Torque to Yield Bolts

With a regular head bolt a technician could torque a bolt to its yield point and it would return to its original form and length. This is because of its elasticity it is able to stretch and return to its original form.

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

1. Elastic deformation: This is the amount of force or torque where you haven't changed the shape or length of the bolt. At this point you are able to reuse the bolt as 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 can not be reused and needs to be replaced.

For example an aluminum cylinder head will be torqued in a sequence say one to ten. The first pass would be a torque of 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.

So in summary you can use the old fashion head bolts over and over but when it comes to torque to yield they need to be replaced with new ones.