Commonly used for head bolts, main bearing caps and other areas requiring a high clamping strength in automotive engines. These bolts are not reusable. Torque to yield bolts are stretched past the elastic stage (where the bolt can return to original shape) to a yield point (stretched to deformity) creating the highest clamping force. This is just before the bolt breaks in two (shear stage). Torque to yield bolts allow fewer bolts to apply more clamping load than conventional bolts used in greater numbers. Torque to yield bolts can be identified by having a more narrow shank than a conventional bolt of the same size.

Torque-to-Yield Bolts

One-time-use, or torque-to-yield (TTY), fasteners take advantage of the torque yield principle. Most torque specifications build in a 25 percent safety margin--they can be overtorqued by 25 percent before damage occurs. TTY bolts do not have this safety zone. They are designed to be tightened just into the yield point but not to the extent that the metal is distorted. The advantage is they deliver 100 percent of their intended strength, versus the 75 percent strength provided by regular bolts. The drawback is that they can only be used one time. Once they are removed from an application, they can no longer support the load they previously did, and they must be replaced with TTY bolts. Installing a conventional fastener in their place will result in a 25 percent weaker clamping force. Because TTY fasteners are used in a variety of applications, such as subframe mountings, it's important to identify these during the estimating process--not just for the additional cost involved but also to alert the technician.