Part of the reason for the marks left when tightening the screw is wear on the screw sleeve. After wear, the contact between the sleeve and the screw changes from a surface connection to a point connection, or the point contact is reduced by half. Screws/nuts leave marks after point stress, and more severe marks when screws are tightened with a solid wrench.

The screw is only stressed on both sides, and the deviation of the solid wrench is large, so it is severely worn during use, and it will leave more obvious marks after tightening. Whether there are marks left on the screws is related to the assembly tool. The high precision sleeve is in tighter contact with the nut. A good quality socket fits over a standard nut and you will find that there is no excessive clearance between the socket and the nut.

At this time, the force of the nut is uniform, and the contact area between the nut and the sleeve is large, so the wear marks are naturally small. Poor force control during tightening. When the screw is subjected to excessive torque, it is not only easy to slide and break the wire, but also the marks left on the surface of the nut will be more obvious.

Therefore, when the screw is twisted with a high precision socket tool and standard torque, the marks on the screw surface become slightly undetectable. Of course, the torque setting is related to the screw parameters. The higher the hardness, the higher the strength, and the surface is not easy to leave distortion marks.

Seeing this, we can guess why there are no traces of screws on the production line! First, we will test the proper torque based on the screws in the different parts. Second, the torque wrench should be set. During the tightening process, there are requirements for speed and angle. When the precision of the tool is high enough, the standard tightening will not leave obvious marks.

At present, most of the screws used by automobile companies are standard parts that meet national standards. The stiffness and strength of these parts meet national standards. No visible marks are left as long as the corresponding torque is not exceeded during operation. Now, after a long period of development in the automobile manufacturing industry, the assembly process has been standardized. According to the experience of employees, there is basically no screwing.

Specifications are set at the beginning of the design, such as where to use what bolts and how much torque to apply, and then communicated to employees through assembly process instructions such as work instructions, and employees need to pass relevant knowledge exams before they can work. Relevant tools will also set parameters according to specific assembly requirements, so this will not happen. In addition, all vehicles will be subject to special quality inspections before leaving the factory, and the supervision of products will be more stringent, especially the inspection of appearance parts.

If something goes wrong, they won't make it to market, so it's not a mystery that OEMs screw up cars without a trace. The key is good tools, good user skills, and brand new standard parts are easy to install; I used to use electric wrenches and air wrenches when I was at the factory. Among them, the Yokota fixed torque has the longest use time, and there is no trace when tightening.