Torque to Yield Bolts

With a standard bolt a technician could torque the bolt to its yield point, and it would return to its original form and length when removed. This is because of its elasticity, it is able to stretch and then return to its original form.

This is not so with torque to yield bolts. There are two terms related to this phenomenon. The first is elastic deformation and the second is plastic deformation:

Torque to yield bolt compared to a standard bolt.

1. Elastic deformation: This is the amount of force or torque where the bolt hasn't changed shape or length. At this point the bolt is reusable because it hasn't changed its shape.

2. Plastic deformation: This is the point past elastic deformation where the bolt has changed its shape and length. At this point the bolt cannot be reused and needs to be replaced.

For Example: An aluminum cylinder head could be torqued in a sequence of one to ten. On the first pass, the head bolts are torqued to 36 foot pounds. This is the elastic deformation point. The second pass would be a 90 degree angle or a quarter turn. At this point the bolt is going into plastic deformation. The third pass would be another 90 degree angle. At this point the bolt has changed its form, and if loosened or removed, it must be replaced with a new torque to yield bolt.

TTY bolts are common and typically used to clamp cylinder heads on late model engines. Be careful, a loss of clamping force will ultimately lead to head gasket failure. Head gasket failure will cause coolant to leak into the combustion chamber resulting in white colored exhaust.