on a cylinder or a cone, or the way a thread is counted: the number of threads per inch.
The angle between the two flanks of a thread, measured in a section that contains the axis.
The name ISO gives to a thread on the outside of a part.
Also written as:
The width of the groove between two teeth, measured along the axis at a stated diameter.
The name ISO gives to a thread inside a hole or a nut.
Also written as:
The width of a tooth, measured along the axis at a stated diameter.
A group of diameter and pitch combinations that follow the same pattern, such as the coarse series, the fine series or a series with one pitch for every diameter.
Also written as:
Also written as:
How many thread crests are counted in one inch of axial length. Divide one inch by it to get the pitch.
The total variation a size is allowed, equal to the difference between its largest and smallest permitted values.
In the ISO system, the pairing of a tolerance grade and a tolerance position, written for example 6g for an external thread or 6H for an internal thread.
In the ISO system, the number that sets how wide the tolerance zone is. A smaller number means a narrower zone and a more accurate thread.
One of the two edges of a tolerance zone: the largest or the smallest value the characteristic may have.
In the ISO system, the letter that shows where the tolerance zone sits relative to basic size. External threads use lower-case letters and internal threads use capitals.
The band between the largest and the smallest permitted value of a characteristic. On a thread it is usually drawn at the pitch diameter.
The whole threaded length, including the complete thread and the incomplete threads at its ends.
Also written as:
A fit that may turn out loose or tight depending on the actual sizes of the two parts, so it can give either a small clearance or a small interference.
The removal of the sharp tip of a crest or of the sharp point of a root. It is measured as the radial distance that is cut away.
The number of full turns the thread makes in one inch of length. For a single start thread it equals the threads per inch.
A tolerance that lets the size vary on one side of the stated size only.
A system in which the tolerance zone lies wholly on one side of basic size. Threads work this way: external zones lie below basic size and internal zones lie at or above it.
Also written as:
The conical surface along which a thread fades out into the unthreaded part, for example where the cutting tool ran out at the end of a threaded length.
Also written as:
The last part of a threaded length where the thread height fades out to zero into the unthreaded part.
A gaging method that gives an actual measured value, not just pass or fail, so the result can be recorded and used to follow a process.
Also written as:
Also written as:
The explanation of this term is being written.
Also written as:
The amount a tapered pipe joint is tightened with a wrench after it has been screwed together by hand.
The explanation of this term is being written.
Lower-case letters belong to the external thread and capitals to the internal thread. s means external (screw), n means internal (nut), c means crest and r means root. “–” means the standard prints no symbol for that member.
| Source | External | Internal | Meaning | Defined in |
|---|---|---|---|---|
| Table 1 · 1 | A | – | Letter that marks an external thread in a class code, as in 2A. | – |
| Table 1 · 2 | – | B | Letter that marks an internal thread in a class code, as in 2B. | – |
| Table 1 · 3 | BUTT | BUTT | Designation of the buttress thread series, pull or push type. | – |
| Table 1 · 4 | c | c | Correction for a wire measurement when the lead angle is large. | – |
| Table 1 · 5 | C | – | The value added to a reading taken across the best-size wires, which turns it into the pitch diameter. | – |
| Table 1 · 6 | d | D | Major diameter: lower case for an external thread, capital for an internal thread. | , , , |
| Table 1 · 7 | d bsc | D bsc | Basic (nominal design) major diameter. | |
| Table 1 · 8 | d max. | – | Largest permitted major diameter of an external thread. | |
| Table 1 · 9 | d min. | – | Smallest permitted major diameter of an external thread. | |
| Table 1 · 10 | d₁ bsc | D₁ bsc | Basic minor diameter. | , , , |
| Table 1 · 11 | d₂ | D₂ | Pitch diameter. | , , , |
| Table 1 · 12 | d₂ bsc | D₂ bsc | Basic pitch diameter, the reference from which the limits are set. | |
| Table 1 · 13 | d₂ min. | D₂ min. | Smallest permitted pitch diameter. | |
