ISO 965-1 General purpose metric screw threads – Tolerances Part 1: Principles and basic data
ISO 965-1:2013 – METRIC SCREW THREADS TOLERANCES
SCOPE & PURPOSE Standard: ISO 965-1:2013 specifies a comprehensive tolerance system for ISO general purpose metric screw threads (M) according to ISO 261.
Purpose: This standard provides the foundational principles and data for ensuring proper fit and interchangeability of threaded components.
Application: The tolerance system applies to metric screw threads and refers to the basic profile according to ISO 68-1.
Coverage:
Internal threads (nuts, tapped holes)
External threads (bolts, screws)
TOLERANCE SYSTEM OVERVIEW The tolerance system consists of two main components:
1. TOLERANCE GRADES (Numbers) Grade Range Category Description 3, 4 Precision Smallest tolerances, tightest control 5, 6 Medium Standard commercial applications 7, 8, 9 Coarse Largest tolerances, manufacturing flexibility
Note: Higher numbers indicate larger tolerances (more permissible variation).
2. TOLERANCE POSITIONS (Letters) Thread Type Position Deviation Description Internal Threads H Zero (EI = 0) Basic position, most common G Positive (EI > 0) Allowance for coating/assembly External Threads h Zero (es = 0) Basic position, maximum metal g Negative Small allowance, general purpose f Negative Medium allowance e Negative Large allowance d Negative Very large allowance c Negative Extra large allowance b Negative Very extra large allowance a Negative Maximum allowance
TOLERANCE CLASS DESIGNATION Format: A tolerance class combines grade (number) + position (letter)
Example Thread Type Meaning 6H Internal Grade 6, Position H 6g External Grade 6, Position g 5H6H Internal Pitch diameter: 5H, Minor diameter: 6H 5g6g External Pitch diameter: 5g, Major diameter: 6g
TOLERANCE POSITIONS DETAILS INTERNAL THREADS Position Fundamental Deviation Characteristics Applications H EI = 0 (Zero) Basic position, most common General purpose, standard applications G EI > 0 (Positive) Additional clearance Easy assembly, coating accommodation
EXTERNAL THREADS Position Fundamental Deviation Characteristics Applications h es = 0 (Zero) Maximum metal, basic position Precision fits, no coating allowance g es = +11 – 15√P Small negative deviation General purpose, standard bolts f es = +11 – 30√P Medium negative deviation Medium coating allowance e es = +11 – 50√P Large negative deviation Large coating allowance, easy assembly d es = +19 – 65√P* Very large negative deviation Very thick coatings c es = +19 – 115√P* Extra large negative deviation Extra thick coatings b es = +19 – 185√P* Very extra large deviation Special coating requirements a es = +19 – 270√P* Maximum negative deviation Maximum coating thickness
*Note: Positions a, b, c, d are only applicable for threads with P > 0.8 mm.
Formula variables:
EI, es = fundamental deviations (micrometers)
P = pitch (millimeters)
THREAD ENGAGEMENT LENGTHS Group Code Description When to Use Short S Relatively short threaded engagement Limited engagement length, requires tighter tolerances for strength Normal N Standard default length group Unknown engagement length, standard bolts (most common) Long L Extended threaded engagement Long engagement, allows wider tolerances due to more contact
LENGTH CALCULATION FORMULAE Parameter Formula Description lN,min ≈ 2.24P × d0.2 Minimum normal engagement length lN,max ≈ 6.7P × d0.2 Maximum normal engagement length
Where:
l = length of engagement (mm)
P = pitch (mm)
d = basic major diameter (mm)
RECOMMENDED TOLERANCE CLASSES INTERNAL THREADS (Nuts, Tapped Holes): Tolerance Quality Short (S) Normal (N) Long (L) Fine – 4H, 5H 5H, 6H Medium 5G 6H, 6G 7H, 7G Coarse – 7H, 7G 8H, 8G
EXTERNAL THREADS (Bolts, Screws): Tolerance Quality Short (S) Normal (N) Long (L) Fine – 4h, 4g 5h4h, 5g4g, 3h4h Medium – 6h, 6g, 6e, 6f 7h6h, 7g6g, 7e6e, 5h6h, 5g6g Coarse – 8g, 8e 9g8g, 9e8e
COMMON FITS: The most frequently used combinations are H/g (general purpose), H/h (precision), and G/h (easy assembly or pre-coating).
THREAD DESIGNATION SYSTEM DESIGNATION COMPONENTS Component Symbol Example Meaning Thread System M M Metric thread Nominal Diameter Number 10 10 mm diameter Pitch × number × 1.25 1.25 mm pitch Tolerance Class Grade + Position 6H / 6g Internal / External Length Group S, N, or L – L Long engagement (optional) Hand LH – LH Left-hand thread (optional)
DESIGNATION EXAMPLES Designation Description M10 × 1.25 – 6H/6g Metric, 10mm dia., 1.25mm pitch, internal 6H, external 6g M8 – 6g Metric, 8mm dia., coarse pitch (omitted), external 6g M10 × 1 – 5g6g Metric, 10mm dia., 1mm pitch, external thread: pitch dia. 5g, major dia. 6g M10 × 1 – 5H6H Metric, 10mm dia., 1mm pitch, internal thread: pitch dia. 5H, minor dia. 6H M6 – 6H/6g Metric, 6mm dia., coarse pitch, fit designation (internal/external) M20 × 2 – 6H/5g6g Metric, 20mm dia., 2mm pitch, internal 6H, external: pitch 5g, major 6g M6 – 7H/7g6g – L Metric, 6mm dia., coarse pitch, long engagement group M20 × 2 – 5H – S Metric, 20mm dia., 2mm pitch, internal 5H, short engagement M8 × 1 – 6g – LH Metric, 8mm dia., 1mm pitch, external 6g, left-hand thread M6 × 0.75 – 5h6h – S – LH Metric, 6mm dia., 0.75mm pitch, external 5h6h, short, left-hand
MULTIPLE-START THREADS Designation Description M16 × Ph3P1.5 – 6H Metric, 16mm dia., Lead 3mm, Pitch 1.5mm (2-start), internal 6H M16 × 3P1.5 – 6H Same as above (Ph can be omitted if no confusion) M16 × Ph3P1.5 (two starts) – 6H Same with explicit number of starts noted M14 × Ph6P2 – 7H – L – LH Metric, 14mm dia., Lead 6mm, Pitch 2mm (3-start), long, left-hand
Note: Number of starts = Lead (Ph) / Pitch (P)Note: For coarse pitch threads, pitch may be omitted from designation.
FUNDAMENTAL DEVIATIONS (FORMULAE) INTERNAL THREADS Position Formula (μm) Notes Position H: EIH = 0 Zero deviation (basic position) Position G: EIG = +11 + 15√P Positive deviation for clearance
EXTERNAL THREADS Position Formula (μm) Applicable For Position h: esh = 0 All pitches Position g: esg = +11 – 15√P All pitches Position f: esf = +11 – 30√P P > 0.3 mm Position e: ese = +11 – 50√P P > 0.45 mm Position d: esd = +19 – 65√P P > 0.8 mm Position c: esc = +19 – 115√P P > 0.8 mm Position b: esb = +19 – 185√P P > 0.8 mm Position a: esa = +19 – 270√P P > 0.8 mm
Formula variables:
EI, es = fundamental deviations (micrometers)
P = pitch (millimeters)
√P = square root of pitch
TOLERANCE GRADE FORMULAE MAJOR DIAMETER (EXTERNAL THREAD) Base Formula – Grade 6: d (6) = 180 ∛(P²) – 3.15/√P (micrometers)
