torque to yeild head bolts

grumpyvette

Administrator
Staff member
MOST manufacturers don,t stay up nights thinking of ways to make you crazy, or change things just for fun, if you research the problem , theres a reason and an answer, if you start working on a project and find your into a new area its best to grab your shop manual and sit down and READ thru the instructions before proceeding further....yes I,M well aware that's not how most guys operate, but IT will save you some grief in the long run to think things thru and read instructions before you proceed

most of you are probably vaguely aware that the new corvette engines and many other engines are now built with aluminum blocks and heads to reduce car weight and improve handling, this has required some changes in the bolt technology as aluminum tends to expand a good deal more under heat cycles than cast iron


standard head bolts are tightened to specs well under the bolts elastic limits which by definition means they return to their
original length, and maintain their strength once the tension is removed, ..for iron blocks even those with aluminum heads this is the route ID suggest, most of us older guys have used a standard torque wrench to tighten ARP head bolts or STUDS with good results, but the newer engines REQUIRE a different approach and new tools

TORQUE TO YIELD
bolts BY DEFINITION, are tightened PAST the ELASTIC LIMIT so they stretch and can not ever return to their original length so its effectively started to fail, the advantage is that its a bit springy at this stage in being excessively stretched ,PROVIDED its JUST STARTING to exceed the bolts elastic limits so that heat cycles, of an engine and the head expanding (LIKE ALUMINUM TENDS TO DO) during heat cycles allows the bolt to move and maintain its clamping force within narrow limits,this is the route most manufactures suggest with aluminum block/heads simply because the much greater range of expansion when both the heads and block are aluminum, as aluminum expands a good deal more with heat , the down side being the bolts are effectively a one time use like a gasket

youll need a good quality torque wrench
torquewrench.png

and a torque angle gauge

41M70Tl7tKL.jpg

http://www.robbos.com.au/myweb/Tech_Bolts.htm

Tech Tips:

TORQUE - TO - YIELD

CYLINDER HEAD BOLTS

Bolt Performance

Under the application of load, all bolts exhibit four main phases; the elastic phase, the plastic phase, the yield point and the shear point. In the elastic phase a bolt will stretch under tension but return to its original length when the load is removed. As we continue to apply load the bolt reaches the plastic phase from which it can no longer recover to its original length and is now permanently stretched, The point that separates the elastic phase from the plastic phase is called the yield point of the bolt. Finally, if we continue to apply load the shear point is reached and the bolt material wastes and breaks.

Torque to yield head bolts

Torque to yield (TTY) bolts, also commonly referred to as angle torque or stretch bolts, are used in many of today's modern engines predominantly for cylinder head bolts but also main bearing and big end caps.

Compared to conventional type bolts, TTY bolts offer the engine manufacturer a number of advantages including greater flexibility of design, reductions in component costs, more accurate assembly and reliability of seal. Engines designed utilising TTY head bolts require fewer head bolts to achieve the desired clamping loads then those using conventional bolts. With fewer bolts the engine manufacturer has more flexibility in cylinder head and block design as well as reducing the cost of the engine.

Whilst TTY bolts are attractive to the engine manufacturer, there are disadvantages to the engine repairer. For the most of us it would be unthinkable to replace a conventional head bolt unless the bolt was damaged, i.e. stripped threads, the bolt head was rounded off, the shank was severely corroded or pitted.

Conventional head bolts simply just did not wear out. Torque to yield head bolts however, by the very nature of their design, do wear out and should NEVER be reused.

Installing Cylinder Head Bolts (General Information)

When installing cylinder head bolts (and any bolt that has to be tightened to a specified torque), the thread of the bolt and under the head of the bolt should be oiled before assembly. This will give 2 - 3 times the loading over a dry assembly. Where head bolts penetrate into the water jacket, coat the threads with a non hardening sealant.

Installing TTY Bolts

TTY head bolts are also tightened in a series of stages and in sequence, however they are not tightened to a predetermined torque, they are tightened through a series of specified angles. This data is provided by the engine manufacturer and should always be adhered to. While the first step in the tightening process is normally stated as a torque figure it is done so only to provide a uniform baseline from which the true load is then applied. This is commonly referred to as a pre-load or snug torque. A typical tightening specification would look as follows:

uniformly tighten in sequence in several passes to 78Nm

tighten in sequence 90°

tighten in sequence a further 90°

This procedure ensures that friction does not cause an uneven bolt loading and that the correct high tension is achieved every time during assembly. It is essential that a quality wrench with an accurate angle gauge be used to achieve the correct angles of turn of the tightening process.

