I just typed this in and re-read it. I'm hoping it is not too preachy, high minded, etc. I think it has value to put out there in front of everyone, but I apologize if anyone falls asleep or strains their eyes from rolling them too much:
Might as well chime in on this subject, put my BSME on the table.
Torque to yield bolts were developed in response to the changing materials used in engine design. When OEM's started putting aluminum cylinder heads on cast iron blocks they found out (the hard way sometimes) that the varying rates of thermal expansion between the cast iron block and the aluminum head could not tolerate the low accuracy of the normal "torque to torque spec" bolts. They needed a better way of ensuring that all of the bolts in the assembly were applying the correct, even preload to the assembly.
When you start working on these type of engines you see this crazy order of operations. Torque to some low torque spec in a particular pattern, then (in the same pattern) an additional 90 degrees, then another 90 degrees in the same pattern. Why is this?
In general, for iron based alloy bolts, during tightening to failure (we have all experienced this at some point) the bolt goes through 4 phases: Elastic region, Yield Point, Plastic Region, and Ultimate Failure Point.
In the Elastic region, as all of our old torque to torque spec bolts were designed for, the bolt is a spring. We tighten it, it stores energy by stretching, but, when we loosen it, it will return to it's original length. Kind of like a torsion spring on the early IFS, it elastically deforms all the time as you drive down the road, but returns to its original position when you take the preload off of it.
The Yield Point is the stress point in the material that results in permanent deformation of the bolt. It is the transition point between Elastic and Plastic.
Once in the Plastic region of the stress/strain curve the bolt will take a permanent deformation to it's length. When you take the load off of a bolt that has been plasticly deformed it will be longer by some amount, depending on how far past the yield point it has been "turned".
The Ultimate Failure Point, is a point that we all know and love. It is that point where the bolt rapidly fails and the applied torque falls off rapidly for any given increase in length/rotation. You know the feeling, oh crap, the wrench is moving easier, now, damn, even easier, like stepping on a rotten plum. Damn, I snapped the bolt.
So, now that I bored everyone to death with the above explanation of Elastic, Yield, Plastic, and Ultimate Failure, why do we use torque to yield bolts?
The tightening of bolts is an in-exact science. The "recorded" torque on a torque wrench is dependant on a litany of factors, including the clearance between the tapped hole and the bolt, the surface finish of the threads, the presence (or not) of lubricant, etc, etc.
To eliminate a significant number of these factors, torque to yield bolts are used.
The initial "snug torque" is used to ensure, as best as possible, that all of the bolts start out at the same frame of reference. All of the bolts and washers should be oiled. The low torque that is specified does not kick in a number of the "uncertainty factors" associated with the thread geometry. So, now you have all the bolts at common reference point.
The next step is to ensure that they are all torqued PAST YIELD POINT, not too yield, not before yield, but past. A stress-strain curve has a flat point in it after yield (iron based bolts). If all of the bolts are torqued past yield into the flat spot in the curve they have (about) the same preload, if you tighten it any more it just permanently stretches, will not store any more energy.
This is where the angle of turn spec comes from. You know the bolts are at a common reference point, so, now you turn them a certain angle, which you know will create a given increase in length of the bolt (thread geometry). If the engineer has done his homework, he knows that if he starts at the common reference point (low torque, oiled bolts) and then turns the bolt a certain number of degrees, all of the bolts will be in the flat portion of the Plastic Region.
If you have done heads with torque to yield bolts you know the feeling when they yield. The force applied to make the bolt turn keeps getting harder, but then, it breaks over and stays the same. Some break over on the first 90 degrees, some on the second, but they all go into the flat region of the stress strain curve.
Now, if you re-use a bolt that has gone through this operation, you are using a bolt that has stretched how far? No way to know, for sure. You can't know, it might have yielded early or late in the sequence.
If it yielded early, and you re-use it, you will push the bolt to far towards the Ultimate Failure Point. It sees a few thermal cylces on the engine, and bam, blown head gasket.
My advice, do not reuse torque to yield bolts. They might work, they might not.
I wonder if this is why everyone sees so many second and third blown head gaskets? Same damn bolts used again, and again, and again.
RockAuto.com, $24 per 8 (3VZE), $50 bones for a full set.
Sorry for the long post, had to do it justice.
Mike