Grade Multiplier Formula Grade 4 0.63 Td (4) = 0.63 × Td (6) Grade 6 1.0 Td (6) = Base value Grade 8 1.6 Td (8) = 1.6 × Td (6)
MINOR DIAMETER (INTERNAL THREAD) Base Formula – Grade 6:
Pitch Range Formula (micrometers) P: 0.2 to 0.8 mm TD1 (6) = 433P – 190P1.22 P: 1 mm and coarser TD1 (6) = 230P0.7
Other Grades: Grade Multiplier Formula Grade 4 0.63 TD1 (4) = 0.63 × TD1 (6) Grade 5 0.8 TD1 (5) = 0.8 × TD1 (6) Grade 6 1.0 TD1 (6) = Base value Grade 7 1.25 TD1 (7) = 1.25 × TD1 (6) Grade 8 1.6 TD1 (8) = 1.6 × TD1 (6)
PITCH DIAMETER (EXTERNAL THREAD) Base Formula – Grade 6: d2 (6) = 90P0.4 × d0.1 (micrometers)
Grade Multiplier Formula Grade 3 0.5 Td2 (3) = 0.5 × Td2 (6) Grade 4 0.63 Td2 (4) = 0.63 × Td2 (6) Grade 5 0.8 Td2 (5) = 0.8 × Td2 (6) Grade 6 1.0 Td2 (6) = Base value Grade 7 1.25 Td2 (7) = 1.25 × Td2 (6) Grade 8 1.6 Td2 (8) = 1.6 × Td2 (6) Grade 9 2.0 Td2 (9) = 2.0 × Td2 (6)
Where: d = geometrical mean value of the diameter range limits
PITCH DIAMETER (INTERNAL THREAD) Derived from External Thread Tolerances:
Grade Multiplier Formula Grade 4 0.85 TD2 (4) = 0.85 × Td2 (6) Grade 5 1.06 TD2 (5) = 1.06 × Td2 (6) Grade 6 1.32 TD2 (6) = 1.32 × Td2 (6) Grade 7 1.7 TD2 (7) = 1.7 × Td2 (6) Grade 8 2.12 TD2 (8) = 2.12 × Td2 (6)
Note: No TD2 values are given when calculated values exceed 0.25P
ROOT CONTOURS & THREAD PROFILE EXTERNAL THREAD ROOT REQUIREMENTS
For Property Class 8.8 and Higher (High-Strength Bolts):
Requirement Specification Purpose Root profile: Non-reversing curvature Prevents stress concentration Minimum radius: Rmin = 0.125P Ensures fatigue resistance Recommended radius: R ≈ 0.144P (H/6 truncation) Optimal stress distribution
Important: Even for lower property classes, aspire to H/6 truncation, especially for fatigue or impact loading applications.
ROOT TRUNCATION FORMULAE Parameter Formula Maximum truncation: Cmax = H/4 – Rmin – Rmin × cos[arccos(1 – 3/4 × Td2 /Rmin ) + π/3] + Td2 /2 Minimum truncation: Cmin = 0.125P ≈ H/7 Fundamental triangle height: H = 0.866025P
INTERNAL THREAD REQUIREMENTS General Rule: The actual root contours must not transgress the maximum material limit defined by the basic profile and tolerance position. This ensures proper thread engagement and strength.
MINIMUM ROOT RADII TABLE:
Pitch P (mm) R(min) (μm) Pitch P (mm) R(min) (μm) 0.2 25 1.5 188 0.25 31 1.75 219 0.3 38 2 250 0.35 44 2.5 313 0.4 50 3 375 0.45 56 3.5 438 0.5 63 4 500 0.6 75 4.5 563 0.7 88 5 625 0.75 94 5.5 688 0.8 100 6 750 1 125 8 1000 1.25 156 – –
COATED THREADS CRITICAL RULE: Tolerances apply to parts BEFORE coating unless otherwise specified.
