Bolt Engineering – Torque to yield and Torque/Angle Tighten (TTY & TAT)

27 Dec 2005 Bolt Engineering – Torque to yield and Torque/Angle Tighten (TTY & TAT)

More great information on bolts and torque from Clare Snyder on the airsoob list:

I’m not going to argue – but I’m not the only one who dissagrees with you.
Google “Torque to yield” and you will find a LOT of informantion. A small
sampling follows.
How about the following:

The secrets of bolt tightening
by Jim Kerr

The way cars are put together has changed. Adhesives, plastic clips and even
Velcro have replaced most of the screws and trim nails used to hold panels
together in the past. Bolts, however, are still used for assembling most of
the mechanical components, but even bolts have changed too. Many engines now
use torque-to-yield bolts to connect major assemblies.

Torque-to-yield bolts are exactly what they sound like. They stretch to the
yield point as they are tightened. These bolts are commonly used where exact
clamping loads are required on parts. You may find them used to hold
cylinder heads, connecting rods, crankshaft main bearing caps, flywheels and
front engine dampers. When a shop manual indicates that a bolt is to be
discarded and new ones used, they are almost always torque-to yield bolts.

Buying new bolts every time one is removed seems expensive and a waste of
time. After all, the old bolts still look good, but looks can be deceiving.
Reusing the old bolts can cause expensive engine failure. During the
tightening process, the bolts have been pulled to their elastic limit. The
bolts actually stretch. Only new bolts can provide the even clamping force
needed on today’s engines.

And:

: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.

And, from a Volvo list:

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 precedure 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
am posting this here now]

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
clampimg 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…

‘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 to the point of over stretching 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 the
problem. With the use of aluminum cylinder heads, the needs for clamp
changed. Instead of a maximum clamp applied 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. [a statement that will probably earn me some
misapprobation]

As stated earlier, the tightening procedure 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 the case
either. 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. Combine the
appearance with the angle tightening and the reasonable conclusion to be
drawn is that they are indeed TTYs.

BUT, 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 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. It feels like you have started to pull the threads. I have
overtightened bolts on other things; I KNOW the feeling of the yield.

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; and I have stated my reasons. I will continue to use
the new style bolts. BECAUSE they are better than the old style bolts.

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.