Torque to Yield Head Bolts
As technology improves, more efficient methods are utilized in the design and construction of modern engines. Many vehicle manufacturers now use torque to yield bolts, or what are commonly known as stretch bolts in engine assembly. Torque to yield bolts have many advantages over conventional bolts. The specialized method of tightening these bolts allows them to be more efficient at clamping. Engine reliability is achieved because the assembly of the engine is far more accurate. Manufactures also prefer these types of bolts as fewer head bolts are required to achieve desired clamping loads. This leads to reductions in component costs and greater flexibility in cylinder head and block design.
Bolts are elastic by nature, meaning that within their elastic range, they’ll stretch as load on the bolt increases. As long as bolt is not stressed beyond its proof load (the maximum load a bolt can withstand and still behave in an elastic manner) it will retract if the torque is relieved. Traditionally, torque specs are calculated to keep fasteners within their elastic range because maximum clamp load is achieved when the bolt reaches its elastic limit, or “yield” point.
Typical bolts can be an issue when you use them on aluminum heads and with a gasket that doesn’t relax, like an MLS gasket. The expansion rate of aluminum heads will stretch typical bolts past their yield point and can snap them. Plus newer engines require high clamping forces (due to increased combustion pressure) which cannot be achieved with the smaller diameter bolts normally found in engines.
Unfortunately, using a larger diameter bolt is not the answer, as the larger a bolt it is, the less it will stretch. Remember bolt stretch is how we get maximum clamping load.
Since Torque-To-Yield head bolts are designed to stretch, reusing them can cause improper or uneven torque and clamping force. Stretched bolts can damage threads in the engine, especially on aluminum blocks, and since the bolts are weakened, they may break if retorqued. You should always replace the bolts.