ISO 965
ISO 965 is a multi-part International Standard published by the International Organization for Standardization (ISO) that defines the tolerance system, including fundamental deviations, limits of sizes, and related parameters, for general purpose metric screw threads designated as "M" and conforming to the basic profile specified in ISO 261.[1] The standard addresses the need for precise control over thread dimensions in manufacturing to ensure reliable assembly, interchangeability, and performance of fasteners in mechanical applications worldwide.[1] It establishes a unified framework for external and internal threads, covering aspects such as pitch diameter, major diameter, and minor diameter tolerances, which are critical for achieving desired fits ranging from loose to interference.[1] ISO 965 comprises several parts, each focusing on specific elements of the tolerance system. Part 1 outlines the principles, basic data, and designation system for the tolerances.[1] Part 2 provides limits of sizes for internal and external threads in medium tolerance qualities. Part 3 details deviations applicable to constructional screw threads.[2] Part 4 specifies limits for hot-dip galvanized external threads, and Part 5 specifies limits for internal threads to mate with them, both to accommodate coating thickness while maintaining mating compatibility.[3][4] As of 2025, Part 6 has been introduced to cover limits of sizes for fine and medium tolerance qualities in internal and external threads, offering first and second choice options for enhanced precision in specialized applications.[5] Key features of ISO 965 include standardized tolerance grades (typically from 3 to 8 for most applications, with finer grades like 4H/6h for precision fits) and position symbols (e.g., h, g for external threads; H, G for internal) that define allowable deviations from nominal sizes.[1] These elements promote consistency across industries such as automotive, aerospace, and construction, reducing defects and facilitating global trade in threaded components.[6]Overview
Scope and Purpose
The ISO 965 series constitutes a multi-part international standard that establishes a comprehensive tolerance system for ISO general purpose metric screw threads, designated as M threads, which conform to the basic profiles defined in ISO 261. This system applies to threads with nominal diameters starting from 1 mm and extending upwards, encompassing both external threads (such as those on bolts and screws) and internal threads (such as those in nuts). The primary objective is to promote interchangeability between mating components, ensure functional reliability under operational loads, and facilitate efficient manufacturing processes by precisely defining permissible deviations and limits of sizes for thread dimensions. Central to the ISO 965 framework are key thread elements, including the major diameter (the largest diameter of the thread), the minor diameter (the smallest diameter of the thread), and the pitch diameter (the diameter where the width of the thread ridge equals the width of the thread groove).[7] Fundamental terminology includes tolerance zones, which specify the allowable variation in thread size based on grades (typically 4 through 9 for general applications); positions, such as h and g for external threads (indicating zero or negative deviations from the basic size) and H and G for internal threads (indicating zero or positive deviations); and fundamental deviations like es (upper limit for external threads) and EI (upper limit for internal threads), which determine the directional positioning of the tolerance zone relative to the basic size.[7] These concepts enable standardized gauging and quality control in mechanical engineering applications, where general purpose threads are widely used for assembly and fastening. By focusing on general purpose applications, ISO 965 supports a range of tolerance classes—such as medium, fine, and coarse—to balance precision with practicality, thereby enhancing compatibility across diverse manufacturing environments without delving into specialized or constructional threads.Historical Development
The development of the ISO 965 standard series originated in the 1960s as part of the International Organization for Standardization's (ISO) efforts to establish a unified tolerance system for metric screw threads, coordinated by Technical Committee ISO/TC 1 on Screw threads.[8] This work built upon earlier national standards, such as the German DIN 13, which had defined metric thread profiles and tolerances since the mid-20th century and served as a foundational influence for international harmonization.[9] In 1964, ISO/TC 1 adopted a proposed tolerance system, leading to the circulation of Draft ISO Recommendation No. 979 in 1966 and its approval as ISO/R 965-1 in 1969, marking the initial publication as an ISO Recommendation rather than a full standard.[8] Subsequent milestones advanced the series toward full standardization and refinement. The first full ISO standard, ISO 965-1, was published in 1980, establishing principles and basic data for general-purpose metric threads conforming to ISO 261.[10] This was technically revised and reissued as the third edition in 1998, which also introduced ISO 965-4 to specify limits for hot-dip galvanized external threads, ensuring compatibility with internal threads tapped to positions H or G after galvanizing.