When Can Metric Fasteners Be Reused?
When looking for ways to reduce waste, cost-conscious and environmentally aware manufacturers and consumers may consider reusing metric bolts and screws. But not all bolts and screws can be safely and effectively reinstalled after being removed from an assembly or component. In fact, an unequivocal “yes” is never the answer to the question of whether a fastener can be reused. Depending on the situation, the answer is always either “no” or “it depends.”
High-strength bolts that have been pre-tensioned into their proof load range – the point at which they permanently deform and do not “snap back” when relieved from their loads – should not be reused. This is the case in many applications, as structural and other bolts used in the manufacturing, construction, and automotive industries are commonly tightened to a torque value that creates a sufficient clamp force to prevent joint failure.
However, in all but the most extreme cases, fasteners that have reached this yield point cannot be identified just by looking at them.
To be safe, ASTM International takes the guess work out of when to use new structural A490M and galvanized A325M metric bolts in each installation. The standards agency notes that these bolts “shall not be reused.” Period. These bolts are not “elastic” enough to return to their pre-use shape and strength after they have undergone a pretensioned installation.
Mr. Metric also advises against reusing any torque-to-yield fasteners. These bolts are designed to be preloaded past this state of plastic deformation.
In other situations, it is best to follow manufacturers’ guidelines when deciding if a bolt is suitable for reuse. Many automotive original equipment manufacturers, for instance, recommend new bolts for any application that attaches steel to a different metal, such as aluminum. These fasteners often employ anti-corrosive coatings that wear off during installation and disassembly.
Other metric bolts that have been pretensioned may or may not have been stressed beyond the limits of reusability. Black ASTM A325M bolts, for instance, are ductile enough to be used several times before failing.
As a first test, simply thread a nut onto the bolt in question. A bolt that can accommodate the nut throughout the entire length of its threads is a candidate for reuse. The ability to screw the nut the whole way onto the bolt indicates – but does not prove – the bolt has not been stretched past its yield point.
To be sure, an engineer must inspect the fastener to ensure it has not suffered any defects to its structural integrity. In determining whether the used bolt can be safely and effectively reinstalled, the engineer will consider several factors:
- The bolt’s grade and material
- Previous installation method
- Internal loads
- External loads and forces such as tension, vibration, shearing, etc.
- Use conditions, such as heat, causticity, and acidity
If you determine that a fastener CAN be reused, the final step is to decide whether it SHOULD be used again in a particular circumstance. Even if you believe fastener failure is only a remote possibility, it may not be worth the risk. Obviously, you should not gamble on repurposed bolts to bear the weight of a bridge. And you wouldn’t use questionable $2 fasteners to secure the parts of a $250,000 metal press. When in doubt, throw it out! Depending on the material, large quantities of unusable fasteners can also be scraped to make back some of the upfront cost.
When the application, potential risk, and financial savings makes reusing bolts economically sound, be aware that the reinstallation almost certainly will require more torque than was needed on the original use in order to achieve the same clamp force.
Mr. Metric can answer all your questions about the proper selection and use of metric fasteners. Call our customer service experts at 1-866-501-9504 for guidance on our vast selection of screws, bolts, and the tools to install them the right way.