What are Torque to Yield Head Bolts?

Do you have to replace torque to yield head 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 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 a torque to yield bolt. There are two terms related to this phenomenon. The first is elastic deformation and the second is plastic deformation:
Torque to Yield Cylinder Head Bolt

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.
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 you can re-use the standard head bolts, but when it comes to torque to yield bolts, if they have reached plastic deformation, they need to be replaced with new ones. Remember, a lightly oiled bolt should be easily screwed all the way in and out of the bore with only your fingertips. If there is excess resistance there is probably something wrong with the bolts threads. Always check with manufacturers specifications and TSB’s when working with engine bolts, especially if you suspect them to be TTY bolts. 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.

Subaru Head Bolts

Now, I have been replacing Subaru Head Gaskets for over 10 years now. These head bolts are torque to yield bolts, but with my experience I have Never replaced Subaru head bolts during head gasket replacement. The same goes with Toyota engines unless I get one of those Toyota Camry’s that pull the threads out of the block. As stated above you should still always inspect the head bolt for distortion and be really skeptical if the bolt don’t run in and out freely.

This post was written by: Martin Hand

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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

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