This month our discussion turns to bolts and screws. The purpose of this article is to provide a cursory overview of the common nomenclature, grades and classes, identification of both SAE Inch and ISO / DIN Metric bolts and screws and a short overview of screw threads.
Generally speaking, we tend to use incorrect terminology when discussing fasteners. This can cause problems when we head down to Joe's Fastener Supply to purchase a particular fastener for some project that we are working on. Generally speaking, most supply houses serve the technical community and are versed in the "geek speak" of the industry. Often times we go to the counter of the fastener store with a certain idea of what we want in our mind. However, the counterperson makes several trips back and forth to the stock room to get that particular item that we are trying to describe. We'll try to make the trips back and forth to a minimum and allow you to get in and out of the store quicker, or where there are no counterpersons to help, the ability to search quickly for what you need. That in mind, here are a couple of definitions:
Screw: A fastener designed to clamp two components together by the fastener mating to a threaded hole or other preformed hole. The first component having a clearance hole through it for passage of the screw and the second component having a threaded hole in it to accept the screw and being tightened or released by torque applied to the head of the screw. A commonly misidentified item is a "Cylinder Head Bolt". Oddly enough in that example, the term "Head Bolt" is so commonly used that it has now become the vernacular, but the technical fact is that it is a Hex Head Cap Screw.
Bolt: A fastener designed to clamp two components together by the fastener passing through the components and mating with a corresponding nut to hold the assembly together. The entire set of components has a clearance hole through them to allow passage of the bolt. The bolt is then secured with a nut and torqued in place to retain the assembly. Either the bolt or the nut can be retained statically during assembly to apply clamping pressure to the assembly.
A diagram is provided to help determine typical bolt and screw configurations.
Grades and Classes / Identification
Grades and Classes of fasteners are one of the most misquoted and misunderstood areas of this type of hardware. We are including the term "Class" in this discussion because it pertains to ISO / DIN Metric hardware. "Class" is the equivalent term of "Grade" in the Metric world. For example, an SAE Grade 8 bolt has a minimum tensile strength of 150,000 psi (the point at which the material must withstand breaking) and a 130,000 psi minimum yield strength (the point at which the material must withstand permanent deformation.) Those combination's of factors along with material alloy composition and heat treatment compose the structural make up of a fastener. Consequently, an ISO / DIN Class 10.9 is roughly the equivalent of an SAE Grade 8. Below is a printable chart defining the identification techniques, basic physical makeup and tensile strengths of the different SAE Grades and ISO / DIN Classes:
Understanding the nomenclature of a thread is important in determining which type of fastener you are going to purchase. Determining factors are Major Diameter (either expressed in a number or fractional size) and threads per inch / MM. For brevity, we'll discuss only common helical "V" threads since it the most common thread form. Simply, a "V" thread is a form in the shape of a "V" that forms an angle of 60 degrees to the complimenting surface of the thread form. For a good visual aid in understanding this concept, go to your store of fasteners in your , pick out the largest bolt / screw you have and examine the profile of the thread. You will see that it does indeed have the basic shape of a "V", likely with a flat surface at the top (crest) and the bottom (root) of the thread form. You will also notice that it spirals (helix). These are the factors that make an external fastener thread, when mated to an internal thread, an effective means of fastening two objects together.
First, some definitions:
Pitch Diameter: The functional diameter of all of the thread form dynamics, including perfect pitch, lead and flank angles, and having a specified length of engagement.
As a guide, see the chart depicting common SAE Inch and ISO / DIN Metric Sizes.
Gauging Thread Size
In the manufacturing world, threads are precisely measured using a combination of expensive and sometimes not commonly available measuring tools. For the common home garage, I would suggest the purchase of a varying degree of nuts, screws and bolts, of as many different sizes as you think you may need. Next, buy a simple plastic container, such as a fishing lure case and marking the applicable compartment with the thread size of the fastener. Using these bolts and screws for only this purpose is an inexpensive way for the "Garage Shop Guy" to have a readily available set of standards on hand. Following are some helpful charts for determining common sizes and the clearances required for tapping and drilling:
|Number Size Drill Chart||Fractional Size Drill Chart|
When we get to the forthcoming article regarding applications, we will delve more into why these different types, shapes, configurations and grades of fasteners are important and where they should and should not be used on our automotive based projects. Keep your bookmarks tuned to this section!
By: Mike Nevitt - February 2004
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