Torque-to-Yield Bolts (TTY)
Fasteners designed to stretch elastically and plastically when torqued, and must be replaced after a single use.
Torque-to-Yield (TTY) bolts, also known as stretch bolts, are fasteners designed to be tightened beyond their elastic limit into their plastic deformation zone. During the final tightening steps, the bolt stretches permanently to apply a highly consistent clamping force across the joint. TTY bolts are widely used for cylinder head bolts, crankshaft main caps, and suspension subframe mounts.
Because TTY bolts stretch plastically and undergo permanent physical deformation when torqued, they lose their tensile strength and elasticity if loosened. Reusing a TTY bolt will lead to joint failure because the bolt cannot stretch properly a second time, resulting in insufficient clamping force. If a reused cylinder head bolt fails, it will cause head gasket leaks and engine overheating.
When replacing head gaskets or rebuilding engines, technicians must always purchase and install brand-new TTY bolts. The tightening procedure is highly specific, requiring the technician to torque the bolt to a baseline inch- or foot-pound value, followed by turning the bolt a specific number of degrees (e.g., 90 degrees plus another 90 degrees) using a torque angle gauge to achieve the correct stretch.