Understanding Torque to Yield (TTY) Bolts

Learn What Torque to Yield (TTY) Bolts Are And Why They’re Used

Torque to Yield bolts are specialized fasteners designed to be tightened beyond their elastic limit, entering the plastic deformation zone. This means they are tightened to a point where they permanently stretch and cannot return to their original length. Unlike standard bolts, which are tightened to a specific torque value, TTY bolts are tightened to a precise angle after an initial torque is applied.

What is the Torque to Yield Bolt’s yield point?

If a bolt is tightened past its point of elasticity, it will not return to its original shape when loosened. This is called the bolt’s “yield point.”

When you tighten a threaded fastener, it can go through four phases:
• Rundown Phase: Spinning the bolt until the bolt head just contacts the component.
• Alignment Phase: The bolt head and mating surfaces are drawn into alignment.
• Elastic Phase: For regular bolts, tightening to the specified torque cause the bolt to stretch to its point of elasticity. The bold is elongated but will return to original length when loosened.
• Plastic or Yield Phase: This is an over-torqued condition for normal bolts. However, for TTY bolts, this is just a part of the tightening process. The initial torque and additional turns cause permanent deformation and elongationof the fastener and/or joint occur. Necking of the fastener occurs.

How TTY Bolts Work

The working principle of TTY bolts revolves around their ability to achieve a more uniform clamping force.

Here’s a step-by-step outline of how they are typically used:

1) Initial Torque: The TTY bolt is first tightened to a specified initial torque value. This initial torque is usually lower than the final required clamping force.

2) Angular Tightening: After reaching the initial torque, the bolt is then tightened further by turning it at a specific angle, typically measured in degrees. This additional angular tightening ensures that the bolt stretches into the plastic deformation zone, providing a more consistent clamping force.

The Advantages of TTY Bolts

The use of TTY bolts offers several significant advantages, especially in applications where consistent clamping force is critical. Some of these advantages include:

1) Uniform Clamping Force: By stretching into the plastic deformation zone, TTY bolts provide a more uniform clamping force across all bolts in an assembly. This is particularly important in applications such as engine cylinder heads, where even distribution of pressure is crucial.

2) Improved Sealing: The consistent clamping force provided by TTY bolts enhances the sealing of components, reducing the likelihood of leaks in critical systems like engines and transmissions.

3) Vibration Resistance: TTY bolts are less likely to loosen under vibration compared to conventional bolts. This makes them ideal for use in high-vibration environments, such as automotive engines.

4) Reduced Risk of Over-Tightening: The angular tightening method reduces the risk of over-tightening, which can lead to bolt failure or damage to the components being fastened.

Where TTY Bolts Are Used On Late Model Vehicles

• The Engine— TTY bolts are commonly used to secure engine cylinder heads, ensuring a uniform and reliable seal between the head and the engine block. This helps maintain proper compression and prevents leaks.
• Transmission Systems— In automotive transmissions, TTY bolts are used to secure components that require precise clamping forces to function correctly.
• Suspension Systems— TTY bolts are used in suspension systems to maintain consistent clamping force, ensuring the stability and safety of the vehicle.

Why newer vehicles use TTY bolts; what was wrong with the old bolts?

When engine blocks and cylinder heads were made from cast iron, both expanded and contracted at the same rate. But newer engines are made with aluminum alloys that expand and contract at different rates. When you install a new gasket, it will relax over time as it “sets” and compresses. If you rely on torque during installation, the connection will loosen as the gasket relaxes. For example, it’s common for a composition head gasket to relax by up to 25%, causing a 0.011 loss of spacing.

The same is true with suspension components. Carmakers have switched from cast iron and steel control arms to lighter magnesium alloys. When you install a steel bolt in a magnesium alloy, you get two different rates of expansion/contraction, so a TTY bolt is required to maintain the proper torque.

The TTY torquing procedure is a multi-step process

Torquing a TTY bolt requires both a torque wrench and a torque angle gauge. You cannot torque these bolts without both or you risk damaging the bolt. Worse yet, you’ll screw up the head gasket and have to do the job all over again.

Step 1: Torque the bolt to a specified torque.
Step 2: Attach a torque angle gauge and turn the bolt the prescribed number of degrees.
Steps 3 & 4: This step may involve another torque step or another rotation.

Once TTY head bolts have been torqued, they generally cannot be re-used. However, some car makers do allow re-use. If you don’t know for sure whether your head bolts can be re-used, don’t risk it; use new head bolts. If you attempt to reuse a TTY head bolt that can’t be reused, you will either break the bolt or screw up the head gasket job. Don’t risk it. Buy new head bolts.

Considerations When Using TTY Bolts

While TTY bolts offer numerous benefits, there are some important considerations to keep in mind when using them:

• Single-Use— TTY bolts are designed for single use only. Once they have been stretched into the plastic deformation zone, they cannot be reused. Reusing TTY bolts can compromise the integrity of the assembly and lead to failure.
•Proper Tools— Proper tools and equipment are required to install TTY bolts correctly. This includes torque wrenches capable of precise angular measurements.
• Manufacturer Specifications— It is crucial to follow the manufacturer’s specifications for initial torque and angular tightening. Deviating from these specifications can result in inadequate or excessive clamping force.
• Inspection— TTY bolts should be inspected before use to ensure they are free of defects and damage. Damaged bolts can fail to provide the necessary clamping force and compromise the assembly.

©, 2019 Rick Muscoplat