While bolts appear very rigid, they are designed to stretch slightly to provide clamping force on the threads. A head gasket is designed to give, or squeeze, slightly to ensure the seal between the head and engine is sufficient. Torque, as a specification for head bolts, refers to the amount of rotational force required to tighten the bolts properly. Torque is measured in foot-pounds.
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Manufacturers provide torque specifications to guide mechanics in properly tightening the bolts connecting the cylinder heads to the main engine. Improper tightening of the bolts on a cylinder head results in uneven distribution of tension across both the bolts and the head gasket. Inadequate seals may result in gasket failure, whereas bolts that are too tight may lead to failure of the threads holding the cylinder head to the engine. Over-tightening cylinder head bolts may lead to stripped threads, damaged gaskets or cylinder head damage.
Torque is typically measured in foot-pounds for vehicles available in the United States. Torque is a measure of the rotational force required to properly secure the bolt in the bolt hole. The foot-pound measure refers to the amount of pressure in pounds required to rotate the bolt from a point one foot away from the pivot point. For example, if a bolt is torqued to 50 foot-pounds, it will take 22.7 Kilogram of force--from a leverage point one foot from the bolt--to rotate the bolt. By providing a uniform measure of force required to turn the bolts to proper tightening, manufacturers are able to ensure mechanics provide sufficient tension on the bolts without damaging the cylinder head, bolt holes or gasket.
A torque wrench is a special wrench that measures the amount of force required to turn a bolt. Typically available with 3/8 inch or 1/2 inch socket ends, the torque wrench allows you to attach the socket or socket extension directly to the wrench to measure how much torque is on the bolt while being tightened. The torque wrench will either use a spring measure or click measure to show when the torque has reached the desired amount. Click-measure torque wrenches are generally more accurate.
The tightening sequence on a cylinder head is a critical part of ensuring your cylinder head is tightened in a way that evenly distributes the tension and pressure across the cylinder head correctly. Many cylinder heads require multiple torque sequences to achieve proper tension. For example, a 10-bolt cylinder head may require you to tighten them all first to 42 foot-pounds in a specified order, then to 75 foot-pounds in the same order. Improper sequencing or torquing of the bolts may result in damage.
The torque sequence and actual foot-pound requirements are based on the engine type, material construction and gasket type. No single torque spec is available for all engine types. Consult online resources, a mechanic or automotive repair manual for specific torque sequencing and foot-pound requirements.
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