Here's some good info explaining the difference between torque to yield and torque plus angle.
How to Identify an Engine with Torque-To-Yield Head Bolts
Early TTY bolts were easy to identify by their undercut shank. Since then, many new TTY designs have been employed. Many of these do not use an undercut shank and therefore cannot be easily recognized as TTY. The most accurate method of determining a TTY engine is based on the head bolt tightening procedure outlined by the manufacturer. If the installation procedure requires tightening the bolt to an angle (vs. torque) specification, the engine requires new TTY head bolts. If torque values are used for tightening, new bolts are not required.
TORQUE - TO - YIELD
CYLINDER HEAD BOLTS
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 manufacturers 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.