Unlike a conventional bolt, TTY bolts are tightened beyond their elastic range past their yield point from which the bolt material can recover to its original length, and into the plastic phase of the bolt material. The bolt is permanently stretched and for this reason should not be reused. The reliability of these bolts once stretched is greatly reduced. If they are reused, they are permanently stretched further a second or third time. It is also for this reason why you should never retorque a torque to yield bolt.

Some engine manufactures provide a measurement within which a head bolt may be reused, however the age and history of the bolt is not taken into account. The bolt may well be within specification to pass a simple measurement test but the bolt could be very close to its shear point. Only one failed bolt can result in serious combustion leakage. The cost of a new set of TTY bolts is well justified when compared to the cost of having to repair an engine for the second time because of insufficient clamping load due to bolt fatigue.

This information was supplied by Gasmiser, suppliers of Gasmiser Head Bolts.

MORE INFO FROM A DIFFERENT SOURCE

http://www.pbase.com/stealthfti/hb

lthfti | profile | all galleries >> headbolts tree view | thumbnails | slideshow
headbolts
Which head bolt is better? the old style or the new style?

A good question. And some would say: forget bolts: go with studs.

The old style bolts were in use up to about 1980. They were superceded by the new style bolts. At the time of the supercession, the head bolt torque specifications and procedures were changed; to reflect the difference in the design of the fastener.

There is an ongoing debate among fans of the turbo'd "Bricks" as to which head bolt is better.

I am firmly in the camp of those preferring to use the new style bolts. There is more to it than just "personal preference" as far as my reasons for using the new style bolts.

****

To begin with: there is a lot of very good information available on fastener theory, design, and application. For in depth information, I would suggest research on the topic.

...because I do not expect people to just accept what I say. Go prove it for yourownself!

****

Here is MY take on the subject:

There are three basic methods to bolt down a cylinder head:

...tightening the head bolt [or stud nut] to a specific torque reading using a torque wrench. This is known as the "Torque" method. It is the method that has been used since engines were first built.

...tightening the head bolt to a specific torque reading, known as a 'snug torque' value, to equalize the pressures on the head gasket, and to establish the basis point for the next step; which is to turn the bolt a specific number of degrees...to turn the bolt a specified angle [example: a 90 degree angle tightening is a quarter of a turn]. This is known as the "Angle Tighten", "Angle Tension", "Torque/Angle", or "Torque/Angle Tighten" method. I will refer to this as TAT.

...or tighten the bolt to a specified torque value for the reasons stated above; then turn the bolt a specified number of degrees [like in the TAT method] either one time, or do the angle tightening procedure twice [as in one round of 90 degree turns of the bolts, followed by another round of 90 degree turns of the bolts]; the purpose of this method being to stretch the bolt to the point of "Yield". This method is known as "Torque to Yield", or "TTY" for short.

Because the procedure for the "Torque/Angle" and the "Torque to Yield" methods are basically identical, they are often confused and mis-identified. I have been guilty of such confusion and mis-identification. [which is WHY I put together this gallery]

Despite the similarity of procedure, the goals of the two methods are QUITE different. Since both the TAT and TTY methods are derived from, and purposed to improve upon, the original "Torque" method; a quick review of the original method is in order.

The "Torque" method has worked for years, in spite of the main deficiency: the torque reading used is dependent on, and very affected by, the friction of the bolt threads/block hole threads and the bolt head base/cylinder head material under the bolt head base. Upwards of 90% of the applied torque to tighten the bolt can be and actually is used to fight the friction. What that means is that only 10% of the torque applied to the bolt ends up as being used to apply the clamping force of the fastener. Besides being a considerable source of loss of applied clamp, the friction is a variable that causes considerable variation in accuracy of the clamp pressure applied. Reports that I have read indicate an accuracy variation of as much as 35% in clamp force applied using the "Torque" method.

