Cylinder Head Torque Specsby Paul Novak
Automobile engines are subjected to intense pressures and stresses created by the process of combustion taking place within the engines' cylinders. In order for the engine to operate reliably, the parts making up the engine must be securely fastened in order to contain these pressures and stresses. Cylinder heads seal the combustion chambers and are subject to some of the most extreme pressures, so the bolts securing them must be torqued to the proper specs.
Cylinder heads are clamped to the engine block according to measurements indicated in "foot-pounds." Foot-pounds refer to the amount of torque applied to the bolts during tightening to produce the clamping force that holds the cylinder head to the engine block. The greater the amount of torque applied to the bolts, the greater the amount of clamping force created. The amount of torque applied to a cylinder head bolt is measured using a torque wrench, which indicates the amount of force applied to the bolt during tightening.
The amount of torque required to secure a cylinder head to an engine block varies from one type of engine to the next. The amount of clamping force and therefore torque needed is determined by the amount of cylinder pressures that will be created by the engine. Each cylinder head bolt creates only one part of the total amount of clamping force necessary to seal the cylinder heads to the engine block. To determine how much torque is needed for each bolt it is necessary to divide the number of bolts by the total amount of cylinder pressure the engine produces. If an engine produces 1,500 pounds of pressure and the cylinder head has 8 bolts, then each bolt must be tightened to 187 foot-pounds of torque to secure the cylinder head to the block.
In order for torque to be evenly applied across the sealing surfaces of the cylinder heads and engine block, the cylinder head bolts must be carefully tightened in an incremental manner. In most applications it is necessary to first hand-tighten each bolt snugly until the head of the bolt is flush with the cylinder head surface. The bolts are then partially tightened to approximately half of their recommended torque loading in the order specified by the particular engine's manufacturer. The process is then performed once again in the same order to bring all of the bolts to their specified torque loading.
Every engine has a specific bolt-torquing pattern that must be followed when tightening cylinder heads. This pattern helps to evenly apply the clamping force along the surfaces where the cylinder head meets the engine block. If this pattern is not followed or the bolts not tightened evenly, the result can be weak spots where cylinder pressure can escape, or warping of the cylinder head surfaces. Either warping or uneven tightening can result in catastrophic failure of the cylinder head gasket and severe engine damage.
Cylinder heads must be tightened to specific torque loads. If too much torque is applied, the cylinder head surfaces may become warped and allow burning gases to escape past the cylinder head gasket. Too much torque can cause bolts to become loaded past their limits when cylinder pressures in the engine rise, resulting in catastrophic failure of the bolts and thus the seal between the cylinder head and the engine block. If too little torque is applied there will not be enough clamping force generated to seal the cylinder heads to the engine block and combustion chamber gases will escape past the head gasket.
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