Check bolts for grades and manufacturer.
Fasteners generally fail for three major reasons. First, counterfeit or substandard fasteners may be used. Second, genuine parts may be used improperly. And third, improper storage, handling, or re-use of fasteners can affect job quality and safety.
Before examining what constitutes a good or bad situation, let's see how fasteners work.
The purpose of a nut and bolt (or stud or cap screw) is to hold two or more objects together by exerting a clamping force. Tightening creates tension inside the fastener, pulling the head and nut together to clamp what is beneath it.
When torque is applied to a nut or bolt head, the threads act like wedges pulling on the bolt. The bolt stretches a tiny bit. Elastic forces inside the bolt cause it to spring back to its original shape. The more it is stretched, the stronger the clamping force that holds the parts together, but only if elastic limits have not been reached. Otherwise, a permanent deformation destroys the ability to clamp. A good-quality, 3/8-in diameter Grade 8 bolt can exert more than 12,000 lbs of clamp force.
The bolt is only part of a system that also includes the nut and any washers. The nut applies the stretch to the bolt. Washers protect the work and keep the system from backing off and relaxing tension. If any of these are mismatched, they will not function as designed, and the assembly may fail.
- For automotive work, I suggest domestic Grade 5 or Grade 8 bolts. You can use Grade 8 in many applications calling for Grade 5, but not in extremely hot areas such as engines, exhaust systems, and brakes. Never use Grade 5 where Grade 8 is specified, and never mix grades on one work piece or part.
- Many shade-tree mechanics operate on the philosophy that if some is good, more is better. Not so with fasteners. Over-tightening can stretch fasteners beyond their elastic limits, resulting in permanent deformation and loss of clamping strength. This is especially true with wheel fasteners.
- Handle air impact wrenches and torque wrenches carefully, and properly maintain and recalibrate them regularly. Fasteners can cause wheel studs to be permanently stretched if out of adjustment, or if applied for just one or two more seconds after starting to slip “just to make sure.” This causes the wheel nut to loosen. Exhaust clamps can be distorted and leak fumes. Manifolds can crack.
- If repair manuals call for oil, use just one drop. Do not overlubricate, or clamping force will be too great. Manuals give torque values for oiled threads. Whenever possible, apply torque to the nut, not the bolt head. If you must torque the bolt head while holding the nut stationary, increase recommended torque 20%. When using lubricated nuts, lower dry torque recommendations as follows: with anti-seize compound, reduce torque 40%;
- with heavy oil or graphite, reduce torque 30%;
- with synthetic white grease or white lead, reduce torque 25%;
- with Loctite compounds, reduce torque 25%;
- with light oil, reduce torque 20% to 25%.
- We've all kept used fasteners that still looked good. But using them in any high-strength application is not wise. Besides not knowing where and how the fastener was previously applied, jostling around in a moving vehicle may damage threads, requiring more torque to reach full clamp strength. Used fasteners have less clamping force.
At a recent TMC meeting, one member reported a new ½-in diameter nut and bolt generated more than 13,000 lbs of clamping force when torqued to 170 ft-lbs. The second time, force dropped to 12,500 lbs. Subsequent tightening resulted in 11,500 lbs, 10,700 lbs, and 9200 lbs. And after six times they got only 7500 lbs, less than 60% of the original clamping force, all at the same torque.
- Store fasteners so threads will not be damaged. Follow instructions about torque and lubrication. Do not apply either excessively. Don't reuse fasteners.