To maintain a tight leak-free seal, the bolts must the tightened in the proper sequence and torqued to specifications
In an engine with four inch cylinder bores and peak combustion pressures of around 1,100 pounds per square inch, each cylinder exerts about 13,827 lbs. of pressure against the cylinder head at full throttle. In fact, head bolts may have to handle loads of more than five tons per bolt at wide-open throttle!
The clamp load that is typically required to keep the head gasket sealed under these operating conditions is about three times the peak pressure exerted against the head (this is called the "lift-off" force). As a result, the bolts around the combustion chamber have to exert a combined force of about 41,500 lbs. to hold the head in place. If each cylinder has four head bolts around it, each bolt has to exert a clamping load of 10,375 lbs. If there are five head bolts per cylinder, the load needs to be 8,300 lbs. per bolt. If there are six bolts per hole, then the load required drops to 6,916 lbs. per bolt.