"Torque to yield" bolts

Help me out here, guys. I come here because I think this is probably the best collection of true and back yard engineers in the Corvette world so I put a lot of stock in what is said here. Most of us know the new LS platform engines use "torque to yield" bolts for all critical fasteners, like main/rod caps and head bolts. What makes, if anything, this design any better than the earlier design with a conventional torque wrench?
I would like a good explanation of the advantages to the newer method. Previously, my procedure has always been to torque all new fasteners to the torque specs to pre-stretch, then relax them and retorque to the listed torque value. I've always felt this was the best way to ensure proper long term torque values when using new fasteners. It's my understanding you can no longer do this with the "torque to yield" since they're a one-and-done fastener.
Learn me sumthin'.

I have ARP bolts all through this 355 I running for some years now, and many head installs while messing with things....it IS a hobby car for christsakes....

and never seen any problems related to TTY or not....

I think it's a method of selling bolts....

but those bolts are made in CHINA these daze, think for a microsecond I trust the TTY spec??

but sometimes there is no choice...so use them and pray....

:club::hunter:

We have had a run of head jobs at work lately and they nearly all use tty bolts. The last one was a GM motor (z14xe) in a holden barina(Australia). It felt like some were going down harder than others but thats what tty are all about,they all go down the same amount as if you use a tention wrench it would click off at a different yeild than others. Dont like em but only for the little extra hassle to do them. As to who makes them dont know dont care as i havnt seen any fail (yet)

That's what they are all about, they all torque down to to yield point, a certain degree of elongation over rotation this takes friction out of the equation, something that with normal torqueingcan cause varying clamping forces between bolts As much as a pita tty is, it does work.

Twin_Turbo said:

That's what they are all about, they all torque down to to yield point, a certain degree of elongation over rotation this takes friction out of the equation, something that with normal torqueingcan cause varying clamping forces between bolts As much as a pita tty is, it does work.

Click to expand...

Maybe I gotta go back and read again, but when I did the Escort engine some years ago, the TTY head bolts came with the gasket set, and so I set them to some ft. lbs. of torque just as per normal, as I recall....so how does friction get eliminated from the equation??

I always use RTV on the threads on head bolts to number ONE seal the water jacket , and add a tad of lube to help on friction...

TTY theory just escapes me, I fail to understand how it's better....


:clobbered:

simple terms,they are made to stretch a certain degree,which is what the angles verses pitch is all about. where as the old torque bolts are harder(less stretchy)so they rely on friction to get to the right tightness(i think)
the toyota hzj cruisers had a decal on them saying to check headbolts for stretch with a measurement and called them plastic bolts!!!!!:thumbs:

shipy59 said:

simple terms,they are made to stretch a certain degree,which is what the angles verses pitch is all about. where as the old torque bolts are harder(less stretchy)so they rely on friction to get to the right tightness(i think)
the toyota hzj cruisers had a decal on them saying to check headbolts for stretch with a measurement and called them plastic bolts!!!!!:thumbs:

Click to expand...

That's the part that bugs me, now in hell you KNOW how far the damn thing stretched.....??? with the engine assembled that is....I suppose once it has 'yielded' you can unbolt it and measure the length differences, but then we not supposed to reuse the same bolt, surely there are many mills difference between bolt lengths to start with...

obviously even my ARP bolts will stretch SOME under tension...and with them hardened washers under there with my RTV, I would have to think it's more accurate....

there has to be something I just can't get my mind bent around on this topic.....

:sos::stirpot:

I did some reading on them yesterday and that was the recurring theme, they take friction out of the equation. I'm not sure I'm sold on that though. I'm not necessarily arguing they don't, but the counter argument is that conventional bolts may have radically different torque values because of friction. So let me ask this, if you prepare the fastening surfaces correctly (i.e. chase the threads in the block, verify the condition of the threads on the bolt, properly lubricate all the friction surfaces), don't you minimize the difference between conventional and TTY bolts?

Many types of steel have properties that are not apparent in the normal experience. One of these properties is called plastic elongation. It is where the steel stretches under a constant force.