[11][12] The 1998 revisions across parts incorporated updates based on practical experience with metric thread manufacturing and assembly. Further evolution occurred in 2013 with the fourth edition of ISO 965-1, which clarified aspects of thread fits and tolerance positions for improved applicability in diverse engineering contexts.[1] Recent updates include Amendment 1 to ISO 965-2 in 2021 and a full revision of that part in 2024, extending coverage to larger diameters up to 100 mm.[13] Additionally, ISO 965-4 was revised in 2021 and again in 2025 to refine limits for galvanized threads, reflecting ongoing adaptations to industrial needs.[14][3] Overall, ISO 965's progression underscores the global shift from unified inch-based threads to metric systems, promoting interoperability in international manufacturing supply chains through ISO/TC 1's continuous oversight.[15]Tolerance System
Fundamental Principles
The ISO 965 tolerance system establishes limits for ISO general purpose metric screw threads (M) by referencing the 6H/6g medium fit as the baseline for general-purpose applications, ensuring compatibility and ease of assembly across standard components.[1] This reference fit defines the positional relationship between internal and external threads, where the internal thread tolerance zone (H) is positioned above the basic size and the external thread tolerance zone (g) is positioned below it, creating a controlled clearance without interference.[1] The system adopts a positional tolerance approach, deriving upper and lower limits algebraically from the basic thread size to accommodate manufacturing variations while maintaining functional interchangeability.[1] Tolerance zones are represented as rectangular areas on deviation charts, bounded by fundamental deviations and tolerance grades that specify the width of the zone.[1] Fundamental deviations denote the maximum material limits relative to the basic size; for external threads, this includes es for the pitch diameter (lower limit ensuring clearance with internal thread), while for internal threads, EI applies to the minor diameter (upper limit).[16] These deviations are calculated as algebraic differences from the basic size, with values derived from standardized tables—for instance, the fundamental deviation es for the pitch diameter of an M6 thread with 6g class is -0.026 mm.[17] The total tolerance T is then the difference between the upper and lower limits of the zone, providing the allowable variation in thread dimensions.[1] Key calculations for tolerances incorporate empirical formulas adjusted for thread geometry. For the pitch diameter tolerance Td2 in grade 6, a basic formula is Td2(6) = 90 P0.4 D0.1 (in μm), where D is the nominal diameter and P is the pitch.[18] This equation accounts for the scaling of tolerances with thread size and pitch. Fundamental deviations for the pitch diameter, such as es, are similarly tabulated based on diameter and class, ensuring the lower limit aligns below the basic size to guarantee clearance.[1] By design, the ISO 965 system prioritizes clearance fits, positioning all standard tolerance zones to avoid interference between mating threads and thereby maximizing assembly reliability in general applications.[1] This default clearance approach simplifies production and inspection, as threads within specified classes are interchangeable without risk of binding, supporting broad industrial use.[1]Tolerance Classes and Designations
The tolerance system in ISO 965 employs a classification based on numerical grades and letter positions to define permissible deviations for screw thread dimensions, ensuring interchangeability and functional performance. Tolerance grades range from 4 (finer) to 8 (coarser, as typically used), with higher grades providing progressively larger tolerance zones to balance manufacturing precision and cost. Grade 6 serves as the standard for general-purpose threads, balancing cost and reliability for medium-quality production.[1] Position letters indicate the location of the tolerance zone relative to the basic thread size. For internal threads, uppercase letters are used: H denotes a position with no negative deviation allowed on the major diameter (tolerance zone at or above basic size), while G allows positive deviations above the basic size. For external threads, lowercase letters apply: g specifies a slight negative deviation on the pitch diameter (tolerance zone below basic size for clearance), h aligns with zero deviation, and other letters like a, b, c, d, e, f position the zone further below for varying fits. These positions ensure the internal thread's tolerance zone encompasses or overlaps with the external thread's to achieve desired assembly characteristics.[1] The designation system combines a numerical grade with a position letter to specify the tolerance class for key diameters, such as the pitch diameter. For example, 6g denotes an external thread pitch diameter with grade 6 tolerance positioned at g, while 6H indicates an internal thread with grade 6 at H. Fits between mating threads are denoted by combining classes, such as 6H/6g for a medium clearance fit suitable for general assembly. This notation fully describes the tolerance requirements without needing additional details for standard applications.