The nut-and-bolt combination has two main functions: to provide a clamping force, and the ability to remove that force when required. The clamping force is generated through tension in the bolt, which occurs as the bolt stretches during the tightening process. Removing the force requires breaking the friction between the threads and allowing the nut to unscrew.
The new hardware was installed using removable threadlocker on all the bolts. Notice that the edge of the flat washer has been ground for clearance around a weld. If the flat washer does not seat correctly, the torque tension relationship will be askew. Threadlockers lubricate the fastener and reduce friction. Wet torque values should be used.
The amount of available clamping force is determined by the grade of fastener and how far it will stretch before losing elasticity. Generally speaking, Grade 5 is adequate for automotive applications. Sometimes, Grade 8 is used. Grade 2, typical of hardware-store bolt bins, is questionable for structural assemblies. Matching the material and size of the flat washer to the nut and bolt is very important. The key to a reliable bolted joint is generating adequate clamping force and then locking the nut in place to maintain tension in the bolt.
The chart on page 66 displays the SAE torque values of the most popular grades and sizes, which should help you decide what you need for any application. There are differences in torque values for bolts based on materials (high-carbon, copper, zinc, and so on) and construction (heat-treated, quench-tempered, case-hardened, and so on), so use this chart only as a ballpark estimate. Remember, too, that stronger is not always better, as some bolts are intentionally designed to fail before extreme torsional stresses can be transferred to other (more expensive) components.
Welding, bonding, or use of a rivet is a way of unitizing parts and preventing unwanted disassembly, without the benefit of easy removal. A liquid threadlocker unitizes the nut-and-bolt assembly by filling the air space between the threads with a chemical resistant plastic.
Vibration and thermal cycling can cause a threaded fastener to come loose with sometimes devastating results. There are many mechanical devices designed to keep a nut or bolt from coming apart. Some work, some don't. The split-ring lock washer is not very effective. Nylon insert nuts have been known to loosen just enough to reduce effective clamp force. The easiest way to ensure the nut stays put is through the use of a liquid chemical threadlocker such as Loctite.
The benefits of using a liquid threadlocker include preventing rust and corrosion and easy removal in the future through the use of force or heat. We've all been there before, fighting with that rusted or heat-seized nut or stud that refuses to come loose. Along with busted knuckles, the scene generally degrades to Vise-Grips, drilling, cutting, or torching. Threadlocker will minimize the aggravation factor.
A hardened flat washer was used to fill the gap between the engine mount and frame mount. The threadlocker will effectively hold the nut in place until you purposely break it free. The majority of the time, removable (blue) threadlocker is adequate. In severe vibration circumstances, permanent (red) threadlocker can be used. To disassemble the red, you need to heat the parts to 500 degrees for five minutes. | 
Here is the transmission-to-transfer-case adapter. The adapter is bolted to the transmission with hardened socket-head cap screws with a black oxide finish. You can't torque the cap screws too much, or you risk damaging the 30-year-old aluminum TH400 transmission case. Removable threadlocker from a stick was applied to the cap screw prior to assembly, and the bolt holes in the transmission were primed with 7649 since both the bolts and transmission case are relatively inactive surfaces. The cap screws were adequately snugged, and the threadlocker will ensure they won't back out. |
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