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Though head bolts seem to be pretty rigid fasteners, they are actually designed to stretch. Stretching a bolt is like stretching a rubber band. It allows the fastener to exert force against a surface to hold the parts together. This is especially important with gasketed surfaces because the gaskets have a certain amount of give that must be overcome by the fasteners to maintain a tight seal.

Head bolts are truly one of the most critical fasteners in any engine. Consequently, it's extremely important that all the head bolts be in perfect condition and are installed and lubricated properly so they are not overloaded.

When new head bolts are used, some experts recommend "cycling" the bolts several times to improve loading accuracy. Run the bolts down to about 50 percent of their final torque value several times before finally tightening them to their full value.