Over the years, the bolts were improved with stronger materials and better thread designs...rolled threads as an example...and improved thread lubricants; all in the pursuit of a stronger and more accurate clamping force applied. "Studs" were developed and implemented to improve upon the basic head bolt/torque down procedure. The classic head bolt or "stud kit" work very well.

The "Torque" method, using either head bolts or studs/nuts, is intended to turn the bolt [or the stud's nut] enough to actually stretch the bolt [or stud] a few thousandths of an inch. It is this stretching [AKA "tensioning"] of the bolt or stud that actually applies the clamping force.

If the bolt or stud is only stretched a certain amount, then when it is loosened it will return to its original length. Stretching the bolt or stud within this area of 'stretchability/return to length' is known as stretching the bolt/stud within it's 'elastic' area. If the bolt or stud are tightened to the point that the stretching cannot "unstretch", the bolt/stud will not return to original length when loosened. This point beyond 'stretch/return to length' is known as the "Yield" point.

Classic head bolts and studs are designed and torque specified to be tightened to a point below the Yield Point, AKA 'percentage of yield"; usually about 75% to 80% of yield.

...in other words, when you torque down a classic head bolt to the specified torque wrench reading, you are trying to stretch the bolt to a point just under the point at which the bolt will permanently stretch out of length. When staying under the Yield Point, the clamping force applied will be at the maximum possible.

...it is this "maximum possible clamping force" that is the reason why many prefer to use either the classic style head bolt, or upgrade to studs.

IF 'maximum possible clamping force' were all we had to be concerned with, then there would be little problem with making a choice regarding type of fastener used: the classic style head bolt or stud would be the premier choice.

BUT....there is more to it than just how much of a clamp is applied. Which is why and where TAT and TTY come into the picture.

The TAT method is actually a refinement of the "Torque" method. Its purpose was to find a way to minimize the variations caused by friction. Knowing the thread size and pitch, it is easy to determine just how many thousandths of an inch the bolt [of a known diameter and material] will stretch if it is turned a certain number of degrees. The angle tightening method is a way of more accurately stretching the bolt. Reports that I have read indicate a accuracy variation in clamping force applied using the angle tightening method to be in the 15% range...a considerable improvement over the "Torque" method.

It is very important to remember that the TAT method, while more accurately stretching the bolt than the "Torque" method, is still a method that only stretches the bolt BELOW the Yield Point:

...TAT stretches the bolt to a percentage of yield...the bolt remains in the elastic area of tension...and returns to length when loosened.

'Torque To Yield' is different from 'Torque' and 'TAT' because:
....with TTY, the goal IS to tighten the bolt to the Yield Point....and actually a bit beyond that into the plastic area of stretch.

WHY?

To stretch the bolt beyond the permanent elongation point seems counterproductive: you are going beyond the maximum clamping force capability [at 75-80% of Yield] and on to the point of having over-stretched the bolt.

Like I said: there is more to it than just maximum applied clamping force.

With the classic style head bolt or stud, the tensioning below yield does provide a very steady clamping force. The steadiness of the clamp is good. The constant-ness of the clamping force is good.

..and is also a source of problems.

With the use of aluminum cylinder heads, the needs for clamp changed. Instead of a 'maximum clamp applied' that is constant and unchanging being optimal, a steady clamp that can allow for the thermal expansion of the aluminum head without exceeding the compressibility of the head gasket became more important. The classic head bolt or stud does expand and allow for some thermal expansion, but the flexibility was not quite optimal. When the aluminum head warms up at full operating temperature, it grows in size, putting more bolt tension on the head gasket, and on the aluminum itself. This results in brinelling of the head surface where the fire rings of the head gasket are located, overcompression of the HG itself, and distortion of the aluminum around the head bolts. If the HG cannot handle the excessive pressure that occurs, it will remain permanently thinner; so that when the engine cools down, the HG does not provide as good a seal between the head and block. Over time, this will lead to HG failure.

Enter TTY:

By stretching the bolt beyond the yield point, and into the plastic range, the maximum clamp applied is reduced; but by being in the plastic range, the bolt can and does give more with the thermal expansion of the aluminum head. The clamp is not as great; but it is steadier throughout the temperature range...a very important thing when using aluminum for head casting material. HG longevity is increased; and brinelling and distortion of the head is reduced.

