TTY bolt questions

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Hello, I have two questions on TTY bolts that I haven't been able to find an answer to.

1. Why are TTY bolts used in the first place? I know the simple answer is "because they provide more clamping force than a standard bolt of the same size", but why would that be? I understand why angle tightening is more precise than torquing, but with all else being equal (torque, thread pitch, length, etc) I don't understand why a TTY bolt would provide more clamping force than a bolt that is of a high enough grade to not stretch past its yield point under the same torque. My understanding is clamping force is a function of If someone could explain that one to me I would really appreciate it.

2. Is any downside to replacing TTY bolts with ARP high strength bolts or studs other than cost? When I take the head off of one of my cars I like to replace the stock TTY bolts with ARP studs because they are more convenient to use (in my opinion) and can be reused if I ever need to remove the head in the future and this strategy seems to have worked well for me so far and I am wondering if this is a bad practice for some reason I am not aware of since ARP studs don't stretch past their yield point at the proper torque.

Thanks in advance as always!
 
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TTY bolts equalize clamping force by effectively limiting it.

You can install studs and nuts if you ensure all of the fasteners are properly and equally torqued.
 
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I know the simple answer is "because they provide more clamping force than a standard bolt of the same size", but why would that be?
That's 100% FALSE.

First, the TTY fastener is a specialized hybrid that's made for a specific application ( dimension and/or load)- they are not to be swapped without a proper engineering analysis

The best way to explain this is with an example- you have a grade 5 bolt (1/2) and it rates say 10,000 psi to yield strength to the grade standard and a 3/8 is about 7000 psi

Your joint ( engineered) only requires say 5,500 psi load in the extreme. Lets say you don't really have the excess room ( or weight) for that 1/2 hole.

You can get a TTY fastener that maxes out around 6000 psi in 5/16. its not "stronger' but its close to requirement but theres a trade off.

To get that metallurgy and temper- it has to be tensioned in the yield range ( it will actually grow) and this usually permanently plastically deforms it ( most are single use if tensioned to the max)

On many applications, they can save cost, weight and dimension. They also have the unique ability to tension almost equally ( due to the stretch) which is often more valuable in a joint clamp than the actual tension in terms of holding.

Is any downside to replacing TTY bolts with ARP high strength bolts or studs other than cost?

A potentially dangerous one. Since the TTY fastener is made for a specific and tight range- you cant just swap out with a different grade without re-engineering the joint. You could overstress the fastener you replaced it with ( asking for catastrophic failure) or invite excessive force damaging the joint.

The point is, TTY fasteners cost more to use (higher degree of engineering on the joint) and are specifically made. They didn't put them in the application because it was easier or cheaper- they did it for a reason.

Make sure you know the reason and joint requirements before just swapping them out.
 

Avery4

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TTY bolts are effectivly a spring which is able to maintain tension over a wide range of heating and cooling cycles either ambient or from component heating. They also eliminate lock and tooth washers.
That makes sense, thanks. ARP said their high strength studs act like springs too but they aren't torqued to their yield point because they are a stronger material, therefore they don't permanently stretch and can be reused if they aren't damaged. I wonder how much truth there is to that?
 

Avery4

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That's 100% FALSE.

First, the TTY fastener is a specialized hybrid that's made for a specific application ( dimension and/or load)- they are not to be swapped without a proper engineering analysis

The best way to explain this is with an example- you have a grade 5 bolt (1/2) and it rates say 10,000 psi to yield strength to the grade standard and a 3/8 is about 7000 psi

Your joint ( engineered) only requires say 5,500 psi load in the extreme. Lets say you don't really have the excess room ( or weight) for that 1/2 hole.

You can get a TTY fastener that maxes out around 6000 psi in 5/16. its not "stronger' but its close to requirement but theres a trade off.

To get that metallurgy and temper- it has to be tensioned in the yield range ( it will actually grow) and this usually permanently plastically deforms it ( most are single use if tensioned to the max)

On many applications, they can save cost, weight and dimension. They also have the unique ability to tension almost equally ( due to the stretch) which is often more valuable in a joint clamp than the actual tension in terms of holding.



A potentially dangerous one. Since the TTY fastener is made for a specific and tight range- you cant just swap out with a different grade without re-engineering the joint. You could overstress the fastener you replaced it with ( asking for catastrophic failure) or invite excessive force damaging the joint.

The point is, TTY fasteners cost more to use (higher degree of engineering on the joint) and are specifically made. They didn't put them in the application because it was easier or cheaper- they did it for a reason.

Make sure you know the reason and joint requirements before just swapping them out.
That sort of makes sense, thank you for explaining. A couple questions though:

1. On that hypothetical joint that requires 5500 PSI of clamp load with room only for a 5/16 bolt, why wouldn't a better solution be to use a higher strength bolt that won't yield under the required torque? Or is the problem that it isn't always possible or economical to design a bolt that is strong enough to not yield under the required torque?

2. Would it be reasonable to assume that a well respected major company like ARP already did an engineering analysis to ensure that the fastener they are selling will work properly for its intended application? It would seem that they did since they provide reccomended torque specs and I haven't heard of anyone complaining about a failure due to replacing OEM fasteners with ARP fasteners, so something must be working right.
 
