Sources:   (1)   U.S. BLM, Metric Handbook, H-9102, Sect. 2.E.8.  (1999-12-29).
  (2)   ISO 898-1, 1999, Sections 3 through 7, and App. A.
  (3)   ISO 965-1, 1998, Sections 5.2, 7, 12.
1999-12-29,  Rev. 2003-01-22.               © 2003 Garrett D. Euler

Metric Bolt Strength    

6.  General Fasteners.

  1. USA industry is now using metric fasteners extensively.  The remainder of the world uses ISO metric fasteners almost exclusively, due to their superiority in proportions, fatigue strength, pitch, size and specification designations, and international availability.

  2. The Thomas Register lists hundreds of firms under Metric Fasteners, Metric Screws, and Metric Bolts.  The Industrial Fasteners Institute (IFI) has guides for fastener types and producers.

  3. Metric fasteners for all aerospace applications are made readily and easily available at "rock" bottom price to all NASA and contractor employees via NASA GSFC Fastener Inventory.

  4. Metric fastener size designation.

    1. Metric fastener size designation nomenclature.  As fully explained in ISO 965-1, Sect. 5, metric fastener size designations always begin with capital M or MJ followed by fastener nominal diameter and thread pitch, both in units of millimeters (mm), separated by the symbol "x", as follows.  M10 x 1.5-6g-S means metric fastener thread profile M, fastener nominal size (nominal major diameter) 10 mm, thread pitch 1.5 mm, external thread tolerance class 6g, and thread engagement length group S ("short").  If referring to internal thread tolerance, "g" would be capitalized.  A fit between threaded parts is indicated by internal thread tolerance class followed by external thread tolerance class separated by a slash; e.g., M10 x 1.5-6H/6g.

    2. Default metric fastener thread pitch and engagement length.  If metric thread pitch designation (e.g., " x 1.5") is omitted, it specifies coarse pitch threads.  For example, M10 or M10-6g, by default, specifies M10 x 1.5.  The standard metric fastener thread series for general purpose threaded components is the M thread profile and the coarse pitch thread series.  If thread engagement length group designation (e.g., "-S") is omitted, it specifies thread engagement length group N meaning "normal."

    3. Default metric fastener thread tolerance class.  If thread tolerance class designation (e.g., "-6g") is omitted (e.g., M10 x 1.5), it specifies "medium" thread tolerance, which is 6H/6g.  The 6H/6g fit is the standard ISO tolerance class for general use.

    4. Equivalent imperial thread tolerance classes.  Imperial internal and external thread tolerance class 2B/2A is essentially equivalent to ISO thread tolerance class and fit 6H/6g.  Imperial tolerance class 3A is approximately equivalent to ISO tolerance class 4g6g, though class fit 3B/3A is approximately equivalent to ISO class fit 4H5H/4h6h.  For full details, see ISO 965-1, Sects. 5.2, 7, and 12.

    5. Metric fastener thread profile compatibility.  Metric fastener thread profile M is the normal, commercially-available thread profile.  Thread profile MJ designates the external thread has an increased root radius (shallower root relative to external M thread profile), thereby having higher fatigue strength (due to reduced stress concentrations), but requires the truncated crest height of the MJ internal thread to prevent interference at the external MJ thread root (just as the UNJ external thread profile requires the UNJ internal thread).  However, M external threads are compatible with M and MJ internal threads (just as UN and UNR external threads are compatible with UN and UNJ internal threads).

  5. ISO metric fastener material strength property classes (grades).  As given in ISO 898-1, ISO metric fastener material property classes (grades) should be used.  For example, fastener material ISO property class 5.8 means nominal (minimum) tensile ultimate strength 500 MPa and nominal (minimum) tensile yield strength 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.  (In a few cases, the actual tensile ultimate strength may be approximately 20 MPa higher than nominal tensile ultimate strength indicated via the nominal property class code.  Consult Table 10, below, for exact values.)  Many anchor bolts (L, J, and U bolts, and threaded rod) are made from low carbon steel grades, such as ISO classes 4.6, 4.8, and 5.8.

  6. Preferred diameters.  Preferred nominal diameters for bolts and threaded rod are as listed below.  The fourth series listed below should be limited to unusual requirements when none of the preceding series can be used.  Reference individual standards prior to specification.  Sizes M5 to M45 are commonly used in construction.