POST-COATING REQUIREMENTS Requirement Specification Maximum material limit: Thread profile must not transgress H or h position limits after coating Coating allowance: Position G (internal) or g, f, e, d (external) provide coating thickness allowance Design consideration: Coating thickness must be factored during design phase for proper fit
TOLERANCE POSITION SELECTION FOR COATING Thread Type Before Coating After Coating Recommended Positions External Threads Select with allowance Verify within H/h limits 6g, 6e, 6f (depending on coating thickness) Internal Threads Select with allowance Verify within H limits 6G, 7G
COATING TYPES & POSITION RECOMMENDATIONS Coating Type Typical Thickness External Thread Position Internal Thread Position Thin coatings (zinc, cadmium) 5-15 μm 6g, 6f 6G Medium coatings 15-30 μm 6e, 6f 6G, 7G Thick coatings (hot-dip galvanizing) 30-100 μm 6e, 8e, special positions 7G, 8G
Note: For hot-dip galvanized threads, refer to ISO 965-4 and ISO 965-5 for specific requirements.
MULTIPLE-START THREADS With the same pitch, the tolerances for multiple-start threads are the same as for single-start threads, with the exception of the pitch diameter tolerances which are enlarged.
MULTIPLICATION FACTORS FOR PITCH DIAMETER TOLERANCES: Number of Starts Factor 2 1.12 3 1.25 4 1.4 5 and larger 1.6
Example: For a 2-start thread with pitch diameter tolerance T(d2) = 100 μm, the actual tolerance becomes 100 × 1.12 = 112 μm
PRACTICAL APPLICATION GUIDELINES SELECTING TOLERANCE QUALITY Quality Tolerance Classes Characteristics Applications Fine 4H, 5H, 4h, 5h, 3h4h Precision threads, minimal fit variation Instrumentation, aerospace, precision machinery, measuring equipment Medium 6H, 6g, 6h, 5g6g General commercial use, standard fasteners General purpose, standard bolts/nuts, automotive, construction (DEFAULT) Coarse 7H, 8H, 8g, 9g8g Manufacturing flexibility, larger tolerances Hot-rolled bars, long blind holes, rough machining, difficult conditions
SELECTING TOLERANCE POSITION (FIT TYPE) Fit Type Designation Characteristics Applications H/h fit 6H/6h, 5H/5h Maximum metal, no intentional allowance, tight fit Precision applications, maximum thread strength required H/g fit 6H/6g, 7H/7g6g Slight clearance, general purpose (MOST COMMON) Standard fasteners, general assembly, commercial products G/h fit 6G/6h, 7G/7h6h Easy assembly, clearance for variations Pre-coating applications, easy assembly required, field assembly H/e, H/f fit 6H/6e, 7H/7e6e Larger allowances, very easy assembly Thick coatings, very easy assembly, significant clearance needed
THREAD SIZE SPECIFIC GUIDELINES Thread Size Recommendation Reason M1.4 and smaller Use 5H/6h, 4H/6h, or finer Guarantee sufficient thread overlap and strength in small sizes M1.6 to M64 Use 6H/6g (default medium quality) Standard commercial applications Standard bolts (unknown engagement) Use Normal (N) engagement group Default when actual engagement length is unknown
TOLERANCE CLASS COMBINATION MATRIX Application Type Internal Thread External Thread Engagement Group Precision instruments 4H, 5H 4h, 5h4h S, N Standard fasteners (uncoated) 6H 6g, 6h N Standard fasteners (coated) 6H, 6G 6g, 6e N Easy assembly / field work 6G, 7G 6h, 6g N, L Hot-rolled / rough machining 7H, 8H 8g, 9g8g N, L Long engagement (pipes, etc.) 7H, 8H 7g6g, 8g L Hot-dip galvanized Special (see ISO 965-4/5) Special before galvanizing Varies
PREFERENCE ORDER FOR SELECTION Priority Internal Threads External Threads When to Use 1st Choice 6H (d≥1.6mm) 6g (d≥1.6mm) Default for commercial threads, most common 2nd Choice 6G, 7H, 5H 7g6g, 5g6g, 6e, 6f Specific requirements (coating, engagement, etc.) 3rd Choice 5G, 7G, 8H, 8G 7e6e, 8g, 9g8g, 8e, 9e8e Special cases only (thick coating, rough conditions)
DEFAULT TOLERANCE CLASSES (WHEN NOT SPECIFIED) If “medium” quality is specified without tolerance class:
Thread Type Diameter Range Default Class Internal threads Up to d ≤ 1.4 mm 5H Internal threads d ≥ 1.6 mm 6H External threads Up to d ≤ 1.4 mm 6h External threads d ≥ 1.6 mm 6g
Exception: For threads with pitch 0.2 mm, only tolerance grade 4 is defined.