| Table 1 · 14 | d₂ max. | D₂ max. | Largest permitted pitch diameter. | |
| Table 1 · 15 | d₃ | – | Minor diameter of an external thread that has a rounded root. | , |
| Table 1 · 16 | d₃ min. | – | Smallest permitted minor diameter of an external thread with a rounded root. | |
| Table 1 · 17 | d₃ max. | – | Largest permitted minor diameter of an external thread with a rounded root. | |
| Table 1 · 18 | – | D min. | Smallest permitted major diameter of an internal thread with a flat root. | |
| Table 1 · 19 | d₁ | – | Minor diameter of an external thread that has a flat root. | , |
| Table 1 · 20 | d₁ min. | – | Smallest permitted minor diameter of an external thread with a flat root. | |
| Table 1 · 21 | d₁ max. | – | Largest permitted minor diameter of an external thread with a flat root. | |
| Table 1 · 22 | – | D₁ min. | Smallest permitted minor diameter of an internal thread. | |
| Table 1 · 23 | – | D₁ max. | Largest permitted minor diameter of an internal thread. | |
| Table 1 · 24 | – | D₃ | Major diameter of an internal thread that has a rounded root. | |
| Table 1 · 25 | – | D₃ min. | Smallest permitted major diameter of an internal thread with a rounded root. | |
| Table 1 · 26 | – | D₃ max. | Largest permitted major diameter of an internal thread with a rounded root. | |
| Table 1 · 27 | es | EI | Allowance: how far the tolerance zone is moved away from basic size, at the major, pitch and minor diameters. The first symbol is for the external thread, the second for the internal thread. | , , , |
| Table 1 · 28 | fcs | fcn | Crest truncation: how much of the sharp crest is cut away. The last letter s means external, n means internal. | |
| Table 1 · 29 | frs | frn | Root truncation: how much of the sharp root is cut away. | |
| Table 1 · 30 | Fcs | Fcn | Crest width: the width of the flat on the crest. | |
| Table 1 · 31 | Frs | Frn | Root width: the width of the flat at the bottom of the groove. | – |
| Table 1 · 32 | h | h | Basic height of the thread. | |
| Table 1 · 33 | hs | hn | Height of the thread as made, for the external thread and the internal thread. | |
| Table 1 · 34 | H | H | Height of the sharp-V triangle from which the profile is built. | |
| Table 1 · 35 | LA | LA | Length of assembly. | |
| Table 1 · 36 | LE | LE | Length of thread engagement. | |
| Table 1 · 37 | LG | LG | Gaging length. | |
| Table 1 · 38 | Mw | Mw | Measurement taken over the measuring wires. The small w stands for wire. | – |
| Table 1 · 39 | n | n | How many threads there are in one inch. | |
| Table 1 · 40 | P | P | Pitch. | |
| Table 1 · 41 | r max. | R max. | Largest permitted root radius. | |
| Table 1 · 42 | r min. | R min. | Smallest permitted root radius. | |
| Table 1 · 43 | rrs min. | – | Smallest permitted root radius of an external thread. | |
| Table 1 · 44 | rrs max. | – | Largest permitted root radius of an external thread. | |
| Table 1 · 45 | rrs | Rrn | Root radius of the external thread (first symbol) and the internal thread (second symbol). | |
| Table 1 · 46 | s | s | Truncation of a rounded form, the distance the sharp form is cut back when the crest or root is rounded. | |
| Table 1 · 47 | scs | scn | Truncation at the crest when the crest is rounded. | |
| Table 1 · 48 | srs | srn | Truncation at the root when the root is rounded. | |
| Table 1 · 49 | s min. | s min. | Smallest permitted rounded truncation. | |
| Table 1 · 50 | s max. | s max. | Largest permitted rounded truncation. | |
| Table 1 · 51 | Td₁ | TD₁ | Tolerance of the minor diameter. | |
| Table 1 · 52 | Td₂ | TD₂ | Tolerance of the pitch diameter. | |
| Table 1 · 53 | Td | TD | Tolerance of the major diameter. | |
| Table 1 · 54 | TPI | TPI | Threads per inch. | |
| Table 1 · 55 | W | W | Diameter of a measuring wire. | |
| Table 1 · 56 | w | w | Average diameter of a set of three measuring wires. | |
| Table 1 · 57 | α | α | Flank angle of a symmetrical thread, measured from a line at right angles to the axis. | |
| Table 1 · 58 | α1 | α1 | Flank angle of the leading flank. | , |
| Table 1 · 59 | α2 | α2 | Flank angle of the following flank. | , |
| Table 1 · 60 | λ | λ | Lead angle. | , |
| Table 1 · 61 | λ' | – | Angle used in the correction of wire measurements. | – |
| Table 1 · 62 | Δd₂α | ΔD₂α | How much a flank angle error moves the pitch diameter of a symmetrical thread. | |
| Table 1 · 63 | Δd₂α1 | ΔD₂α1 | How much an error of the leading flank angle moves the pitch diameter. | |
| Table 1 · 64 | Δd₂α2 | ΔD₂α2 | How much an error of the following flank angle moves the pitch diameter. | |
