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.

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.