Many current engine designs use bolts that are called "Torque To Yield". As the name implies, these bolts are designed to stretch a certain amount when tightened correctly. During the Heat Cooling Cycle these bolts often take a permanent set and are not capable of regaining the correct torque if re-used. It is recommended that on all engines requiring Torque To Yield Bolts that the bolts are replaced every time the head is removed. Refer to the factory manuals for the correct tightening specifications for each engine. SBI sells TTY Bolts in engine sets for 4 cylinder engines because they normally have only one cylinder head. In the case of the "V" style engines where only one head may have been removed we sell head bolts in single head packs. This means that in most applications two sets will be required for a complete engine. The Notes column of the catalog application section will indicate how many sets are required for that engine.