Do this, get one of the plastic six pack rings.

Yank on it a little bit and it stretches, let go and it springs back. This the elastic range of steel and the top of the range is the yield strength. Say fy, 60,000 psi.

Now pull on it harder, slowly with more force. At some point, one of the loops will neck down and start to stretch. And at this point you can continue to stretch the plastic without using any more of your strength. HOWEVER if you let go the plastic will not go back to its original shape.

Some steels do not have this ability to stretch and these are brittle steels, others have a wonderful capacity to stretch and these are called ductile steel. Cast iron differential covers that just break v. deep draw steels to stamp out Corvette chassis parts on the older cars.

And finally continue to pull and then the plastic snaps. This is the ultimate strength of the steel, maybe, fu 80,000 psi.

Typical bolts stay in the first range. Stretch to yield bolts are in the second range. In the second range, the clamping force on the head gasket is very well defined and is constant over a range of twists. Problem is you have no easy way of knowing once you have stretched the bolt, how much stretch is has left in it for a second use without doing some fancy measuring that takes longer than just buying another bolt. And you have no resurve capicity, where a typcal bolt has the whole of the yeild range to be in before if breaks under load.

George

Corvettes White said:

Many types of steel have properties that are not apparent in the normal experience. One of these properties is called plastic elongation. It is where the steel stretches under a constant force.

Do this, get one of the plastic six pack rings.

Yank on it a little bit and it stretches, let go and it springs back. This the elastic range of steel and the top of the range is the yield strength. Say fy, 60,000 psi.

Now pull on it harder, slowly with more force. At some point, one of the loops will neck down and start to stretch. And at this point you can continue to stretch the plastic without using any more of your strength. HOWEVER if you let go the plastic will not go back to its original shape.

Some steels do not have this ability to stretch and these are brittle steels, others have a wonderful capacity to stretch and these are called ductile steel. Cast iron differential covers that just break v. deep draw steels to stamp out Corvette chassis parts on the older cars.

And finally continue to pull and then the plastic snaps. This is the ultimate strength of the steel, maybe, fu 80,000 psi.

Typical bolts stay in the first range. Stretch to yield bolts are in the second range. In the second range, the clamping force on the head gasket is very well defined and is constant over a range of twists. Problem is you have no easy way of knowing once you have stretched the bolt, how much stretch is has left in it for a second use without doing some fancy measuring that takes longer than just buying another bolt. And you have no resurve capicity, where a typcal bolt has the whole of the yeild range to be in before if breaks under load.

George

Click to expand...

I appreciate the help there man, trick is, aren't they the same deal with having to tighten them?? as I recall I tightened to like 65 ft lbs on the Escort...whatever the figger was, due to friction in the threads and head, how do I know the bolt yielded enough??
or too much maybe??

seems like such a black art to me, :crutches:

That’s the magic part, the steel flat lines. It is hard to visualize which is why I suggested the experiment. You can feel it in your muscles. Once the bolt starts to stretch, it will continue to stretch as long as the force is the same, you do not need to apply more force.

In the first range of the steel it is like a spring. 1000 pounds pulls it 0.01 inch, 2000 pounds pulls it to 0.02 inch and so forth until you reach yield. Pretend yield is 5000 pounds and we have now stretched the bolt to 0.05. But now as long as I am hanging a 5000 pound weight on the bolt, it will continue to stretch until it breaks or strain hardens. More like taffy. So at 0.06 or 0.07 inches long we are still at 5000 pounds. Now when the bolts get very close to snapping the load may increase a bit to say 6000 pounds but that may be at a total stretch of 0.10 inches. (Numbers are totally made up but they should get the idea across.)