[1] ISO 965 defines tolerance classes based on 8 positions (a to h) for external threads, forming classes such as 3a (fine on minor diameter), 4h, and 6g (standard clearance on pitch diameter). For internal threads, classes include 4H to 8H (position H) and 6G to 8G (position G), providing progressively looser tolerances while maintaining positions for reliable engagement. These classes are selected to achieve specific functional properties: for instance, 4H/4h enables close fits with minimal clearance, whereas 7H/8g supports free-running fits for easy insertion and disassembly in non-critical applications.[1] The core concept of these classes lies in positioning the tolerance zone relative to the basic size, as defined in tables within the standard. For an M10 thread (nominal diameter 10 mm, pitch 1.5 mm), the 6g class for external pitch diameter places the lower limit at approximately -0.202 mm from the basic size, ensuring a controlled clearance while avoiding excessive play. This approach, grounded in fundamental principles of tolerance zones, allows designers to select classes that optimize fit without overlapping into interference unless intended.[19]Standard Parts
ISO 965-1: Principles and General Data
ISO 965-1 establishes the foundational principles and basic data for the tolerance system applicable to ISO general purpose metric screw threads (M threads) as defined in ISO 261. This part outlines the architecture of the tolerance system, which combines numerical tolerance grades ranging from 3 to 9—with grade 3 offering the finest control and grade 9 the coarsest—to specify permissible variations in thread dimensions, alongside letter designations for positions that determine the location of the tolerance zone relative to the basic thread profile. The system covers external (bolt) and internal (nut) threads, ensuring interchangeability and functional fits across a wide range of applications.[1] The standard addresses thread series including coarse pitch (preferred for general use) and fine pitch series, with diameter ranges spanning from M1 to M600, where the nominal major diameter d (for external threads) or D (for internal threads) is designated in millimeters. Pitch selections are standardized based on diameter size, for instance, coarse pitches increase from 0.25 mm for M1 to 4 mm for diameters up to M52, while fine pitches provide closer increments such as 0.2 mm steps for smaller sizes to enhance precision in specific assemblies. Basic major diameters and corresponding pitches are compiled in tables that serve as reference data, calculated at a standardized reference temperature of 20°C to account for thermal expansion effects in measurements.[1][18] Fundamental deviations, which position the tolerance zones, are detailed in deviation tables for grades 3 through 9, providing upper (ES, EI) and lower (es, ei) limits as functions of pitch P and diameter d; for example, the fundamental deviation es for position g in external threads is given by +15P^{1/2} μm for P ≤ 1 mm, ensuring a close fit with minimal clearance. These tables include separate values for pitch diameter, major diameter, and minor diameter deviations, with tolerances like TD2 (pitch diameter tolerance) scaling with grade number, such as TD2 = 190(P)^{2/3}(d)^{1/6} μm for grade 6 external threads. The length of engagement is a key consideration, categorized into short (S), normal (N), and long (L) groups based on formulas like LE_N = (1.5 d^{2/3} P^{1/3}) to (5 d^{1/2} P^{1/2}) mm, which influence the selection of tolerance grades to prevent excessive wear or seizure.[1][18][16] For internal threads, the standard introduces the concept of truncated threads in nuts, where the minor diameter is truncated to a flat crest to facilitate assembly and avoid interference, with root fillet radii not exceeding 0.1443P and minimum radii of 0.125P to maintain strength. Normative annexes provide tolerance zone diagrams illustrating the positional relationships for various grade-position combinations, such as 6g for external pitch diameter where the maximum limit is the basic pitch diameter plus the fundamental deviation es plus the grade tolerance TD2. Calculation methods in these annexes enable derivation of upper and lower limits, for instance, the upper pitch diameter limit for an external 6g thread is d2_max = d2_basic + es + TD2, with TD2 values sourced from grade-specific tables to ensure precise manufacturing control.[1][18][16]ISO 965-2: Limits for General Purpose Threads
ISO 965-2 establishes the dimensional limits for general purpose metric screw threads, ensuring compatibility and interchangeability between external threads on bolts and screws and internal threads on nuts. It focuses on medium quality tolerances, specifically class 6g for external threads and 6H for internal threads (or 5H/6h for smaller sizes up to M1.4), with thread profiles conforming to ISO 68-1 and sizes per ISO 262. These limits apply to pitch diameters and crest diameters, promoting reliable assembly in standard fastening applications without requiring specialized fits.[20] The standard includes comprehensive tables detailing maximum and minimum values for major, pitch, and minor diameters across coarse and fine pitch series. For instance, in the M8×1.25 coarse series, which represents a common general purpose thread:| Thread Type | Tolerance Class | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) |
|---|---|---|---|---|
| External | 6g | Max: 7.972 Min: 7.760 | Max: 7.160 Min: 7.042 | Max: 6.619 Min: 6.272 |
| Internal | 6H | N/A (not controlled) | Max: 7.348 Min: 7.188 | Max: 6.912 Min: 6.647 |