Torque-to-yield (TTY) bolts, also referred to as angle torque or stretch bolts, are fasteners designed to be tightened beyond their elastic range and right into a controlled yield zone. Their job is to use a really precise and consistent clamping load and keep it there.
All bolts stretch when tightened. In a standard bolt, that stretch stays inside the elastic phase. TTY bolts have a distinct working principle. Their tightening continues past the elastic limit, which causes them to physically deform. At this point, the bolt is permanently stretched and won’t return to its original length.
To succeed in this controlled stretch, TTY bolts will not be tightened to a final torque figure. As an alternative, they’re delivered to a decent base value to seat the components, then turned a specified variety of degrees. Since the stretch is everlasting, torque-to-yield bolts are single-use fasteners. Once removed, they need to get replaced, making them one in every of those automobile parts you need to never low-cost out on.
Why automakers use torque-to-yield bolts
The design of engines is what led to the widespread use of torque-to-yield bolts. Prior to now, engines commonly combined iron blocks with aluminum cylinder heads (though newest engines have moved to aluminum blocks completely) and multi-layered metal head gaskets. Normal fasteners cannot maintain a consistent head gasket seal due to quicker expansion rate of aluminum over iron.
TTY bolts are engineered to operate near their yield zone, essentially turning them into very long, controlled springs. This controlled stretch allows the bolt to raised accommodate different thermal expansion rates while maintaining a more uniform clamping load across the block.
Torque-to-yield fasteners allow engine designers to attain higher clamping forces with smaller bolts. Since these stretch greater than larger ones, the consistency of clamp load can be improved, allowing the identical job to be done by fewer fasteners. This provides designers more room to play with the cylinder head and block layout.
What this implies for engine repair and assembly
While manufacturers could have quite a bit to achieve from the usage of torque-to-yield bolts, mechanics must adhere to strict protocols due to them. For one, reusing a torque-to-yield bolt is a definite no-go. This might cause the clamping force to cut back to the purpose of completely failing. It’s because, unlike conventional bolts, TTY bolts wear out, with everlasting 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 should be followed to a tee. An initial torque value is used only to ascertain a uniform baseline. All meaningful load is applied throughout the subsequent angle-tightening steps. A top 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 due to its failures. Dirty or corroded threads increase friction and reduce effective clamping force. Bolt holes needs to be cleaned, threads needs to be calmly oiled unless otherwise specified, and sealant needs to be applied where bolts enter water jackets.
This Article First Appeared At www.jalopnik.com
