"Torque to yield" bolts

[Corvettes White]

I read the ARP stuff some time ago and also found it very helpful. Take a look at the second graph. The dashed line is telling you that even in the plastic range, it will spring back. It just will not spring back to where it started. So within a range of movement; compression stroke or thermal, you are covered by a stiff spring action.

 

[Ralphy]

What I remember about TTY bolts is as Corvettes Whites says A TTY bolt can handle expansion and contraction better. Such as when your engine heats up and those aluminum heads grow at higher rates then cast. Or over age your gaskets diminish the TTY's will keep on holding.

I also know TTY's are used in some aircraft engine parts. Remember though they cannot be reused.

 

[clutchdust]

Thought you guys might appreciate this insight from an expert.


Hello,
I have been having a lively discussion on a forum I frequent (http://www.vettemod.com/forum/showthread.php?t=7425) regarding the merits of "Torque to Yield" bolts versus conventional fasteners. I have read much of the technical information on your, and other, websites. I am among a few others that are not sold on the benefits of the new method. Could you please elaborate on why this method is superior to conventional bolts? Specifically, when conventional bolts are properly prepared and torqued, is TTY still significantly superior? What about when fastening aluminum heads to iron block? What about aluminum to aluminum? If TTY is technically superior, do you offer TTY for conventional applications? Are there conventional bolts to replace TTY bolts on later (LSx) engine designs?
Thanks in advance for your insight,

Thanks for the Question and hopefully I will be able to give you a adequate answer, Most factory service manuals state that Torque to Yield bolts (TTY) should not be re-used, The Technician or researchers who set up the angle or degree of force needed for a bolt to get to its maximum clamping force or its (Yield) which means that bolt is stretched to its maximum yield amount to achieve the desired clamping force, will set the amount of force or angle of torque that is needed to be applied for a specific applications and due to the materiel used it will “not” go back to the original length and should be replaced when removed. One of the down falls of the TTY bolt is that if you are running a Turbo, Supercharger or have a high compression engine you may lift the Cylinder head, the TTY bolt will normally not take the stretch caused from the head lift and yield out to a relaxed state, thus not keeping adequate pressure on the head gasket. With the higher grade materials that ARP uses we have the capability of setting the torque of the fastener to a percentage of the yield, say 75% rather than going to the maximum yield that is needed for a factory bolt, and we will end up with a higher clamping force plus the fastener will still have the capability of stretching for aluminum head expansion, and since we do not torque our material to its yield point they should spring back to their original length when removed and can be re-used. That in conjunction with our ARP Ultra Torque lube should give you a far superior clamping load and a more accurate torque on each individual fastener.



I do have a couple of different Head Stud kits available for the LSX motor depending on which configuration you have:



Factory Block and heads LSX Head Stud kit # 234-4319



Aftermarket block or heads: Call me for info.

Sam,

Thanks for the quick response. If I understand you correctly, you pretty much confirm the conclusion I have, being that the method is superior, but is also a compromise in cost and weight from an OEM standpoint. Would you agree that given proper preparation conventional bolting could rival TTY in effectiveness? For your torque to yield fasteners are there conventional torque specs also? Or do they require a degree spec only? Given the different expansion rates, what are your opinions on conventional fasteners compared to TTY given an iron block and aluminum head application?

Also, you mention that your bolts don't reach maximum yield when torqued to proper specs. Does that mean that your bolts reach the plastic phase at spec? Or do they remain in the elastic phase? During my reading I understand that if the fastener rebounds to its original length that it does not reach plastic phase. If that's the case, does that indicate that the bolts are not technically "torque to yield"? The bolt/stud kits you offer for the LSx engines, are they TTY or conventional bolts with a torque specs?

Thanks again,

We may lose some conventionally when we start dealing with higher grade materials, but Yes the conventional fastener would rival the TTY if it goes through the right preparation and is made from the right material. For your torque question about TTY torque methods, ARP does “not” use TTY material so most all of our kits will have a specific torque that will get our fasteners to the desired clamping force, and most TTY fasteners from the factory will have an angle or degree torque method to get to a desired fastener yield point and as I stated before the TTY fasteners are usually set up to its maximum clamping force when it reaches its full yield point so is does not have much more stretch available for Aluminum contraction or head lift caused by a Turbo or Supercharger.



For you second question here is a copy from our catalog that I hope will help:





The Stretch Factor

It is important to note that in order for a fastener to function properly it must be “stretched” a specific amount. The material’s ability to “rebound” like a spring is what provides the clamping force. If you were to simply “finger-tighten” a bolt there would be no preload. However, when you apply torque or rotate a fastener a specific amount and stretch it, you will be applying clamping force. The amount of force or preload you can achieve from any bolt or stud depends on the material being used and its ductility, the heat treat, and the diameter of the fastener. Of course, every fastener has a “yield” point! The yield point or yield strength of a fastener is the point at which the fastener has been overtightened and stretched too much, and will not return to its original manufactured length. As a rule of thumb, if you measure a fastener and it is .001˝ (or more) longer than its original length it has been compromised and must be replaced.





Thanks,

Sam Benson

Quality and Customer Service Supervisor

ARP, (Automotive Racing Products)