What Are Torque-To-Yield Bolts And Why Do Automakers Use Them?

Torque-to-yield (TTY) bolts, also known as angle torque or stretch bolts, are fasteners designed to be tightened beyond their elastic range and into a controlled yield zone. Their job is to apply a very precise and consistent clamping load and keep it there.

All bolts stretch when tightened. In a conventional bolt, that stretch stays within the elastic phase. TTY bolts have a different working principle. Their tightening continues past the elastic limit, which causes them to physically deform. At this point, the bolt is permanently stretched and will not return to its original length.

To reach this controlled stretch, TTY bolts are not tightened to a final torque figure. Instead, they are brought to a tight base value to seat the components, then turned a specified number of degrees. Because the stretch is permanent, torque-to-yield bolts are single-use fasteners. Once removed, they must be replaced, making them one of those car parts you should never cheap out on.

The design of engines is what led to the widespread use of torque-to-yield bolts. In the past, engines commonly combined iron blocks with aluminum cylinder heads (though most modern engines have moved to aluminum blocks completely) and multi-layered metal head gaskets. Normal fasteners cannot maintain a consistent head gasket seal because of the quicker expansion rate of aluminum over iron.

TTY bolts are engineered to operate close to their yield zone, essentially turning them into very long, controlled springs. This controlled stretch allows the bolt to better accommodate different thermal expansion rates while maintaining a more uniform clamping load across the block.

Torque-to-yield fasteners allow engine designers to achieve higher clamping forces with smaller bolts. Since these stretch more than larger ones, the consistency of clamp load is also improved, allowing the same job to be done by fewer fasteners. This gives designers more room to play with the cylinder head and block layout.

While manufacturers may have a lot to gain from the use of torque-to-yield bolts, mechanics need to adhere to strict protocols because of them. For one, reusing a torque-to-yield bolt is a definite no-go. This might cause the clamping force to reduce to the point of completely failing. This is because, unlike conventional bolts, TTY bolts wear out, with permanent changes to their elastic properties occurring once they're tightened past their yield point.

TTY bolts are tightened in a specified sequence throughout multiple stages, and thus, the manufacturer's installation procedure must be followed to a tee. An initial torque value is used only to establish a uniform baseline. All meaningful load is applied during the subsequent angle-tightening steps. A quality torque wrench and an accurate angle gauge are essential.

Thread condition matters, too. Just ask Cadillac's Northstar engine, which got a seriously bad rap because of its failures. Dirty or corroded threads increase friction and reduce effective clamping force. Bolt holes should be cleaned, threads should be lightly oiled unless otherwise specified, and sealant should be applied where bolts enter water jackets.