Use of Fasteners

Use of fasteners

July 21, 2025
By Bob Lund | Engineering Manager

This article continues our ongoing series focused on fastener performance and engineering best practices. After discussing torque fundamentals in a previous installment—including the importance of clamp load and the role of lubricity and “K” factors—this piece addresses several follow-up questions received from engineers and quality professionals across the industry.

Torque in multi-bolt applications like flanges

In multi-bolt assemblies such as flanges, you need to look at the pattern of assembly and the material of both the joint as well as the bolting materials. The most common multi-bolt patterns are circular flange applications. Rather than tightening in a circular pattern taking bolts directly to the suggested torque, a “star” pattern, moving across the flange rather than around it and tightening incrementally, is recommended. This approach helps prevent joint relaxation. A typical tightening sequence might include:

First pass: 40% of final torque value
Second pass: 70% of final torque value
Final pass: 100% of final torque value

Re-tightening after 24–48 hours can further improve joint integrity by compensating for embedment and relaxation effects.

Removal torque vs. installation torque

A common question is whether the torque required to loosen a fastener matches the original installation torque. In practice, removal torque is usually lower due to factors like lubrication, corrosion, and other factors. Because of this variability, removal torque is not a reliable method for verifying installation torque. If you’re looking for a way to verify installation torque, reach out to the engineers at Fastenal. We have some better options for you.

Reuse of fasteners and torque consistency

Reusing fasteners can significantly affect torque performance. In training sessions, we’ve demonstrated how re-tightening a fastener—even immediately after initial installation—requires more torque to achieve the same clamp load. For example, in a recent session we used Grade 5 bolts (3/8” diameter) with hardened washers and Grade 5 nuts, all brand new and straight out of the package. The initial torque to reach a 5,000 lb clamp load was 25–30 ft-lbs (it is normal to see at least 20% variation in torque to get the same clamp load on a bolt). On loosening the bolt and re-tightening, torque for every assembly increased by 5 and often 10 ft-lbs to get the same clap load, and a third cycle required even more.

This variation can lead to under-tightening during field service, resulting in fatigue failures. For critical applications, fastener reuse is not recommended. Using lubricants during both installation and re-tightening can help maintain consistent clamp loads and reduce the risk of failure.

How is torque different with stainless steel

Stainless steel fasteners present unique challenges. While torque formulas apply broadly, stainless fasteners often deviate due to their tendency to gall or seize before reaching full clamp load. Fastenal’s torque recommendations for stainless fasteners are based on a combination of testing and calculation and are values you should be able to achieve without galling. Lubrication or coating can significantly improve performance, allowing for higher clamp loads, less galling, and more formula-based torque values.

Determining torque without matching nut and bolt strength

In applications involving tapped holes (e.g., castings, extrusions) or pilot holes (sheet metal screws), determining torque without a matching nut requires testing. A practical method may involve tightening a dozen or so fasteners until failure. If the tapped hole acting like a nut is stronger than the bolt, the bolt will break. If the bolt is the stronger of the two, the threads of the hole will strip. Record the torque values at which failure occurred and find an average. You should be safe using an installation torque that is 60-70% of that average.

If destructive testing isn’t feasible, torque can be estimated through stripping strength analysis—a topic we’ll explore in a future article.

Engineering support and training

Fastenal’s engineering team is here to support your fastener-related needs—from torque guidance and material selection to training and troubleshooting. We offer in-person sessions, online resources, and direct consultation to help you make informed decisions and avoid costly mistakes.

For more information, visit Fastenal Engineering & Design Support or contact our engineering team at [email protected].

Fastenal Company disclaims all warranties, both express and implied, relating to the information, reports, opinions and analysis disclosed to the Customer by Fastenal Company. Fastenal Company shall not be liable for any errors or omissions or any losses, injuries or damages arising from the use of such information, reports, opinions and analysis by the Customer.