...[sounds good to me]...

There is one other good result of using TTY bolts: a further reduction in the variation of applied clamp force between the bolts. Reports that I have read indicate that TTY bolts reduce the variation to the 7% range; a very considerable improvement over the classic style "Torque" head bolts. This means that using TTY bolts can provide a very improved uniformity of clamp around the head.

...[and that sounds real good to me as well]...

So far, it sounds like going with TTYs is a good choice; and that Volvo did that when they superceded the old style bolts with the new style bolts. And based on appearances of the new style bolts, and the revised tightening procedure, it sure looks like they are TTYs.

AND, up until recently, I viewed them as TTYs, called them TTYs, and defended their use as being better than the old style bolts based on the assumption that they were TTYs.

I WAS WRONG!!!

THEY ARE NOT TTYs!!!

As stated earlier, the tightening procedures for TATs and TTYs are nearly identical: an initial torque; usually followed by another torque; then a final angle tightening. Often, TTYs receive two angle tightening rounds, but not always.

Normally, TTY bolts are a one time use; but that is not necessarily always the case. So, the fact that the new style bolts CAN be reused up to four times [as per the green manual] is not absolute proof that the new style bolts are NOT TTYs.

The biggest reason that most [myself included until recently] consider the new style bolts to be TTYs is the appearance of the bolt itself: it has the reduced diameter section between the head and the threads: like most TTYs have. Combine the appearance of the bolts themselves with the angle tightening procedure and the reasonable conclusion to be drawn is that they are indeed TTYs.

BUT.........[and this is one of those Bertha Class size of 'buts': a really BIG one].........
...there is one very important thing that needs to happen when tightening down a TTY bolt that does NOT happen when tightening down the new style bolts on a Volvo redblock:

....the bolts DO NOT YIELD.

In all the motors on which I have tightened down the new style bolts, following the factory procedure [15 lbs-ft, 45 lbs-ft, angle tighten 90 degrees], I have never felt the bolts yield.

And when a bolt yields, you CAN feel it "yield". It feels like you have started to pull the threads: like it went soft. I HAVE overtightened bolts on other things; I KNOW the feeling of the yield. And when you feel a bolt yield when it should not have, it gives you a certain kind of sick, sinking feeling in the stomach...

The new style bolts are NOT TTYs; they are TATs.

...The final angle tightening part of the procedure is to improve the clamp force accuracy.
...The reduced diameter section of the bolt body is to provide the flexibility needed to respond to the thermal expansion of the aluminum head; without having to go with a 'one-time-use-only' bolt.
...The new style bolts can be reused up to four times, IF there is no evidence of the bolt having stretched! By its very nature, a TTY bolt WILL be stretched when loosened.
...The new style Volvo bolts are not supposed to stretch: if they do, you replace them.

In Conclusion:

That is how I see it; I have stated my reasons.

I will continue to use the new style bolts: BECAUSE they are better than the old style bolts at APPLYING and MAINTAINING a MORE UNIFORM CLAMP [throughout the temperature range] on my aluminum cylinder heads.

Footnote for the skeptical:

...in the Penta redblocks, the same HG and the same part number head bolts are used. Penta gives a head bolt tightening procedure of: 15 lbs-ft; 45 lbs-ft; angle tighten 120 degrees. The Penta manual also states that those head bolts CAN be reused up to four times, if they have not stretched.

Hmmmm...sounds like even a 120 degree angle tightening is less than the yield point.

UPDATE: A point that I have confirmed in practice: NO YIELD at 120 degrees angle tightening.

Anyway....just something for the skeptics to ponder and mutter about.
the two choices
 
Last edited by a moderator:

grumpyvette

Administrator
Staff member
http://www.enginebuildermag.com/Article ... angle.aspx

Threaded Fasteners Torque-to-Yield and Torque-to-Angle


By Bill McKnight


Understand torque-to-yield (TTY) fasteners, you need a good fundamental understanding of threaded fasteners in general. The threaded fastener topic is a huge one.

All the original equipment manufacturers (OEMs) have fastener labs with lots of sophisticated equipment and well-educated people working on fastening issues. They even have their own professional association – The Bolting Technology Council – which holds meetings and seminars about fasteners.

