Getting to the precise yield zone (for maximum clamping force) is accomplished by tightening bolts to a certain torque spec, then turning the bolts a pre-determined number of degrees. The bolts are stretched into their elastic range, and in many cases, the stretching approaches the bolts' elastic limit, permanently stretching it. Once they reach this zone, they maintain a more precise and consistent level of clamping force across the entire head-to-block mating surface. Used by many manufacturers, especially on engines with aluminum heads and in conjunction with Multi-Layer Steel (MLS) head gaskets, T-T-Y (Torque-To-Yield) head bolts are engineered to stretch within a controlled yield zone. Remember, bolt stretch is how we get maximum clamping load. Unfortunately, using a larger diameter bolt is not the answer, as the larger a bolt it is, the less it will stretch.
Today’s engines require higher clamping forces (due to increased combustion pressure) which cannot be achieved with conventional small diameter bolts normally found in these engines. This torque specification applies the proper clamp load to the joint while accounting for friction provided by the threads and the underside of the bolt head. Traditionally, a torque specification is a calculated number. This can be overcome by designing and offering better fasteners to address these conditions. However, conventional bolts can be an issue when they are used on aluminum heads, given that the expansion rate of this type of head can stretch typical bolts past their yield point. As long as the bolt is not stressed beyond its proof load (the maximum load a bolt can withstand and still behave in an elastic manner), it will return to its original length once the torque is relieved.
Bolts are elastic by nature, meaning that within their elastic range, they'll stretch as the load on the bolt increases.
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