     First choice:  M2 2.5 3 4 5 6 8 10 12 16 20 24 30 36 42
    Second choice: M3.5 14 18 22 27 33 39 45
    Third choice: M15 17 25 40
    Avoid: M7 9 11 26 28 32 35 38
  7. Bolt versus screw definition.  The correct definition of bolt and screw is as follows.  Bolts are headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as M, MJ, UN, UNR, and UNJ) such that they can accept a nontapered nut.  Screws are headed, externally-threaded fasteners that do not meet the above definition of bolts.  For full discussion of misdefinitions and corresponding confusion regarding these two words, see details.

  8. Handy conversion factors.  Imperial conversion factors, verified accurate to the decimal places shown via multiple, independent, credible sources, are 25.4 mm/inch (exact), 4.4482216152605 N/lbf, 6.89475729318 MPa/ksi, 47.880259 Pa/psf, 112.98483 (N mm)/(in lbf), 157.08746 (N/m^3)/pcf, 16.0184634 (kg/m^3)/(lbm/ft^3), 27679.9047 (kg/m^3)/(lbm/in^3), 9.80665 (m/s^2)/gravity (exact).  Rounding these conversion factors to a few less decimal places, we have 4.448222 N/lbf, 6.89476 MPa/ksi, 47.8803 Pa/psf, 113.0 (N mm)/(in lbf), 157.087 (N/m^3)/pcf, 16.01846 (kg/m^3)/(lbm/ft^3), 27679.9 (kg/m^3)/(lbm/in^3).

  9. Metric system (SI).  The abbreviation for the metric system is SI, the International System of Units (from the French, Systeme International d'Unites).  It evolved from the original French metric system and is currently being used virtually worldwide.  Long the language universally used in science and among technically adept individuals, SI has also become the dominant language of international commerce and trade.  All new USA standards (ASTM, ANSI, SAE, IEEE, ASME, etc.) are now written in metric, as the lead engineers in these organizations recognize the importance of trying to get the USA on track with technically advanced countries, in an effort to regain lost USA competitiveness in a global economy, as there is essentially no global market for the archaic, oddball, incompatible product dimensions USA arbitrarily comes up with, while they forfeit industries and jobs to third-world countries who have no problem understanding something so simple and fulfilling the need efficiently.  IEEE was intelligent enough to recognize this decades ago.  Japan also was intelligent enough to recognize simple matters such as this long ago.  This small country, defeated in WWII only 60 years ago, has since captured a large portion of the global economy due to their intelligent progress, and consequently has become a major global financier, while USA has become a world-class debtor to the tune of trillions due to inefficient business practices, low educational level, slackerism, and inability to solve or understand even simple problems such as metric conversion.

8.  Fastener Data.  Tables 9 and 10 provide much of the data available for different metric fasteners.  Table 9 comes verbatim from Ref. 1, including what appear to be a few typos, marked "[sic]," below.  Table 10, on the other hand, has been verified accurate per ISO 898-1 and ASTM F 568M.

Table 9  

Basic Product Product Type and Head Style Available Size Range For thread and dimension details refer to: For mechanical property details refer to Table 10 or:
Metric Bolts hex M5-M100 ANSI/ASME B18.2.3.5M ASTM F568M
heavy hex M12-M36 ANSI/ASME B18.2.3.6M
round head short square neck (carriage) M8-M20 ANSI/ASME B18.5.2.1M
round head square neck (carriage) M5-M24 ANSI/ASME B18.5.2.2M
bent M5 and larger IFI 528 [sic]
heavy hex structural M12-M36 ANSI/ASME B18.2.3.7M ASTM A325M
hex transmission tower M16-M24 IFI 541 [sic] IFI 541 [sic]
Metric Screws hex cap M5-M100 ANSI/ASME B18.2.3.1M ASTM F568M
formed hex M5-M24 ANSI/ASME B18.2.3.2M
heavy hex M12-M36 ANSI/ASME B18.2.3.3M
hex flange M5-M16 ANSI/ASME B18.2.3.4M
heavy hex flange M10-M20 ANSI/ASME B18.2.3.9M
hex lag M5-M24 ANSI/ASME B18.2.3.8M see note 3 [sic]
Metric Studs double end M5-M100 IFI 528 [sic] ASTM F568M
continuous thread M5-M100
Metric Locking Screws prevailing torque, non-metallic insert M1.6-M36 see note 3 [sic] IFI 524
chemical coated M6-M20 see note 3 [sic] IFI 525
Metric Socket Screws socket head cap M1.6-M48 ANSI/ASME B18.3.1M ASTM A574M
socket head shoulder M6.5-M25 ANSI/ASME B18.3.3M ASTM F835M
socket button head cap M3-M16 ANSI/ASME B18.3.4M
socket countersunk head cap M3-M20 ANSI/ASME B18.3.5M
socket set M1.6-M24 ANSI/ASME B18.3.6M ANSI/ASME B18.3.6M
Metric Nuts hex, style 1 M1.6-M36 ANSI/ASME B18.2.4.1M ASTM A563M
hex, style 2 M3-M36 ANSI/ASME B18.2.4.2M
slotted hex M5-M36 ANSI/ASME B18.2.4.3M
hex flange M5-M20 ANSI/ASME B18.2.4.4M
hex jam M5-M36 ANSI/ASME B18.2.4.5M
heavy hex M12-M100 ANSI/ASME B18.2.4.6M
Metric Prevailing-
Torque Nuts
hex, steel M3-M36 ANSI/ASME B18.16.3M ANSI/ASME B18.16.1M
hex flange, steel M6-M20

Notes for Table 10.

  1. When only the ISO property class number is shown in Table 10, below, the class is standard in both ISO 898-1 and ASTM documents.  Properties specified in each are identical except for minor exceptions.  Where differences exist, the ASTM F 568M values are given.

  2. To compute the tensile proof load, tensile yield strength, or tensile ultimate strength in kilonewtons (kN) for a bolt, screw, or stud, multiply the stress value (MPa) in Table 10 by the tensile stress area (mm^2) of the product's screw thread as given in Table 9 or Standard Metric Bolt Shank Dimensions, then divide this result by 1000.

  3. In general, identification markings are located on the top of the head and preferably are raised.

  4. Class 5.8 products are available in lengths 150 mm and less.

  5. Caution is advised when considering the use of property class 12.9 products.  The capabilities of the fastener manufacturer, as well as the anticipated service environment, should be carefully considered.  Some environments may cause stress corrosion cracking of nonplated, as well as electroplated, products.

Table 10  

Property Class Designation Nominal Size of Product Material and Treatment Mechanical Requirements Property Class Ident. Marking
Proof Load Stress, MPa Tensile Yield Strength, MPa, Min. Tensile Ultimate Strength, MPa, Min. Prod. Hardness, Rockwell
Surface, Max. Core
Min. Max.
4.6 M5-M100 low or medium carbon steel 225 240 400 -- B67 B95 4.6
4.8 M1.6-M16 low or medium carbon steel, fully or partially annealed 310 340 420 -- B71 B95 4.8
5.8 M5-M24 low or medium carbon steel, cold worked 380 420 520 -- B82 B95 5.8
8.8 M16-M72 medium carbon steel, quenched and tempered 600 660 830 30N56 C23 C34 8.8
A325M Type 1 M16-M36 A325M  8S
8.8 M16-M36 low carbon boron steel, quenched and tempered 600 660 830 30N56 C23 C34 8.8
A325M Type 2 A325M  8S
A325M Type 3 M16-M36 atmospheric corrosion resistant steel, quenched and tempered 600 660 830 30N56 C23 C34 A325M  8S3
9.8 M1.6-M16 medium carbon steel, quenched and tempered 650 720 900 30N58 C27 C36 9.8
9.8 M1.6-M16 low carbon boron steel, quenched and tempered 650 720 900 30N58 C27 C36 9.8
10.9 M5-M20 medium carbon steel, quenched and tempered 830 940 1040 30N59 C33 C39 10.9
10.9 M5-M100 medium carbon alloy steel, quenched and tempered 830 940 1040 30N59 C33 C39 10.9
A490M Type 1 M12-M36 A490M  10S
10.9 M5-M36 low carbon boron steel, quenched and tempered 830 940 1040 30N59 C33 C39 10.9
A490M Type 2 M12-M36 A490M  10S
A490M Type 3 M12-M36 atmospheric corrosion resistant steel, quenched and tempered 830 940 1040 30N59 C33 C39 A490M  10S3
12.9 M1.6-M100 alloy steel, quenched and tempered 970 1100 1220 30N63 C38 C44 12.9
Return to Structural Analysis Reference Library. © 2003 Garrett D. Euler