Torque to yield bolts, also commonly referred to as angle torque or stretch bolts, are used in many of today’s modern engines, predominantly for cylinder head bolts but also main bearing caps. Torque to yield bolts, also commonly referred to as angle torque or stretch bolts, are used in many of today’s modern engines, predominantly for cylinder head bolts but also main bearing caps. Compared to conventional type bolts, torque to yield head bolts offer the engine manufacturer a number of advantages: greater flexibility of design reductions in component costs more accurate assembly and reliability of seal Engines designed utilising torque to yield (TTY) heads bolts require fewer head bolts to achieve the desired clamping loads than those of conventional bolts. With fewer bolts the engine manufacturer has more flexibility in the cylinder head and block design as well as reducing the cost of the engine. While torque to yield head bolts are attractive to the engine manufacturer there are disadvantages to the engine repairer of which we all need to be aware. For most of us it would be unthinkable to replace a conventional head bolt unless the bolt was damaged: i.e. stripping threads, the bolt head was rounded off, the shank was severely corroded or pitted. Conventional head bolts simply just did not wear out. Torque to yield head bolts however, by the very nature of their design do wear out and should never be reused. Under the application of load, all bolts exhibit four main phases: the