QUICK SELECTION GUIDE Question Answer Selection Will threads be coated? Yes Use G for internal, g/e/f for external Will threads be coated? No Use H for internal, h/g for external Is precision critical? Yes Use grades 4-5 Is precision critical? No Use grades 6-8 Manufacturing difficulties expected? Yes Use coarse quality (7-8) Manufacturing difficulties expected? No Use medium quality (6) Is engagement length known? Yes Select S, N, or L accordingly Is engagement length known? No Use N (Normal) as default Thread size ≤ M1.4? Yes Use 5H/6h or finer Not sure what to use? – Use 6H/6g with N engagement (safest default)
KEY SYMBOLS AND DEFINITIONS DIAMETER SYMBOLS Symbol Description Thread Type D Basic major diameter Internal thread (nut) D1 Basic minor diameter Internal thread (nut) D2 Basic pitch diameter Internal thread (nut) d Basic major diameter External thread (bolt) d1 Basic minor diameter External thread (bolt) d2 Basic pitch diameter External thread (bolt) d3 Minor diameter (actual root) External thread (bolt)
DIMENSIONAL SYMBOLS Symbol Description Definition P Pitch Distance between corresponding points on adjacent threads Ph Lead Axial advance of thread in one revolution H Fundamental triangle height H = 0.866025P (for 60° thread angle)
TOLERANCE SYMBOLS Symbol Description Application T Tolerance (general) Magnitude of permissible variation TD1 Minor diameter tolerance Internal thread TD2 Pitch diameter tolerance Internal thread Td Major diameter tolerance External thread Td2 Pitch diameter tolerance External thread
DEVIATION SYMBOLS Symbol Description Case Application EI Lower limit deviation Uppercase Internal threads ES Upper limit deviation Uppercase Internal threads ei Lower limit deviation Lowercase External threads es Upper limit deviation Lowercase External threads
THREAD PROFILE SYMBOLS Symbol Description Application R Root radius External thread root contour C Root truncation External thread root flat Rmin Minimum root radius Rmin = 0.125P (for property class ≥ 8.8)
ENGAGEMENT GROUP SYMBOLS Symbol Description Usage S Short engagement group Limited threaded length N Normal engagement group Standard length (default) L Long engagement group Extended threaded length
SUMMARY KEY BENEFITS OF ISO 965-1:2013 Benefit Description Interchangeability Components from different manufacturers can be assembled together Functionality Proper thread engagement and strength characteristics ensured Manufacturing Flexibility Range of tolerance grades and positions suit different production methods Application Versatility Options for precision, general purpose, and coarse applications Coating Accommodation Tolerance positions allow for surface finishing without interference
MOST COMMONLY USED COMBINATIONS Fit Type Designation When to Use General Purpose 6H/6g Standard commercial applications, uncoated threads Precision 6H/6h, 5H/5h Tight fit, precision requirements, maximum strength Easy Assembly/Coated 6G/6h, 6H/6e Pre-coating, easy assembly, field work
QUICK REFERENCE: DEFAULT SELECTION Condition Recommendation When in doubt Select medium quality (grade 6) with normal engagement (N) Standard applications 6H/6g – N Small threads (≤M1.4) 5H/6h or finer Coated threads 6G/6g or 6H/6e Unknown engagement Use N (Normal) group
TOLERANCE QUALITY SELECTION SUMMARY Quality Grades Best For Fine 3, 4, 5 Precision work, aerospace, instrumentation Medium (DEFAULT) 6 General commercial, standard fasteners, automotive Coarse 7, 8, 9 Difficult manufacturing, hot-rolled, long blind holes
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