Hex cap screws dimensions are shown in the following chart according to ASME B18.2.1 standard. All dimensions are in inches.
Hex cap screw is a hexagonal head shape screw with a flat and washer faced bearing surface under the head. Heavy hex head cap screw is a screw type with a larger screw head and head bearing area. Hex cap screws are widely used in machine applications.
Some important notes from the ASME B18.2.1:
- The diameter range of hex head cap screws is from ¼ inch to 3 inches according to standard. For heavy hex cap screws, the range is from 3/8 inch to 6 inches in the standard.
- Screw threads, when rolled shall be Unified inch coarse, fine or 8 thread series with tolerance class of 2A (UNRC, UNRF, or UNR-8). Threads that produced with other production methods may be Unified inch coarse, fine, or 8 thread series (UNC, UNF, or UN-8.)
- Unless otherwise specified,chemical and mechanical properties of steel screws shall conform to ASTM A 449, ASTM A 354, or SAE J 429.Stainless steel screws shall conform to the requirements of ASTM F 593.Non ferrous screws shall conform to the requirements of ASTM F 468.
- Bearing surface shall be flat and washer faced
Note: 1 See the "Definitions" section for the drawing of the fillet.
Hex Cap Screw - Hex Bolt Differences:
- The bearing surface under the head of hex cap screw is flat and washer faced with thickness around 0.015 in to 0.035 in depends on the diameter of the screw. There is a tight run out tolerance for this face defined in ASME B18.2.1.
- Dimensional tolerance for body (or shank) diameter and head height is tighter in hex cap screws.
- There are different material options for hex bolt and hex cap screw: Hex bolts shall conform to ASTM A307, Grade A steel. For heavy hex bolts, it shall conform to ASTM A 307, Grade B. Hex cap screw shall conform to ASTM A307, ASTM A449, ASTM A354 GRADE BD or SAE J429.
- It’s permissible to have die seam on the body of hex bolt within the specified tolerance values.
As a summary, hex cap screws are generally used in more precise applications due to better dimensional properties. Hex bolts are used in construction works and generally intended to be used with a nuts.
Fillet: For the details of the fillet for long and short screws, visit ASME B18.2.1 standard.
Long Screw: Screw which is not threaded for full length.
Unified Screw Thread: A thread form used by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. It has 60° thread angle and dimensions are stated in inch units.
UN form screw threads: The UN thread is intended for general purpose fastening operations.
UNC (Unified National Coarse): Most commonly used type for general engineering applications. This thread form is used in materials with low tensile strength which makes threads more resistance against stripping (internal threads) .UNC give possibility for quick assembly.
UNF (Unified National Fine): External threads of this Fine Series have greater tensile stress area than comparable sizes of the Coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.
UNEF (Unified National Extra Fine): Used when finer pitches than UNF are needed (Ex: Short engagement length).
UNS (Unified National Special): UNS are special series. Bolt / screw selection shall be done from standard series unless there is a special requirement to use a special series.
Short Screw: A screw which is threaded for full length.
Thread class: A classification system to classify the threads for interchangeability and manufacturability. Class 1 threads are loosely fitting thread and used for ease of assembly. Most commonly used class is 2 and this class is designed for general use. Class 3 is used for closer tolerances.
Thread Gages and Gaging : Specifications and dimensions for the gages used on Unified inch screw thread is defined in the ANSI/ASME B1.2-1983 standard. Types of gages including thread plug gages, snap gages, thread ring gages and plain diameter gages for internal and external threads are covered in same standard. Screw thread gaging systems for acceptability is covered in the ASME B1.3-2007 standard.