What to Do If the Screws on a Car's Body Rust or Become Damaged and Cannot Be Opened

In automotive repair work, the disassembly and assembly of various types of screws are common tasks, but they can be physically demanding and mentally exhausting. Screws on a car are often exposed to road grime and moisture over time, leading to rust and corrosion. Regular driving can also cause components to collide, shifting the screws out of place. Additionally, in the event of a traffic accident, screws may become deformed due to impact. In such cases, conventional methods of loosening screws often fail to apply sufficient force, and forced disassembly can easily result in broken bolts or damaged nuts, bringing subsequent repair work to a halt. Below, we will share knowledge on the use of tap and die sets in automotive repair.

From forceful removal to repairing the screw hole, we typically avoid simply increasing force. Instead, based on the extent of damage to the screw hole (such as rust, complete fusion, or grime adhesion), we adopt the following approaches:

1. Initial Conservative Repair
   Before starting the disassembly process, first apply a high-quality penetrating lubricant and allow sufficient time for it to seep in and soften the rust. Then, wipe the area clean with a cloth and use tools like a screwdriver to gently tap and loosen the bond between the screw and the component before using specialized tools to unscrew it. Another handy trick is to insert nylon thread or a thin cloth strip into the hole that needs repair before using a screwdriver to unscrew it. If this fails to provide sufficient grip, abandon the method and switch to a more formal approach.

2. If Conservative Repair Fails: Professional Repair with Taps and Dies
   A. Repairing Damaged Nuts or Internal Threads (Thread Restoration with Taps)
   When welded nuts on the car body or threaded holes become damaged due to the reasons mentioned above, making it impossible to insert screws or other fastening tools, taps are used for repair.
   A tap is a standard tool for cutting internal threads. During operation, select a tap that matches the original thread specifications (e.g., M6×1.0, M10×1.25). Insert it into a tap wrench and carefully position it in the damaged screw hole. Turn it clockwise half to one full turn, then reverse it a quarter turn counterclockwise to remove debris and prevent clogging (sedimentation) in the repair hole. For severely damaged threaded holes, consider using the "enlarge and rethread" technique, where a larger tap is used to cut new threads, and correspondingly larger screws are used.

B. Repairing Damaged Bolts or External Threads (Thread Restoration with Dies)
When the external threads on a bolt shaft are damaged—such as in special cases where replacement is not possible (e.g., exhaust hanger bolts, engine mount studs)—a die is used for repair.
A die acts as a cutting tool in the shape of a nut. Place the die over the damaged bolt and rotate it slowly, following the "turn three times, reverse once" rule. Through the use of the die, residual rust and grime on the bolt can be removed, and threads can be restored to nearly their original condition. This technique helps preserve the original components as much as possible.

3. Emergency Measures for Special Situations
   Removing Broken Screws: If a screw has broken off inside a hole, a screw extractor (reverse taper) is needed. Drill a pilot hole in the center of the broken screw's cross-section, insert the reverse taper counterclockwise, and use reverse torque to extract the broken screw.
   Heating Method: Use an oxyacetylene torch or heat gun to locally heat the nut, causing it to expand. The difference in thermal expansion coefficients can break the rust bond between the threads.

Taps and dies are precision cutting tools, and their material and accuracy determine the success rate of the repair. It is recommended to use tools made of high-speed steel (HSS) or cobalt high-speed steel (HSS-E), as these materials offer good hardness and wear resistance, making them less prone to chipping. Even for infrequent repairs, high-quality steel is sufficient. Finally, if conventional methods fail to repair the area, consider drilling a new hole for fastening in a location that does not affect the normal connection and use of adjacent components. If the work is physically demanding, tools like creeper with wheels and an endoscope can be used to facilitate the repair.

A professional set of thread repair tools for automotive work is an essential instrument for skilled automotive technicians and 4S shops, showcasing their expertise and capability.