Alright George, what you posted looks very much like what I've been reading. So it sounds like every body is on the same page. The one thing I read last night I haven't seen posted here is that many times the TTY bolts are smaller than their conventional counterparts. I infer that the reason is because a conventional bolt is typically well beyond the strength required to accomplish the job without going into the plastic range. So, if that is the case, aren't conventional bolts typically far stronger than what is required to apply sufficient clamping force? If so, that would also indicate that you cannot use the TTY method with a conventional bolt because you would likely exceed the limit of the material into which the bolt is fastened, right? Additionally, even if the material withstood the force, it is unknown what type of deformation it may cause in either the base material or any material (as in gaskets) that may be within the clamping range.
I'm beginning to get a better appreciation for this method, but I'm still not sold that it's so far superior to conventional bolts.

clutchdust said:

Alright George, what you posted looks very much like what I've been reading. So it sounds like every body is on the same page. The one thing I read last night I haven't seen posted here is that many times the TTY bolts are smaller than their conventional counterparts. I infer that the reason is because a conventional bolt is typically well beyond the strength required to accomplish the job without going into the plastic range. So, if that is the case, aren't conventional bolts typically far stronger than what is required to apply sufficient clamping force? If so, that would also indicate that you cannot use the TTY method with a conventional bolt because you would likely exceed the limit of the material into which the bolt is fastened, right? Additionally, even if the material withstood the force, it is unknown what type of deformation it may cause in either the base material or any material (as in gaskets) that may be within the clamping range.
I'm beginning to get a better appreciation for this method, but I'm still not sold that it's so far superior to conventional bolts.

Click to expand...

:ghost::suspicious: I agree....:crutches:

Sorry, Now you are making me think about it. Your observations sound correct, But it is Sunday and my brain is in “OFF” That TTY bolts would be a smaller diameter makes sense to me. And the diameter would need to be very accurately manufactured I think it all depends on the steel the bolts are made of. High strength steels typically do not have as much plastic yield as lower strength steel, but there are so many steel in the market place today.

My C5 has studs in it, replaced the yield bolts. I simply hate tossing things away. They were made by another company in LA that does high strength bolts. I think I bought their last set.

Corvettes White said:

Do this, get one of the plastic six pack rings.

Yank on it a little bit and it stretches, let go and it springs back. This the elastic range of steel and the top of the range is the yield strength. Say fy, 60,000 psi.

Now pull on it harder, slowly with more force. At some point, one of the loops will neck down and start to stretch. And at this point you can continue to stretch the plastic without using any more of your strength. HOWEVER if you let go the plastic will not go back to its original shape.

And finally continue to pull and then the plastic snaps. This is the ultimate strength of the steel, maybe, fu 80,000 psi.

George

Click to expand...

Thanks but I just don't understand. I know that if you have a bolt, and you tighten it up, it will stretch. It's stretch to me is roughly like a rubber band. As you tighten, the bolt will stretch, and if you loosen the bolt it will return to it's unstressed length. However, if you stretch a rubber band too much it will break, and if you stretch a bolt too much if will have internal fractures were it will no not return to it's original length. I think the terminology is that the bolt has exceeded it's elastic limit. Once it's exceeded it's elastic limit, if the bolt hasn't snapped into, it nevertheless is worthless and needs to be destroyed. I've seen instructions to destroy these bolts with a sledge hammer.

Ok What does Torque to the Limit mean? Is this something different that just the old common knowledge that you don't torque a bolt past it's elastic limit?
.............................

I once worked on a project where we had an assembly that came in two halves. Each half made by a separate contractor. The halves were bolted together by 8 bolts. The bolts were approximately 1 1/4 inch in diameter and perhaps 9 inches long. The bolt set was also accompanied by nuts. OK...how much did each set of 8 bolts/nuts cost?....they cost $65,000!...and this was $65,000, 30 years ago!

These bolts were each grown in some sort of a vacuum furnace where each bolt was essentially a single pure crystal of steel. The crystal was then machined into a bolt shape. Also, each bolt was drilled with a small hole running down the center of the bolt. A strain gauge was installed in each bolt. At the factory, each bolt was torqued and stretch was measured and the internal strain gauge was calibrated. When the two halves of the assembly were bolted together, the strain gauge readings told the installers how much stretch was occurring in each bolt. The bolts were all stretched to specification limits at installation.

