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.
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.{Nulon L90 } 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.{ 3j Permatex }
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.The tightening specification for the RF - R2 Mazda-Ford Engine is as follows:
uniformly tighten in sequence to 30Nm- 22ft lbs
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.
