Stuck because your brain is used to the metric system? Is the inch measurement confusing you? Read the short blog on bobber.center about US standard screws!
Let’s dive in! Simply!
The standard sizes of American inch-based screws are based on the guidelines set by the ASME (American Society of Mechanical Engineers) and the Unified Thread Standard (UTS) system.
To accurately determine the size of an inch screw or properly size an US standard screw, you need to understand a few basic concepts and definitions:
THREAD TYPE:
For inch screws, there are primarily 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).
SCREW DIAMETER:
The largest diameter of the threaded part or the diameter of the unthreaded part (if present). Examples of common sizes: #0, #2, #4, #6, #8, #10, 1/4”, 5/16”, 3/8”, 1/2”, 5/8”, 3/4”, 1”, etc. Smaller diameters are measured with a screw gauge number, and larger ones are measured in fractions of an inch.
THREAD PITCH:
The TPI denotes Threads Per Inch, which is the number of threads per inch along the screw’s length. 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 screw shank that is threaded. The standard thread length depends on the screw diameter and total length. If the screw length is ≤ 6 inches: Thread length = 2 × diameter + 1/4”. If the screw length is > 6 inches: Thread length = 2 × diameter + 1/2”.
SCREW LENGTH:
The total length of the screw measured from the bottom of the head to the tip. For countersunk screws, the head is also included in the length.
SCREW HEAD SIZE:
Distance between flats: The distance between two opposite sides of a hexagonal or square head. This is standardized depending on the screw diameter. Example for a hexagonal screw: A 1/4” screw has a head measurement of 7/16”. Head height: The height of the head measured from the bottom to the top. Example: A 1/4” hexagonal screw has a head height of 3/16”.
DRIVE TYPE:
For example, with hexagonal screws, the drive is external (using a wrench). Other screws may have internal drive types, such as slotted, Phillips, Torx, etc.
MATERIAL GRADES AND MARKINGS:
The material grade of American inch screws is marked on the head in standard markings. You may encounter three types of markings that indicate different tensile strength classifications (and of course, this is reflected in the price):
• Grade 2: No markings.
• Grade 5: Three radial lines.
• Grade 8: Six radial lines.
We don’t aim to bore you with detailed engineering data, so simplifying the core of the classification: Grade 5 has a minimum tensile strength at least twice that of Grade 2, and Grade 8 has a minimum tensile strength two and a half times that of Grade 2.
These sizes ensure compatibility with nuts, washers, and other fasteners, guaranteeing the standardized applicability of the screws.
INTERPRETING INCH SCREW SIZE:
Here’s an example:
• Allen bolt 1/4”-20 x 1.5”, grade 8
• Allen bolt: hex socket screw
• 1/4” (0.25 inch): screw diameter
• 20: thread pitch (20 threads per inch)
• 1.5”: screw length (one and a half inches)
• Grade 8: classified in the highest grade with the greatest tensile strength.
You can find all the inch fasteners you need for your American V-Twin motorcycle at the bobber.center!
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