Saturn head bolts are designed to stretch. The stretching allows the fastener to exert force against a surface to hold the parts together. This is critical with gasketed surfaces because the gaskets have a certain amount of give that must be overcome by the fasteners to maintain a tight seal. Like a rubber band, a Saturn head bolt under load will stretch and spring back to its original length when the load is released. However, if it stretched so far the bolt will either become permanently elongated or break. A head bolt exerts its maximum clamping force when it is stretched to its "yield point." This is the point where the bolt can stretch no further without being permanently deformed.
Torque-To-Yield (TTY) is one special type of head bolt. They are typically used on engines with aluminum cylinder heads (where there is a lot of thermal expansion) and with multi-layer steel (MLS) head gaskets. TTY head bolts are designed to deform, in a controlled way and can not be reused.
It s extremely important that all the Saturn head bolts be in perfect condition and are installed and lubricated properly so they are not overloaded. Installing head bolts dry can lead to trouble because the increased friction in the threads can reduce clamping force up to 25 percent or more. Inspection of head bolts at the top of the threaded area or in the shank can help in revealing if they have too be replaced already. Equally important is to remember not to reuse deformed head bolts because this has been proven to be risky and dangerous. Recycled head bolts may not hold proper torque and may eventually fail if retightened. Moreover, head bolts that are too stretched may break or deform and cause head gasket leakage.
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