Torque to Yield (TTY) Fasteners
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Table of Contents
TTY Fastener Overview
Automotive powertrains have become lighter and smaller, yet more powerful than ever before. This trend pushed the limits of existing fastener technology so a new fastener torquing technology, know as the torque-to-yield (TTY), was developed. This lesson will cover the TTY bolt technology along with the procedure used to properly tighten TTY bolts.
The concept of torquing a bolt until it approached the failure point to achieve maximum clamping power started many years before it was introduced in production engines. Most race engine connecting rod bolts had machined divots in each end so their length can be accurately measured with a bolt stretch gauge as shown in the image below.
The connecting rod bolts are initially torqued to a low level to remove all “slack”. Their length is measured and compared to the manufacturer’s specifications. The bolts are tightened, as needed, to bring the bolt’s installed length into specs.
Load Vs Yield Point Graph
The graph, shown above, is a typical load graph for a TTY fastener. The horizontal axis is degrees of rotation of the fastener. The vertical axis is the load on the fastener.
Blue Section – Bolt Run Down
During the blue section at the start of the graph, the fastener is being run down and hasn’t developed any significant clamping force.
Green Section – Bolt Tighten/Stretch
The green section is where the bolt creates clamping force and starts to stretch. This is the elastic region of the bolt. It’s called elastic because when the load is relaxed, the bolt will return to its original length. Most bolts are designed to be used in this section because the fastener can be used over and over again with no worry about the bolt failing.
Yellow Section – Bolt Yield Point
After crossing the yield point, the stretch of the bolt becomes permanent and it will not return to its original length. When following the tightening instructions from the manufacturer, the bolt should be close to the start of the yield (failure) point.
- The bolt is first tightened to a low torque value to ensure it is properly seated.
- The bolt is then turned a specified number of degrees (using a torque angle gauge or an electronic torque wrench) that puts the fastener close to the yield point.
Red Section – Bolt Failure Point
The red section of the graph is where the fastener stretches to the point that it will quickly lose strength and fail.
How to Identify a TTY Bolt
At first look, there is little difference between a highly engineered TTY bolt and a standard bolt sold at a hardware store. However, a TTY bolt is precisely engineered and manufactured to deliver the maximum clamping power for it’s given size. TTY bolts are typically sold for a specific use and include detailed information about the proper way to torque the bolt.
Can a TTY Bolt Be Reused?
Maybe. The majority of the bolt manufacturers state that the bolt should be replaced; however, some manufacturers publish a procedure to test a TTY bolt for reuse. The two most popular test procedures are the bolt stretch test and the minimum diameter test as covered in the two sections below.
Bolt Stretch Test
The most common test published by fastener manufacturers to determine if a TTY bolt can be reused is based on bolt stretch. The bolt’s length is precisely measured and compared against a maximum length spec.
Minimum Diameter Test
Some manufacturers publish a minimum bolt diameter at a specific point on the bolt to determine if the bolt can be reused. For example, some Honda TTY head bolts can be reused if they are larger than a specified minimum diameter at a certain point on the bolt.
Proper Installation of a TTY Bolt
Torque Angle Gauge
The least expensive way to confirm you have turned a bolt a specific number of degrees is to use an add-on torque angle gauge (shown in image below). It is a degree wheel that is installed on the socket, or extension and then used to determine how many degrees the bolt is being turned. For a better understanding of how this works, see the YouTube Playlist below.
Electronic Torque Wrench (With Angle Feature)
The best way to turn a TTY bolt a specified number of degrees is to use an electronic (digital) torque wrench that supports angle measurements (as shown in the image below).