Metric fastener threads are described by the pitch rather than the thread density. Thread Pitch is the distance from one thread to the next, measured from crest to crest. Like inch fastener descriptions, the thread parameter of metric fastener descriptions comes after the thread diameter.
The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter and its pitch. ISO metric threads consist of symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other.
Class 5 fit is not a loose thread. It is an interference thread, used on items like spring shackles on an automobile. The letter suffix "A" or "B" denotes whether the threads are external or internal, respectively. Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.
Most (but certainly not all) threaded items are made to a classification standard called the Unified Screw Thread Standard Series. This system is analogous to the fits used with assembled parts.
0.313 8mmSAE to Metric Conversion ChartSAEMetricInch5/16"0.3138mm0.31511/32"0.3449mm0.35447 more rows•Apr 29, 2020
7.9US to Metric Conversion TableUS (inches)Metric (millimeters)5/167.93/89.57/16111/21315 more rows
7.94Bolts and Screws: Conversion from Fractions of an Inch to MillimetersInchmm5/167.9421/648.3311/328.7323/649.1360 more rows
To sum up: M8 and 5/16 are different bolts. M8 and 5/16 are not compatible. It is not recommended to use a M8 bolt into a 5/16 thread (and vice-versa), otherwise you run the risk of forcing and damaging the thread.
8mm = 5/16 inch. 9mm = almost 3/8 inch. 10mm = just over 3/8 inch. 11mm = almost 7/16 inch.
7.95 mmConversion table inches to mmDimensions — Inches to Metric0.313”5/16”7.95 mm0.375”3/8”9.53 mm0.438”7/16”11.13 mm0.500”1/2”12.70 mm21 more rows
The size of a metric screw or bolt is specified as diameter, pitch and length, in millimeters (millimeters is abbreviated "mm"). For nuts, size will appear as diameter and pitch....Table 3.MetricClosest InchM8-1.255/16-18 (Smaller)M10-1.53/8-16 (Smaller)M12-1.751/2-13 (Larger)M14-2.09/16-12 (Larger)16 more rows
Tap sizeBasic major dia (mm)Basic major dia (inch)M16 x 1.516mm.6299M18 x 2.518mm.7087M18 x 1.518mm.7087M20 x 2.520mm.787424 more rows
Dimensions for DIN931 Hex Head BoltDiameter (d)PitchAcross Flats (s)M121.75mm19mmM142mm22mmM162mm24mmM182.5mm27mm17 more rows
The defined diameter of metric bolts is actually slightly larger than the actual diameter of the bolt shaft. So an M8 bolt would have a shaft diameter that's slightly under 8mm which means that the bolt should fit through an 8mm hole.
US Nut Size TableSizeDiameter*Hex NutMachine Screw Nut5/16"1/2"9/16"3/8"9/16"5/8"7/16"11/16"-17 more rows
US Bolt Head/Wrench SizeBolt DiameterHead and Wrench SizeHex Bolt - Lag Bolt - Square BoltHeavy Hex Bolt5/16"1/2"-3/8"9/16"-7/16"5/8"-13 more rows
Thread class refers to the acceptable range of pitch diameter for any given thread. The pitch diameter is indicated as Dp in the figure shown above. Several methods are used to measure the pitch diameter. The most common method used in production is by way of a go/no go gauge.
Class 1 threads are loosely fitting threads intended for ease of assembly or use in a dirty environment. Class 2 threads are the most common. They are designed to maximize strength considering typical machine shop capability and machine practice. Class 3 threads are used for closer tolerances.
Thread Count (TPI) TPI stands for Threads Per Inch, Which means the total number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric Fasteners use a thread Pitch. In general, smaller fasteners have finer threads, so the thread count is higher.
The Unified Thread Standard (UTS) defines a standard thread form and series along with allowances, tolerances, and designations, for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries.
A classification system exists for ease of manufacture and interchangeability of fabricated threaded items. Most (but certainly not all) threaded items are made to a classification standard called the Unified Screw Thread Standard Series. This system is analogous to the fits used with assembled parts.
Class 4 thread fit is even tighter than a Class 3.