| Table 1 · 65 | Δd₂λ | ΔD₂λ | How much a lead error moves the pitch diameter. | , |
| Table 2 | Ph | L | The ISO symbol for lead. The U.S. symbol is L. | |
| Table 2 | ψ | λ | The ISO symbol for lead angle. The U.S. symbol is λ. | |
| Table 2 | β | α | The ISO symbol for flank angle. The U.S. symbol is α. | |
| Table 2 | α | α1 + α2 [Note (1)]; 2α [Note (2)] | The ISO symbol for thread angle, the angle between the two flanks. | |
| Figures and text | es | Upper deviation of an external thread. | – | |
| Figures and text | ei | Lower deviation of an external thread. | – | |
| Figures and text | ES | Upper deviation of an internal thread. | – | |
| Figures and text | EI | Lower deviation of an internal thread. | – | |
| Figures and text | T | Tolerance, the width of a tolerance zone. | – | |
| Figures and text | G, H | Letters for where an internal thread's tolerance zone sits in the metric system: H lies on basic size, G is moved off it slightly. | – | |
| Figures and text | e, f, g, h | Tolerance positions of external threads in the metric system. h has no allowance; e, f and g have increasing small to large allowances. | – | |
| Figures and text | P/2, d, d₂, d₁, D, D₂, D₁ (bsc / max / min) | Labels used on the fundamental deviation figure: half the pitch and the basic, maximum and minimum diameters. | – | |
| Figures and text | R, r | Root radius of an internal thread (R) and of an external thread (r). | – | |
| Figures and text | a | The centre line position used on the taper thread figures, at which diameters are read. | – | |
| Figures and text | θ, β | Angle labels on the pitch cone figure. | – | |
| Figures and text | j, 2j | Distances on the best wire figure that locate where the wire touches the flanks. | – | |
| Figures and text | LH, RH | Abbreviations for left-hand thread and right-hand thread. | – |
59 figures drawn with code, one for each figure of the standard, with the terms that refer to it. Click a drawing to enlarge it.
| Designation | What it names | Standard | Status |
|---|---|---|---|
| ACME-C | Acme thread of the centralizing type, where the major diameter fit keeps the screw centred in its nut. | B1.5 | Current |
| ACME-G | Standard Acme form for everyday power screws, with clearance at the major diameter. | B1.5 | Current |
| AMO | Thread used on microscope objectives. | B1.11 | Current |
| ANPT | Taper pipe thread of the aeronautical type, a variant of NPT. | AS71051 | Current |
| BUTT | Buttress thread, pull type. | B1.9 | Current |
| PUSH-BUTT | Buttress thread, push type. | B1.9 | Current |
| F-PTF | Dryseal taper pipe thread of the fine series. | B1.20.3 (Appendix C) | Current |
| M | Metric screw thread built on the ISO basic profile. | B1.13M | Current |
| MJ | Metric screw thread with a rounded, controlled root radius. | B1.21M | Current |
| MJS | Special series of the MJ metric thread. | B1.21M | Current |
| NC5 HF | Class 5 interference-fit thread for driving into hard ferrous material. | B1.12 | Current |
| NC5 CSF | Class 5 interference-fit thread for copper alloys and soft ferrous material. | B1.12 | Current |
| NC5 ONF | Class 5 interference-fit thread for driving into other nonferrous material. | B1.12 | Current |
| NC5 IF | Class 5 interference-fit external thread for the whole ferrous range. | B1.12 | Current |
| NC5 INF | Class 5 interference-fit external thread for the whole nonferrous range. | B1.12 | Current |
| NGO | Thread for gas cylinder outlets; the hand, right or left, must always be stated. | ANSI/CGA V-1 | Current |
| NGS | National gas straight thread. | ANSI/CGA V-1 | Current |
| NGT | National gas taper thread. | ANSI/CGA V-1 | Current |
| NH | Hose coupling thread of full form. | B1.20.7 | Current |
| NHR | Hose coupling thread for garden hose. | B1.20.7 | Current |
| NPSC | Straight pipe thread cut inside pipe couplings. | B1.20.1 | Current |
| NPSF | Dryseal fuel thread, internal and straight. | B1.20.3 | Current |
| NPSH | Straight hose coupling thread for joining. | B1.20.7 | Current |
| NPSI | Dryseal intermediate thread, internal and straight. | B1.20.3 | Current |
| NPSL | Straight pipe thread for loose joints held with a locknut. | B1.20.1 | Current |
| NPSM | Parallel pipe thread for loose mechanical joints, for example on fixtures. | B1.20.1 | Current |
| NPT | Taper pipe thread for general use. | B1.20.1 | Current |
| NPTF | Dryseal taper pipe thread. | B1.20.3 | Current |
| NPTR | Taper pipe thread for railing joints. | B1.20.1 | Current |
| PTF-SAE Short | Dryseal taper pipe thread, SAE short series. | B1.20.3 | Current |