I’m not a fastener engineer, and I’m not going to make you into one. I’ll keep this article thorough but fairly basic, giving you a good solid working knowledge of the business of bolted joints.

Fasteners function in an engine to hold parts together. For example, a rod bolt and nut hold the rod and cap together. Fasteners are also, in the case of head gaskets, used to load the gasket with the necessary force to seal the gasket under the forces of combustion as well as thermal expansion and contraction. Understanding some of the physics of fasteners and fastener tightening is necessary for an engine rebuilder who wants to keep fastener failures and engine failures to an absolute minimum.

Threaded fasteners in an engine can be divided into two general categories: critical and non-critical. Rod bolts, main bolts and head bolts are examples of critical fasteners. Critical fasteners can be identified because the repair procedure for the engine details exact tightening information. Pan bolts, timing cover bolts and valve cover bolts are examples of non-critical fasteners having no detailed fastening procedures. Here, we’ll focus on critical fasteners.

Bolt stretch
Bolts are elastic. When you tighten a critical bolt to specs, you’re actually stretching the bolt. As you stretch the bolt, it wants to return to its original length. Based on the quality of steel used in the fastener, the diameter of the fastener and how far you stretch it, the load or force applied to the joint (the two pieces being fastened together) changes.

Think about this for a minute. If you don’t stretch the rod bolts on the next engine you build, what would keep the rod nuts from vibrating loose and falling off as the engine runs? Yes, most of us have experienced just this sort of problem at some time in our lives!

Bolt load applied to the joint by the fasteners seals a head gasket through head lift-off during firing and changes in temperature that occur as an engine runs. To show you how important this is, I’m going to show you a sample calculation our Victor Reinz engineers used to calculate bolt load needed on an engine:

* General approximation (GA) for clamp load to seal a gasket is three times the lift-off force.
* Lift-off force for a 4.250" bore race motor with 1,400 psi firing pressure is 19,861 lbs.
* GA is 19,861 x 3 or 59,583 lbs. per cylinder. With a 5-bolt pattern, 11,917 lbs. of force is needed per bolt.
* With a 6-bolt pattern, 9,930 lbs. of force is needed per bolt.

This then becomes the initial load needed from each head bolt in order to seal the gasket. Specifying the diameter of the bolts and their tensile strength, the engineer calculates a tightening procedure that will provide the desired load to the gasket. Obviously, I’m leaving some factors out of this basic model. Hardware, cylinder head stiffness and gasket relaxation factors would also be considered and factored into the calculations. But, hopefully, you get the idea.

Yield
This is probably a good time to bring up finite elasticity in fasteners. Unfortunately, every fastener has an elastic limit, commonly referred to as its yield point, or more properly, "the threshold of yield." Up to this point, if the load on a fastener is released, the fastener will spring back to its original length. When a fastener is stretched into the yield zone, some of the elasticity is permanently lost, and the fastener will remain somewhat elongated when the load is removed. The further we stretch the fastener into the yield zone, the more elongation we get.

Many of us have observed severe elongation in fasteners as a "necking down." This occurs in the threaded area (the root diameter of a fastener is smallest in the threaded area), usually about one thread above where the fastener is engaged in the threads of the nut or the block (the threads of the nut or engine block support the fastener resisting yield). As most of you have experienced, if you stretch a fastener far enough into the yield zone, it will actually pull into two pieces.

Occasionally in automotive engine applications, the threads in the block or nut will yield before the fastener does, especially where a large number of rundowns (tightenings) have occurred. However, most of the time the bolt yields first. As you can see from the graph in Illustration 1 maximum clamp load from a fastener comes at the threshold of yield or shortly thereafter. Once a fastener is stretched farther into the yield zone, very little additional clamp load is generated and the risk of ultimate failure becomes greater. Consequently, we’d like to have some means of tightening fasteners to get the elasticity we need for load without yielding them.

Tightening methods
Tightening critical fasteners introduces numerous additional factors into our discussion. Traditional methods have all used some means of measuring the resistance needed to turn the fastener. We’ve all used the most basic of those: "seat of the pants," "experience" or whatever you want to call it. The farther we tightened the fastener, the harder it turned, and experience (some bolts loosening and coming apart and breaking a few bolts off) taught us when to stop. Not real scientific, not very repeatable and probably not too reliable!