George, Thank you for that excellent anology.

But I think we're talking at cross purposes here.

IMHO, TTY bolts are for rapid manufacturing or clueless repairs. They will torque to a value predetermined by the cross section and provide adequate clamping force for the calculated application.

For the variations this board produces all bets are off and the TTY can screw you fast. Do you have higher CR for example?

The members of this board are typically going to be measuring as opposed to twisting. TTY is for assembly line work whether at the factory or at home....

As for what I've been reading, I think Steve may be a little more correct than he realizes. With TTY, all the onus is on the manufacturer. Rather than a bolt manufacturer saying, "this bolt will hold at least this much force under these conditions", they are now engineering bolts that have a defined variation. They will give "this" amount but not give "that" amount, if you get my meaning. And as a result all the end assembler has to do is set it to a specific torque limit far below the elastic limit, then turn it a specified amount of degrees and walk away.
Like many of you here, I'm far too anal for that when assembling critical engine parts. I chase/deburr threads, preload new bolts several times, use specific bolt assembly lube, etc. And most importantly, I know how to use a torque wrench correctly. On a side note, the other day I did a brake job for a guy. Removing the tires I really had to crank on my 4-way lug to get the lugnuts off. He tells me the wheels were last tightened by some knuckledragger at Big O tires. Of course, they tell him they can't over torque the wheels because they use a "torque stick". Yeah right.
I do see some advantage to using the TTY method in that it gets the bolts into that plastic range, especially with using aluminum heads. But people have been using conventional bolts successfully in the same applications for years.

GT6Steve said:

George, Thank you for that excellent anology.

But I think we're talking at cross purposes here.

IMHO, TTY bolts are for rapid manufacturing or clueless repairs. They will torque to a value predetermined by the cross section and provide adequate clamping force for the calculated application.

For the variations this board produces all bets are off and the TTY can screw you fast. Do you have higher CR for example?

The members of this board are typically going to be measuring as opposed to twisting. TTY is for assembly line work whether at the factory or at home....

Click to expand...

That's what I kinda thought, but not to go there myself....

:hissyfit:

I will acknowledge that I do not know why the manufactures have changed to TTY bolts. I would guess that is more of a technical reason than a cost reason. I have seen those multiple automatic torque sockets machines they use in the factory that tighten all the bolts on a head or a wheel at one time, so I doubt that it is just a factory assembly issue. And I do not think they care that much about those of us in the driveway.

As for the rubber band example, that is a bad example for this case. I found the graphs below on the internet. The rubberband is more like the brittle line in the first graph. It stretches and then just snaps. A TTY bolt is more like the drawn out line on the right of the first graph. It has a long flat spot on the curve. For normal bolts, you stay on the straight-line portion of the graph with the loads. The “Elastic Range” that is it springs back. For the TTY bolts you move to the plateau. This is the “Plastic Range,” hence as someone said, they are called plastic bolts. The difference is once you pull a something into its plastic range, it will not spring back to where was. This is the dashed line in the second graph. An elastic rubber band lacks this plastic range. Wood, glass, concrete, cast iron; all very different materials, lack this plastic range. Some plastics; some metals, gold, silver, steel; and chewing gum, have this plastic range

I am not in the automotive business, so I do know a lot of why things are done in the industry, Hell, I can barely keep my car running.

George

The more I read about it the more I remain a fan of the conventional bolts. Check out the ARP website, they have some really good information on not just TTY, but fasteners and clamping forces in general.
The deal with TTY is that they go into that "plastic" stage where they will no longer rebound to their original form. Now the manufacturers say that's a good thing when talking about thermal expansion but that just doesn't make sense to me. It would seem that we would want a fastener that remains elastic through the thermal expansion/contraction stage for more consistent clamping. That's the part I haven't had really clarified yet. Maybe I'll e-mail ARP and see what they have to say on the matter.
One other point ARP makes, which makes total sense but I doubt most of us take into account is regardless of bolt type, is that using the example of a head the various length bolts would have different torque specs to achieve the same torque load.