| PTF-SPL Short | Dryseal taper pipe thread, special short series. | B1.20.3 (Appendix C) | Current |
| PTF-SPL Extra short | Dryseal taper pipe thread, special extra-short series. | B1.20.3 (Appendix C) | Current |
| SGT | Special gas taper thread. | ANSI/CGA V-1 | Current |
| SPL-PTF | Special-purpose taper pipe thread of the Dryseal family. | B1.20.3 (Appendix C) | Current |
| STUB ACME | Acme thread with shorter teeth. | B1.8 | Current |
| UN | Unified inch thread that keeps one pitch over many diameters. | B1.1 | Current |
| UNC | Coarse-pitch member of the Unified inch family. | B1.1 | Current |
| UNF | Fine-pitch member of the Unified inch family. | B1.1 | Current |
| UNEF | Extra-fine-pitch member of the Unified inch family. | B1.1 | Current |
| UNJ | Unified form with a controlled rounded root, one pitch across many diameters. | B1.15 | Current |
| UNJC | Coarse-pitch version of UNJ. | B1.15 | Current |
| UNJF | Fine-pitch version of UNJ. | B1.15 | Current |
| UNJEF | Extra-fine-pitch version of UNJ. | B1.15 | Current |
| UNR | Unified form with a rounded root on the external thread, one pitch across many diameters. | B1.1 | Current |
| UNRC | Coarse-pitch version of UNR. | B1.1 | Current |
| UNRF | Fine-pitch version of UNR. | B1.1 | Current |
| UNREF | Extra-fine-pitch version of UNR. | B1.1 | Current |
| UNM | Unified thread family for miniature sizes. | B1.10M | Current |
| UNS | A Unified thread cut to a special size: a diameter, pitch or engagement length outside the standard series. | B1.1 | Current |
| G | ISO straight pipe thread of the 55° Whitworth form, where the joint does not seal on the threads. | ISO 228-1 | Current (ISO) |
| R | ISO taper external pipe thread, 55° Whitworth form, where the joint seals on the threads. | ISO 7-1 | Current (ISO) |
| Rc | ISO taper internal pipe thread that mates with R. | ISO 7-1 | Current (ISO) |
| Rp | ISO straight internal pipe thread that mates with R. | ISO 7-1 | Current (ISO) |
| S | ISO miniature screw thread for very small diameters. | ISO/R1501 | Current (ISO) |
| Tr | ISO metric trapezoidal screw thread with a 30° angle. | ISO 2901, ISO 2902, ISO 2903, ISO 2904 | Current (ISO) |
| 8N | Superseded American National thread with 8 threads per inch. | – | Superseded |
| 12N | Superseded American National thread with 12 threads per inch. | – | Superseded |
| 16N | Superseded American National thread with 16 threads per inch. | – | Superseded |
| NC | Superseded American National coarse series; replaced by the Unified series. | – | Superseded |
| NEF | Superseded American National extra-fine series; replaced by the Unified series. | – | Superseded |
| NF | Superseded American National fine series; replaced by the Unified series. | – | Superseded |
| NR | Obsolete American National form that controls the root radius. | – | Superseded |
| NS | Obsolete American National thread cut to a special size, outside the standard series. | – | Superseded |
| Pitch P = 1 / TPI (in.) | 0.0769231 |
|---|---|
| Lead L = P x starts (in.) | 0.0769231 |
| Turns per inch = 1 / L | 13 |
| Sharp-V height H = (sqrt(3)/2) x P (in.)Geometry of the 60 deg fundamental triangle (Fig. 18). The numeric factor is not printed in B1.7; B1.7 only defines H. | 0.0666173 |
| Basic height of thread, UN and M = 0.625 H (in.) | 0.0416358 |
| Basic height of thread, UNJ and MJ = 0.5625 H (in.) | 0.0374723 |
| Percent of thread (obsolete) = h actual / (0.75 H) x 100 | 80.0592373 |
| UN/M maximum achievable percent of thread | 83.3333333 |
| UNJ/MJ root radius, minimum = 0.15011 P (in.)Table 3, UNJ series note: rounded root radius 0.15011 P to 0.18042 P | 0.0115469 |
| UNJ/MJ root radius, maximum = 0.18042 P (in.)Table 3, UNJ series | 0.0138785 |
| UNR root radius, not less than 0.108 P (in.)Table 3, UNR series | 0.0083077 |
| NR (superseded) root radius range 0.108 p to 0.144 p, upper (in.)Table A-1, NR | 0.0110769 |
| Tolerance TD = D max. - D min. (in.) | 0.025 |
|---|---|
| Allowance (internal minimum minus external maximum, in.) | 0.002 |
| Upper deviation es = max. limit of external part - basic size (in.) | -0.002 |
| (a) Shear area, internal thread = 3.1416 d x section x (LE x n) [as printed] | 0.306306 |
|---|---|
| (b) Shear area, external thread = 3.1416 D x section x (LE x n) [as printed] | 0.2756754 |
The thread explorer is an illustration drawn from the values you enter; it is not a thread of any standard. For real sizes, limits and tolerances open a thread page, or check a pair in the Fit Check.