How Lockthread Technology Prevents Fastener Loosening in Extreme Heat

In high-heat environments, fastener loosening is often a predictable outcome when the wrong fastener type is specified. Exhaust manifolds, turbocharger flanges, and engine mounts cycle through extreme temperature swings every time a vehicle runs.

Each cycle expands and contracts the metal surrounding a fastener joint, and with each cycle, a conventional fastener can lose a fraction of its clamp load. Over thousands of cycles, that loss becomes a real problem.

This article explains how locking fasteners work mechanically, why chemical thread lockers fall short in high-temperature service, and how MacLean-Fogg's Lockthread system addresses the problem with a purely mechanical approach that holds up through repeated thermal cycling.

Key Takeaways

• In high-temperature exhaust and turbocharger environments, conventional fasteners lose clamp load incrementally with each heat cycle. Over thousands of cycles, that cumulative loss becomes a measurable failure risk.
• Chemical thread lockers are not a reliable fix. Standard anaerobic adhesives degrade above roughly 150°C, and even high-temperature formulations fail well below the 600°C+ temperatures common at exhaust fastener locations.
• Mechanical locking through thread geometry is the correct solution for thermal applications. Prevailing torque resistance is built into the thread form itself, so it does not degrade with heat the way adhesive-based lockers do.
• MacLean-Fogg's SALT (Self-Aligning Lockthread) system combines a locking thread form with MAThread geometry to address two failure modes simultaneously: loosening under vibration and fatigue from thermal misalignment loads.
• SALT fasteners are available in A286 for exhaust manifold and turbocharger flange applications up to approximately 650°C, and Inconel 718 for extreme-duty applications up to approximately 980°C.

The Mechanics Behind Locking Fasteners

A fastener holds a joint together by stretching slightly when torqued. That elastic stretch creates a clamping force. The joint stays tight as long as the bolt remains in tension. The threat in any joint is any force, thermal or mechanical, that reduces that tension.

Vibration is one threat. Under sustained vibration, a standard fastener can rotate backward incrementally. Self-locking fasteners resist this by creating a prevailing torque, a resisting force that must be overcome before the fastener will turn in either direction. That torque is not applied by a tool. It is built into the thread geometry itself.

The most common mechanical method is thread deformation. The thread form of a prevailing torque lock nut or self-locking bolt is manufactured with a deliberate interference condition. When the fastener is installed, the male and female threads deflect against each other, producing friction that resists loosening. No adhesive. No secondary locking element. The threads themselves are the lock.

Anti-vibration fasteners built on this principle can perform consistently across a wide range of temperatures because the friction mechanism does not depend on a chemical compound that can degrade.

Why Chemical Thread Lockers Fall Short at High Temperatures

Liquid thread lockers such as anaerobic adhesives fill the space between thread flanks and cure to a hard film that prevents relative motion. They work well at ambient temperatures and in applications where the joint sees only modest thermal variation.

The problem is heat. Most standard-grade anaerobic adhesives lose significant holding strength above roughly 150°C. High-temperature formulations push that threshold to around 232°C, but exhaust system temperatures routinely exceed 600°C at the fastener. Turbocharger housing fasteners can see even higher localized temperatures. At those temperatures, any adhesive-based locker has carbonized and is no longer functional.

Once the adhesive breaks down, the fastener behaves as though no locker was ever applied. Vibration and thermal cycling resume their work, and the fastener loosens.

Prevailing torque lock nuts and self-locking bolts avoid this failure mode entirely because their resistance is mechanical, not chemical.

Specify the right locking fastener for your application. MacLean-Fogg's engineering team can help you select the correct specification for exhaust, turbocharger, and high-heat structural joints. Contact us to discuss your requirements.

What Is Lockthread?

Lockthread is MacLean-Fogg's mechanical thread-locking technology. Rather than relying on a chemical compound or a separate locking element, Lockthread builds the locking function directly into the thread form of the fastener.

The thread is manufactured with a deliberate interference condition. When the fastener is installed, the male and female threads deflect against each other, producing a prevailing torque - a resisting force that must be overcome before the fastener will turn in either direction.

That resistance is present at installation and remains present throughout the service life of the joint. Because the mechanism is geometric rather than chemical, it does not degrade with heat, and the fastener can be removed and reinstalled up to its recommended reuse limit without losing its locking characteristic.

SALT (Self-Aligning Lockthread) is a variant of Lockthread. It uses the same locking thread form as standard Lockthread and delivers the same prevailing-torque resistance to loosening. What SALT adds is one thing: MAThread geometry, which gives the fastener an anti-cross-threading, self-aligning characteristic.

MAThread modifies the lead thread so the fastener finds true alignment as it starts, rather than catching and cross-threading on a misaligned or hard-to-reach joint. In assembly environments where fasteners are installed at awkward angles, driven by automated tooling, or seated against surfaces that are not perfectly perpendicular, this feature prevents the cross-threading and off-axis seating that can damage threads and introduce bending stress.

What Is the SALT System?

SALT stands for Self-Aligning Lockthread. It is a proprietary fastener technology developed by MacLean-Fogg that combines two distinct thread geometry features into a single fastener.

The first feature is the locking thread form. Like other vibration resistant fasteners built on prevailing torque principles, the SALT thread is manufactured with an interference condition that creates mechanical resistance when the fastener is driven. This resistance is present at installation and remains present throughout the service life of the joint.

The second feature is MAThread geometry, which gives SALT its self-aligning characteristic. MAThread modifies the thread flank angle to allow minor angular correction as the fastener seats. In applications where mating surfaces are not perfectly perpendicular, or where thermal expansion causes slight misalignment between joint faces, MAThread geometry compensates without placing bending stress on the fastener shank. Bending stress is one of the primary causes of fatigue failure in high-cycle, high-temperature fastener joints.

Together, the two features address two of the most common failure modes in thermal applications: loosening under vibration, and fatigue from misalignment loads.

How SALT Maintains Clamp Load Through Thermal Cycling

When a joint heats up, every component expands. Steel flanges, cast iron manifolds, and stainless fasteners all have different coefficients of thermal expansion. As these components expand at different rates, the joint geometry changes. A fastener torqued to specification at room temperature may see a different load distribution at operating temperature.

On the contraction side, the issue is relaxation. As the joint cools, components contract and microscopic surface asperities on the mating faces can settle and compress. This settling reduces the height of the joint stack, which reduces bolt stretch, which reduces clamp load. Repeat this process enough times and the fastener that was once at full preload can sit at a fraction of its original tension.

SALT addresses both sides of the cycle. The prevailing torque from the Lockthread means that even if the fastener experiences a net loosening tendency as the joint cools, it must overcome the built-in thread resistance before any rotation occurs. In practice, this can prevent the micro-rotation events that accumulate into measurable loosening over hundreds of thermal cycles.

The MAThread self-alignment feature addresses the secondary failure mode: fatigue cracking at the thread root caused by off-axis loading during uneven thermal expansion. By allowing the fastener to self-correct its seating angle, SALT reduces the stress concentration that would otherwise accelerate crack initiation.

Materials Available for SALT Production

MacLean-Fogg produces SALT fasteners in high-temperature alloys selected for specific service environments: