US INCH THREADED BOLTS: EASY GUIDE FOR METRIC FOLKS!
Updated: December 25, 2024
Are you stuck because your brain is used to the metric system? Do inches confuse you? Read bobber.center's short blog about US standard bolts!
Let's dive in! Simply!
The standard sizes of American inch bolts are based on guidelines defined by the ASME (American Society of Mechanical Engineers) and the Unified Thread Standard (UTS) system.
To accurately determine the size of an inch bolt, or to accurately size an American standard bolt, you need to know some basic concepts and definitions:
THREAD TYPE:
For inch bolts, you will primarily encounter two types of threads: (1) Coarse thread (UNC - Unified National Coarse): Fewer threads per inch, e.g., 1/4”-20 (20 threads per inch). (2) Fine thread (UNF - Unified National Fine): More threads per inch, e.g., 1/4”-28 (28 threads per inch).
BOLT DIAMETER:
The largest diameter of the threaded part or the diameter of the unthreaded part (if any). Examples of common sizes: #0, #2, #4, #6, #8, #10, 1/4”, 5/16”, 3/8”, 1/2”, 5/8”, 3/4”, 1”, etc. For small diameters, a bolt size number is used; for larger ones, fractional inches.
THREAD PITCH:
The Anglo-Saxons refer to this as TPI (Threads Per Inch), which is simply the number of threads per inch along the length of the bolt. Examples: coarse thread 1/4”-20 UNC (20 threads per inch) or fine thread 1/4”-28 UNF (28 threads per inch).
THREAD LENGTH:
The part of the bolt shank that is threaded. Standard thread length depends on the bolt diameter and overall length. If bolt length ≤ 6 inches: Thread length = 2 × diameter + 1/4”. If bolt length > 6 inches: Thread length = 2 × diameter + 1/2”.
BOLT LENGTH:
The total length of the bolt measured from the underside of the head to the tip. For countersunk head bolts, the head is included in the length.
BOLT HEAD SIZE:
Distance across flats: The distance between two opposing sides of a hexagonal or square head. This is standardized depending on the bolt diameter. Example for a hexagonal bolt: A 1/4” bolt has a head measurement of 7/16”.
Bolt head height: The height of the head measured from its bottom to its top. Example: A 1/4” hexagonal bolt has a head height of 3/16”.
DRIVE TYPE:
For example, for hexagonal bolts, the drive is external (with a wrench) or, for Allen key bolts, an Allen key. Other bolts may have internal drive types, such as slotted (single), Phillips, Torx, etc.
MATERIAL GRADES AND MARKINGS:
The material grade of American inch bolts is indicated by standard markings stamped on the head. You may encounter three types of markings, which denote different tensile strength classifications (and of course, this is reflected in the price):
Grade 2: No marking.
Grade 5: Three radial lines.
Grade 8: Six radial lines.
We do not aim to bore you with detailed engineering data, so simplifying the essence of interpreting the classification, Grade 5 has a minimum tensile strength at least twice, while Grade 8 has two and a half times that of Grade 2.
These dimensions ensure compatibility with nuts, washers, and other fasteners, thereby guaranteeing the standardized applicability of bolts..
INTERPRETING INCH BOLT SIZES
Finally, an example:
Allen bolt 1/4”-20 x 1.5”, grade 8
Allen bolt: hex socket head cap screw
1/4”(0.25 inches): bolt diameter
20: thread pitch (20 threads per inch).
1.5”: bolt length (one and a half inches)
grade 8: highest-rated bolt, with the greatest tensile strength.
You can find all inch bolts for your American iron at the bobber.center bolt shop!
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