Torque wrenches improved this procedure immensely. We use scientific terms like Newton.meters or ft.lbs., to gain repeatability and improve reliability. We continue to rely on torque wrenches today to tighten many critical fasteners. The one thing we need to keep in mind is that we’re measuring resistance to turn.

Friction on bolted joints is the biggest factor causing resistance to turn (Illustration 2). In automotive engines, about 90% of the effort required to tighten a critical fastener is used to overcome friction. Ten percent actually stretches the fastener. This is a fairly standard number for the rigid joints we have in automotive engines. For example, a new fastener lubed both under the head and on the threads may exhibit the 90/10 relationship, while a used fastener or one with damaged threads will be 92/8.

Think about this. The more effort needed to overcome friction, the less stretch we get on the fastener and the less load on the joint. What will happen on a joint with multiple fasteners (like a cylinder head) is load scatter (variances in load from bolt-to-bolt) because of minor deviations from the 90/10 relationship. This load scatter causes uneven loads on head gaskets and may also have a negative affect on bore distortion. What we’d like to do as engine rebuilders is minimize the variances from bolt to bolt as we use conventional "resistance to turn" to tighten fasteners.

Unless specified otherwise, 30W motor oil is the standard lubricant for automotive fasteners. If we want to achieve loads similar to the OEMs. we need to lubricate our fasteners with 30W oil. Don’t forget that underhead and thread friction both need to be controlled, so lubricate both areas. In the case of head bolts going into the water jacket, the sealer on the threads will provide the lubrication needed, so just apply oil to the underside of the head of the bolt. Super lubricants may actually get you in trouble by relieving too much friction, leading to over-tightening.

Also remember that the OE engineer did the development work with new fasteners and new threaded holes (or nuts). We need to approximate that work by chasing threads in the block and using new nuts and (or) bolts when we can. Remember damaged threads will increase resistance to turning (friction) and thus decrease load.

It’s very important to engine builders to control friction variables to their best ability to ensure even load across the joint! As an example: race engine builders routinely use studs with hardened washers for mains and heads. The hardened washer gives a very uniform surface for the nut to turn against and keeps friction variances low.

Torque-to-yield
In the mid 1980s, we started to see a move in engine fasteners to a new process called torque-to-yield (TTY). Head bolts were the first fasteners affected, although the technology has trickled down to other critical fasteners. The theory holds that the farther we stretch a fastener toward the threshold of yield, the more load it exerts on the joint.

Now you might say, "If we want more load, we can always use a bigger diameter fastener." That’s correct. Let’s use our (hypothetical) gasket example from Victor Reinz. We need 11,900 lbs. of load on each bolt. We can get that load by stretching a 7/16" diameter bolt to the threshold of yield or by putting a very moderate load (requiring very little stretch) on a 9/16" diameter bolt. The concern is on a head bolt application is that you get lots of change in the joint. Both gasket relaxation on a new installation, as well as thermal expansion on bi-metal designs will cause changes to the joint dimension once the installation is complete. Head gasket relaxation causes loss of load from the fastener. The less stretch you have on the fastener, the more the loss of load. Let’s work our theoretical example:

* 7/16" fastener stretched .070" equals 11,900 lbs. of load;
* 9/16" fastener stretched .030" equals 11,900 lbs. of load;
* A composition gasket installed at .045" relaxes 25%, for a net loss of .011";
* 7/16" fastener loses 1/7 of the load, leaving 10,200 lbs.; and
* 9/16" fastener loses 1/3 of the load, leaving 7,933 lbs.

As you can see, we’ve got a major sealing issue with the 9/16" fastener. Obviously, it’s a big advantage to keep the fastener diameter small and use maximum stretch to seal engines. Also, keep in mind that the longer in length the fastener is, the more it stretches to get the desired load. Just look at modern engine designs today. We have a predominance of long yet relatively small diameter head bolts. You’ll also notice that on the good designs all the bolts are the same length. This makes only one engineering exercise to do rather than two or three as a tightening theory is developed.

