Originally, this was considered the most basic mechanical common sense, something even a vocational school teacher would dismiss. But in reality, bolt loosening is one of the most frequent repair issues in machinery. If ignored, it often leads to loss of fastening strength in machine parts, and in severe cases, failure of the related components.

An Example from the Past

Many years ago, there was a major trunk asphalt road in Northeast China—the Harbin–Dalian line. It was rebuilt and paved with asphalt (tar at the time) by the Japanese on the original base. Although its load-bearing capacity was not as good as today’s asphalt roads, it at least solved the problem of all-weather passage. At that time, even provincial highways weren’t paved with asphalt! Until I left the countryside, even G101 was still a gravel road.

When I was a teenager, during the rainy season in the countryside, farmers who wanted to transport crops to town had to add three or four more horses to the usual three-horse cart, sometimes six or eight in total, just to move through the muddy loess roads. The carts moved a little, stopped a little, often slipping into roadside ditches. Only then did I realize why Japanese roads in the occupied zones could not support all-weather automobile traffic.

On that “first asphalt road in Northeast China,” every summer under the blazing sun, I noticed countless tiny bright reflections on the pavement. Upon closer inspection, they were bolts, nuts, washers, and cotter pins that had fallen from passing vehicles, pressed into the asphalt, then polished shiny by car and cart wheels. This showed how common bolt loosening and loss were in those days!

Why Bolts Often Loosened in the Past

Back then, tightening bolts was something repairmen and drivers constantly had to do. Roads were rough, causing heavy vibration. In addition, bolt quality was poor, with little attention to strength grade. As long as the bolt could be screwed in, it was used. Consequently, bolt loss was serious.

Today, everyone knows that high-strength bolts must be used in critical locations. But due to local conditions, the quality of high-strength bolts varies greatly. Many bolts only meet strength requirements but not precision standards, leaving users confused. Even after switching to high-strength bolts, loosening still occurs.

Based on decades of experience, I will share some solutions for mechanics and enthusiasts, hoping to reduce this problem.

Key Points for Dealing with Bolt Loosening

1. Choosing Qualified High-Strength Bolts

   • Hardness: Should meet requirements. If you don’t have a hardness tester, use a simple comparison method: saw or file the old failed bolt and your new one. The one that resists filing more is harder.

   • Head Design: A proper high-strength bolt should have a stepped surface under the head (replacing a flat washer) with enough surface finish. If a “high-strength” bolt lacks this, don’t buy it.

   • Thread Precision: Use a high-precision nut to test. If the fit is loose with noticeable play, it’s substandard.

2. Do Not Reuse High-Strength Bolts Indefinitely

   • Many modern engines (e.g., cylinder head bolts, main bearing cap bolts, flywheel bolts) use torque + yield tightening methods and specify a limited number of uses. Beyond that, bolts must be replaced to avoid breakage.

3. Don’t Confuse High-Strength and High-Precision Bolts

   • A high-precision bolt must also be high-strength, but not all high-strength bolts are high-precision.

4. Lubrication Before Assembly

   • Unless otherwise specified, apply oil to the thread and mating surfaces before installation.

5. Check Mating Surfaces

   • For through-bolts with nuts (e.g., drive shaft flanges), if loosening persists, check whether the flange or contact surface is worn or burred. If yes, repair or replace the part. Simply using stronger bolts won’t solve it.

6. Repairing Internal Threads

   • If a bolt screws into a threaded part and loosens due to thread damage, just replacing the bolt won’t work. Use thread inserts (Heli-Coil or sleeve method). Do not attempt to repair connecting rod big-end threads—replace the rod.

7. Anaerobic Adhesives (Thread Lockers)

   • They can work for small-diameter, low-vibration nonferrous metal fasteners, but are ineffective for large steel parts with strong vibration.

8. Welding Loose Bolts Is Not a Solution

   • Welding often cannot restore function, especially between dissimilar metals (cast iron and steel).

9. Thread Inserts Are Effective

   • Originally used in aviation, thread inserts are now standardized. They can restore strength beyond original levels, especially useful in engine blocks, hydraulic housings, and structural parts.

10. Installation Precautions

    • Do not arbitrarily add spring washers or ordinary flat washers.

    • Do not increase torque beyond specifications, or you risk damaging threads or mating parts.

    • Use proper torque wrenches, not adjustable wrenches.

    • If a torque-angle tightening method is specified, always follow it.

Final Thoughts

Because this issue is considered “too basic,” it is rarely covered in technical manuals. But in real-world maintenance, bolt loosening is a frequent and serious problem. Technical discussion never ends—indeed, we learn as long as we live.