And so we continue our discussion on threaded fasteners that we started last month. Without further adieu, let's get right down to the nuts and bolts of it all.
How Fine Is Coarse?
Essentially, a thread is an inclined plane cut along the surface of a fastener. Varying the angle of the plane determines the cut or thread-increasing the angle produces a coarse thread, while decreasing it results in a fine thread.
Fasteners are graded according to how many threads there are to the inch for SAE fasteners, or centimeters for metric ones. Most threaded fasteners are available with either coarse threads conforming to Unified National Coarse (UNC) standards or Unified National Fine (UNF) threads. Fine threads have more threads per inch than coarse.
The long and short of it is this: Coarse threads are easier and faster to use. They provide an easier "start" of the fastener, with less likelihood of cross-threading. Nicks and burrs from handling are less likely to affect assembly, and they are less likely to seize in temperature applications and in joints where corrosion is likely. Additionally, coarse threads are less likely to "strip" and are more easily tapped into brittle materials.
On the other hand, fine threads provide superior fastening (typically 10-percent stronger holding power than coarse) in hard materials, and they can be adjusted more precisely due to their shallower helix angle. They are also better in situations where length of engagement (depth) is limited and where wall thickness is limited, again because of their smaller thread cross-section (coarse threads are cut deeper into the shaft of the fastener than fine.)
While all of this is very interesting, it isn't worth a hill of beans if the fastener isn't properly installed, and that leads us to a different twist on the subject, if you will.
How Tight Is Tight?
All fasteners have to be correctly tightened in order to perform the job for which they are intended, and that degree of tightness is referred to as the torque specification. Let's say, for instance, that two steel plates need to be tightened together with a force of 100 pounds. Let's also say that a single 3/4-inch-diameter bolt is used to create the binding force. Once tightened, the two plates are held together just as if a 100-pound weight were sitting on top of them.
However, if the bolt was loose, external loading, vibration, and temperature change would eventually cause the plates to come apart because they would fatigue (the simplest form of fatigue is that of metal being bent back and forth, but in this case the bolt would be elongated enough for the joint to loosen). The strength of any joint is dependent upon the two factors: the strength of the fastener itself, and the degree to which it is tightened. Tightness can be accurately controlled by the measurement of the torque (twisting force, measured in pound-feet) to which it is tightened. Torque applied to a fastener creates inner tension (stretching) that, in turn, creates the holding power desired.
Torque wrenches perform this task. The strength of the fastener is determined by the raised grade markings on the head of the bolt or screw. These head markings were developed by SAE International (formerly the Society of Automotive Engineers) for automotive applications, and ASTM International (formerly the American Society of Testing and Materials) for structural applications.
A one-pound weight or force applied to a lever arm one-foot long is equal to one 1 lb-ft, or 12-inch pounds, of torque.
The way to read the marking system is to add two to the number of marks. No marks on the head indicate a grade 1 or 2, three marks indicate a grade 5, four marks a grade 6, and six marks a grade 8. The first thing to know about grade markings is that no markings are considered to mean the fastener is made of mild steel. Conversely, the more marks on the head, the higher the quality and strength. Therefore, bolts of the same diameter will vary in strength depending upon the material they are made of and the number of threads per inch.
Hex Head Bolt Markings
The strength and type of steel used in a bolt should be indicated by a raised mark on the head of the bolt. The type of mark depends on the standard to which the bolt was manufactured. Most often, bolts used in machinery are made to SAE standard J429, and bolts used in structures are made to various ASTM standards. The accompanying tables give the head markings and some of the most commonly needed information concerning the bolts. For further information, see the appropriate standard.
There are often "extra" marks on a bolt head-marks in addition to those shown in the charts-and these marks usually indicate the bolt's manufacturer.
Please note that ASTM A325 Type 2 bolts have been discontinued, but are included in the chart because they can be found in existing structures. Their properties can be important in failure investigations. While this isn't germane to automotive purposes, I decided to err on the side of completeness.
While the bolts shown are among the most common in the U.S., this list is far from complete. In addition to the other bolts covered by the SAE and ASTM standards, there are a host of international standards, of which ISO is perhaps the most well known.
A more detailed explanation of bolt torque specifications and additional related information on these and other automotive fasteners is covered in depth in my book, Garage and Workshop Gear Guide, published by Motorbooks and available from your favorite bookseller.
So, hopefully, you now know a whole lot more about threaded fasteners and what those markings on their heads mean.
Log onto boltdepot.com and click on the Printable Fastener Tools link for an assortment of useful charts, tools, measuring devices, thread sizes, and other handy items you can print out for free. These are great to have on hand for reference in your garage.