Now, let’s look at the other side of this equation. Our head bolt will be pulled or stretched further than the installation dimension because of the thermal expansion rate of an aluminum head versus a steel bolt (Illustration 3). This can be an issue, especially with a fastener installed at the threshold of yield and a gasket that doesn’t relax (Multi-Layer Steel). On a typical cylinder head operating at 250° F, the head bolt will stretch another .005" or so as the engine reaches operating temperature. This will often result in the fastener being moved significantly farther into the post yield zone. Repeated movement of the fastener into the post yield zone can ultimately lead to work hardening of the fastener and sudden failure (Illustration 4). You may remember the 2.5L GM engine with a head bolt near the exhaust manifold that broke during service. This was a prime example of this problem.

Torque Turn to Tighten
One thing that should be obvious by now is that if we’re going to tighten fasteners to the threshold of yield, we need a better method than measuring resistance to turn. Friction variances could easily get us into trouble.

Fortunately there is a method of tightening a fastener that is much more accurate than measuring resistance to turn. It’s called Torque Turn to Tighten (TTT), often referred to as angle turn. With this method, you use a relative low torque to run down and align the fastener (Illustration 5), then rely solely on a measured turn to tighten the fastener to the desired level. What we’ve done has not affected the friction in our fastener, it has taken it out of the equation when it comes to tightening.

For instance, 90 degrees of turn is 90 degrees of turn; old bolt, new bolt, rough threads, new threads, it doesn’t matter. The amount of stretch will be extremely uniform from bolt-to-bolt across the joint. Load scatter is kept to a minimum.

TTT is a far superior method of tightening critical fasteners regardless of whether you tighten them to yield or not. Fastener engineers use sophisticated mathematical models to calculate the amount of turn needed to get a desired load, but what has really fueled the rapid growth in this area is sophisticated electronic equipment. Sensitive electronic load sensing cells coupled with angle encoders using advanced software programs have allowed engineers to test their theories watching run down curves in real time as they tighten fasteners (Illustration 6).

Fastener quality
Some articles I’ve read indicate that TTY fasteners are somehow "special", metallurgically speaking. If you’re comparing them to the garden variety bolt from your local hardware store, then, yes, they are. If you’re comparing them to other critical fasteners in an engine, then, no, they are not. They’re high-grade fasteners, typically grade 8 for English and class 10.9 for metric applications (Illustration 7).

One bit of confusion is that there are true TTY fasteners (Illustration 8), designed with a reduced shank area (Cummins rod bolts and Porsche rod bolts, for example), and there are standard high-grade fasteners tightened to yield. Both styles are tightened to the threshold of yield; the reduced shank style directs the elongation to the shank, where the others elongate in the threaded area. The second style is much more common in most automotive engines.

A final subject is the relative merits to re-using critical fasteners. If I had a dollar for every head bolt I’ve wire brushed and reused I could afford a pretty nice vacation next year. There are very few of us in this industry that haven’t reused critical fasteners!

However, times change, engines change, technology changes, I’ve changed. My policy is that if new critical fasteners – especially head bolts – are readily available, old ones are replaced. Understanding much more about fasteners and engine operating conditions today, I’m reluctant to reuse them.

A well-respected OE engineer specializing in engines tells me that critical fasteners have about six rundowns in their useful life. They use four of those at the OE manufacturing operations, leaving rebuilders just two. One rundown for checking sizes puts us on the last rundown during final assembly. My thinking is: why take the chance? Replace the fasteners! The relative cost compared to the total engine job is small and the peace of mind is high.

I’d like to thank Ralph Shoberg, President of RS Technology, Ltd. (www.rstechltd.com), and Otto Kossuth of General Fasteners, Inc. These two men gave me a fastener education and a pretty good layman’s view of a complicated subject.

Bill McKnight is Director of Training for Clevite Engine Parts.
<
 

grumpyvette

Administrator
Staff member
http://www.engineproblem.com.au/torque.htm

TORQUE TO YIELD HEAD BOLTS

First service requirements on reconditioned engines, in chassis rebuilds and head gasket replacement.

Back ground:

Traditionally in the past all first engine services included a cylinder head retension. The reason behind this stipulation was very simple and straightforward. Cylinder head gaskets by their nature of construction change thickness due to clamping load changes that occur from a cold engine to a normal running temperature situation.