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1. On that hypothetical joint that requires 5500 PSI of clamp load with room only for a 5/16 bolt, why wouldn't a better solution be to use a higher strength bolt that won't yield under the required torque? Or is the problem that it isn't always possible or economical to design a bolt that is strong enough to not yield under the required torque?
When you are dealing with an engineered joint you have to calculate working load, thermal expansion, shock load, shear forces and possibly tolerances and items such as a gasket crush into a linear load ( the clamping force of the fastener)

To get the most out of that then you have to calculate the shank/thread ratio, force cone of the head and hole fit to achieve and maintain the rigidity under that required tension.

"Stronger' is a non term- the fastener has to have the right ratio of hardness (resistance to deformation) and temper ( ability to stretch in a given range) to a given geometry. Those 2 qualities share an inverse relationship then add in dimensions you have a narrow window of selections.

Then of course comes the price. ( totally different set of requirements)

That's why when you change a joint property of bolt selections, they often snap heads, shear and all kinds of other bad stuff.

2. Would it be reasonable to assume that a well respected major company like ARP already did an engineering analysis to ensure that the fastener they are selling will work properly for its intended application? It would seem that they did since they provide reccomended torque specs and I haven't heard of anyone complaining about a failure due to replacing OEM fasteners with ARP fasteners, so something must be working right.

No because they have no possible idea of the applications for their individual fastener UNLESS they were on the design team. This cant really be done after the fact.

Remember, the "torque spec' the fastener OEM provides is for the FASTENER- NOT THE REQUIREMENTS OF THE JOINT. Many bolts if torqued to max will deform and damage the joint they are in.
 
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I am 100% in favor of using ARP studs where available. Considering they are used by almost every major performance engine manufacturer and some OEMs, they are worth it. I just wish they would release the head stud installer they keep teasing.
 

Avery4

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It would seem to me that TTY bolts could permanently stretch while in service under certain conditions. For example, if a cylinder head is held down by TTY bolts and there is a massive spike in cylinder pressure that tries to lift the head, it would seem that TTY bolts would have more of a tenancy to permanently overstretch and result in the gasket failing than standard bolts or studs since they are already stretched past their yield point, so any more stretching would be permanent. This clearly isn't a problem on correctly designed stock engines since most modern engines use TTY head bolts and head gasket failures are quite rare unless the engine has been abused, but it may be a disadvantage for people who are boosting their engine.
 
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It would seem to me that TTY bolts could permanently stretch while in service under certain conditions.

They are supposed to, that's by design. That's why it is ill advised to randomly swap them out or attempt to reuse them.

They usually stretch .0001-.0006 ish at design tension ( about the max before necking begins)
 

Avery4

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They are supposed to, that's by design. That's why it is ill advised to randomly swap them out or attempt to reuse them.

They usually stretch .0001-.0006 ish at design tension ( about the max before necking begins)
I understand that they are designed to permanently stretch while they are torqued, what I was talking about was being forced to stretch farther than that by higher than normal loads such as if someone installs a turbo on an engine that didn't come with one. The higher cylinder pressure can force the head to slightly lift off of the block and cause the gasket to fail.

My understanding was that the main reason people installed head studs on high performance engines was because they are better at holding the head down under extreme cylinder pressures, in part because they are torqued within their elastic range and are therefore less likely to be yielded and lose tension from the head forcing them to stretch more than they are designed to. Please correct me if that is incorrect though as I am far from an expert on fasteners.
 
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This is a bit of a generalization because TTY bolts are normally sized very close to max tolerance by design.

what I was talking about was being forced to stretch farther than that by higher than normal loads such as if someone installs a turbo on an engine that didn't come with one. The higher cylinder pressure can force the head to slightly lift off of the block and cause the gasket to fail.
If a TTY bolt stretches much more than 2-3 ten thousandths beyond design yield ( its max tension) its going to fail ( there will be 2 pieces of a former fastener)- these fasteners have virtually zero ability for additional stretch.

My understanding was that the main reason people installed head studs on high performance engines was because they are better at holding the head down under extreme cylinder pressures, in part because they are torqued within their elastic range and are therefore less likely to be yielded and lose tension from the head forcing them to stretch more than they are designed to.
OK, a "head stud" may or may NOT be a TTY design ( most are not)

A lot of people who don't deal with ASME code (B16.5 & ASTM A193) don't realize that a stud ( designated as such) is a graded fastener whereas a piece of threaded rod or threaded rod with a center portion is not. ( even though they are visually indistinguishable)

The difference is in how they are manufactured and rolled because the center piece gives the hardened mass ( like a bolt head) to pull 2 different tensions against. The major shank diameter of the center versus the minor root diameter of the thread. They don't stretch any more than a normal fastener of the grade but their "super power" is that they can handle 2 different loads simultaneously whereas a "bolt" would normalize over its length to just one.

The most common cause of stud failure is that the stud is counterfeit, ( a cheap bolt dressed up in a stud's clothing so you never actually used a proper stud). Its the easiest one to counterfeit because the differences are all internal and the average person cannot distinguish between them.

If the PROPER graded stud is used in an engine head you would have to run nitro glycerin through it to stretch the stud to where it loosened. If one of those failed, something else was done wrong and the stud was just the scapegoat of a misdiagnosis or the result of another failure mechanism like any other fastener.

ETA- on many studs for hot applications ( hot flanges and many engine head/pressure vessel uses)- many of them have a have a higher fit class thread to deliberately deform them to prevent thermal swelling and loosening during cycles. ( which is why they say they are single use)

Technically that's not a true TTY design because you didn't put it anywhere near its yield level- just a tighter fit.

It depends on the specific design as to which one is being used. I recommend asking to make sure which is there because the designer most likely had a reason for the selection.
 
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