With so many demands on today’s technician, the fundamentals of bolts and tightening can get neglected. Everyone knows about torque and specifications, but this is only the beginning. They scatter information and sometimes finding it is difficult. This guide may answer some basic questions about bolts and proper tightening techniques.
Is it a bolt or a screw?
Experts disagree on an exact definition of a screw and a bolt and the terms are almost interchangeable today. The most widely accepted classification is, a bolt uses a separate nut to attach, while a screw, threads into the part it retains. By this definition, many fasteners, such as head bolts, are a screw. Most fasteners in automotive use fall into this clarification. A hex-headed cap-screw is the common name. A stud is a sort of hybrid, as it treads into a part and has threads for a nut as well.
How tight should a bolt be?
When threaded automotive-fasteners come loose, a problem will occur. In simple cases this may be a rattle. In more critical areas an engine or transmission failure may result. Suspension and brake failure may be far worse. An exact tension, exceeding the maximum load, provides a joint that stays tight. Engineers use complex formulas to decide the clamp force necessary.
They design automotive fasteners for clamp loads between 65% and 90% of their yield strength. If too little tension is present, vibration and force will loosen the fastener. Excess tension may also allow the fastener to loosen or fail.
Torque wrenches tighten fasteners but for many reasons, are less than ideal. Fortunately, most automotive bolts are small in diameter and short. This makes torque an acceptable method usually.
Torque is different from tightness
Torque is a rotational force, while tightness is tension or pressure exerted on the parts clamped by the screw. Threaded fasteners are very powerful springs. Tightening the threads cause the fastener to stretch. This stretching provides the tension that holds things together. A torque wrench measures the rotational resistance of the screw, not how tight it is.
Torque is expressed in foot-pounds in the American system and newton-meters in the metric system. Applying torque causes the screw to rotate. The greater the rotation, the greater the tension. Unaccounted for is friction between the fastener and the object into which we thread it. This changes the outcome and is a flaw in using torque to measure clamping force.
Identical fasteners with the same torque, may provide far different tension.
Many factors affect the tension that results from torque. Understanding these factors helps achieve the proper tightness. Because torque is a resistance to rotation, lubrication drastically changes the equation. Most torque-specifications are for un-lubricated assembly, unless otherwise stated. A lubricated bolt may exceed the yield point when we apply dry torque.
Installing a washer or leaving a washer out can also change torque requirements. A hardened washer may act as a bearing, allowing a screw to rotate far more easily. Leaving a washer out can result in a fastener that is too loose. The same may happen when substituting a serrated nut or bolt for a standard type. Proper tension requires an exact-replacement fastener, and the safest bet is an OEM part.
Other factors that affect bolt tightness
Bolts come in many different hardness and ductility ratings. Engineers use each rating because of characteristics making it suitable for the application. Understanding the factors that influence the ratings is not necessary, but using the specified fastener is imperative.
Federal law, 101-592, specifies a standard rating system. A pattern, stamped into the head, shows the rating of an automotive bolt. The Society of Automotive Engineers or SAE assigns ratings to American course and fine thread screws. The International Standards Organization or ISO gives metric fasteners equivalent ratings.
Because some is good, does not mean more is better. A harder bolt is also more brittle and may break if required to flex too much. A grade eight bolt is not a superior substitute for a grade five. Bolt use is based on the application. Equal torque may produce different results, on bolts with a different grade. An example is substituting grade-eight bolts, for the grade-two, in an oil pan. The harder bolt will stretch less and provide too little tension, at the specified torque. Vibration could cause them to loosen, as they lack the proper tension. Tightening the grade-eight bolt for proper stretch would overload and destroy the gasket.
Bolts and nuts should also be of the same grade. Using a grade 8.8 nut on a 10.9 bolt, lowers the yield-point of the fastener to grade 8.8, and is dangerous. Manufacturers sometimes use a nut, harder than the stud, such as on wheel lugs. The wheel nut is harder than the stud. This is for safety. If over tightened, the stud will break, which is obvious. Stripped lug nuts are less apparent and could come off in operation.
More precise tightening methods and special fasteners are the subject of the next Detailed Topic.