E.g. If the thickness of the unfitted gasket in a relaxed state measures 2mm. the thickness in its cold assembled state could be decreased to as low as 1.5mm. Once the engine is run up to normal operating temperature extra clamping load will be applied due to heat expansion of the head against the head bolts. After the gasket has been subject to this changed increase in clamping load for a number of times the relaxed state of the compressed gasket will measure considerably less than when first tensioned. This reduced thickness that has developed during the first 800km has now reduced the cold clamping pressure to a point that is unacceptable. To add to the condition we have to also consider a small amount of bolt stretch and recession……………….So the justification for a cylinder head retension has never been in doubt.

Slowly all future production engines will change from using head bolts that are tensioned to a predetermined torque setting to the use of "torque to yield" head bolts.

The reasons behind the change are:
Engine design has reduced the number of head bolts.
Engine design has increased the length of head bolts.
Higher performance outputs require higher clamping loads.
Lighter castings require more consistent clamping loads.

So torque to yield bolts and angled tightening achieve almost all the requirements for a modern engine.

A simple explanation of the difference between "torque bolts" and "torque to yield" head bolts.

Traditional head bolts are tensioned to a predetermined torque measured by a torque wrench. This torque reading is not an accurate measurement of the downward clamping load but rather an accumulated measurement of friction resistance between the two threads and the spot face and bolt head and the clamping pressure. With this system the accuracy and consistency of clamping loads are very unreliable. Each head bolt could have varied amounts of friction created due to differing contact surfaces, thread conditions and bolt contact surface finishes. This method often results in inconsistent and inadequate gasket clamping load.

These bolts are always tensioned up to specifications while still in the elastic phase of tension. (That is where the relaxed bolt still returns back to the original free length.)

As this allows the clamping load to decrease as the head gasket crushes during service a head re-tension at 800km. is always recommended. (alloy cold & cast iron hot.) Some manufacturers stipulate a head retention at every 20,000km. interval due to loss of clamping pressure during service.

Angle tensioning is even a good practice with head bolts that are tensioned within the elastic phase of tensioning. This will eliminate any shortfall caused by friction differences.

Angle tensioning uses a tension wrench to establish a snug torque position for all the head bolts. Snug torque is the term given to the torque applied to establish even direct contact between two components being assembled together. In our industry the head, the gasket and the block face. This position is established at a low torque of around 35ft.lbf. From this equal position, clamping loads can be established equally on every bolt by tightening each bolt a specified number of degrees. This is usually done in a series of steps. This method eliminates any influence on clamping load created by differing amounts of friction present at each head bolt.

This method like the first method reaches the required tension while the bolt is still in the elastic stage. (That is where the relaxed bolt still returns back to the original free length.) As also this allows the clamping load to decrease as the head gasket crushes during service a head re-tension at 800km. is again always recommended.

Now lets talk about the third method that does not look that much different from the angle tensioning method. This method is called tension to yield. (TTY) This method uses a low torque application to establish an equal snug torque position. As in angle tensioning. The main difference in the tension to yield method is the composition of the head bolt and the fact that the required clamping load is established when the bolt is in the plastic stage not the elastic stage.

This is the stage of tightening when you think the bolt is about to break or snap. The difference with this plastic stage is that at this point the bolt may not return to the original length when removed. In the plastic stage not a lot of extra clamping tension is reached but the bolt will tend to hold the required clamping tension even as the gasket crushes in service. This feature has allowed manufacturers to suggest that a head re-tension is not necessary.

If the bolts are replaced or within given specifications a first service head re-tension can be considered to keep the clamping load further into the plastic range of the bolt maintaining maximum clamping pressure for much longer.

Some manufacturers also insist upon TTY head bolts being replaced on every removal while others only require the used TTY bolt to meet a relaxed length specification. While other manufacturers insist upon replacement yet supply a relaxed length specification. A fair amount of confusion still exists in this area.

If in doubt replace is a safe method but be warned these bolts are not always available. The after-market parts suppliers have just upgraded their range in 2002 to a level that covers the most common applications.

Considering all of the above and after consulting some of the largest re-manufacturers in Australia our conclusions are that UMR ENGINES will remove the head re-tension as a requirement of the first service on engines with TTY head bolts. Every other UMR Engine application where the head bolts are tensioned in the elastic stage only will still require head re-tensioning.
 
Top