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Automotive engineering has come a long way over the years. Although the American V8s we love have the look, sound, and performance of storied classic powerplants from the heyday of the muscle-car era, the engineering that goes into them is about as different as a stagecoach and a spaceship.
Even seemingly simple components like bolts have undergone radical transformation over the years. While the fasteners holding together, for example, an LS motor may look about the same as those of a 1950s or ‘60s small-block Chevy, they are in fact radically different. Today’s fasteners are often made from specialized materials that demand their own exacting techniques for using them. This is especially true of torque-to-yield (TTY) fasteners. These specialized fasteners require unique procedures, which can vary considerably between engines, manufacturers, and the components they’re used on.
The key difference between TTY fasteners and conventional fasteners is elasticity. Whenever a fastener, such as a bolt, is tightened some degree of stretching and lengthening occurs. A conventional bolt has a small yield zone – the torque-value range needed to provide sufficient clamping – and it only requires a quick snug to produce its clamping force. It typically operates in an elastic manner, not reaching the point of permanent yield. A conventional bolt returns to its original length and overall diameter when it’s removed so it’s reusable.
Torque-to-yield fasteners are designed to perform closer to or within their permanent yield zone. TTY fasteners are engineered to be tightened into the yield zone. Compared to conventional fasteners TTY fasteners are stretched more, and a more consistent clamping load is realized. For a bolted joint, applying and maintaining, in operation, a specific clamping load is of vital importance.
Yield zones in TTY fasteners are designed into a defined area in the shank of the fastener and the preload achieved is determined by the material used, heat-treating techniques employed, thread design, diameter, and the geometry of the fastener. These specs vary from application to application and the TTY fastener is tightened to and operates in the permanent yield zone. Once, by design, a TTY fastener is stretched beyond its elastic zone and into permanent yield, it won’t return to its original length/diameter and cannot be reused.
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Expansion rate is the bone of contention here. Since the expansion rate of different metals will vary, TTY fasteners are commonly found in engines where different metals are working in close proximity. The most popular application is cylinder heads, when an aluminum head is installed on an iron block using a multi-layered metal head gasket for proper sealing.
As the engine warms up, the metals expand at different rates. Aluminum expands at nearly twice the rate of iron, opening the door to sealing challenges and head lift. The TTY bolt’s specific shank geometry and diameter allow it to handle the differing expansion rates. Being tightened to a specific permanent yield condition allows TTY bolts to maintain a specific clamping load on the engine block/head junction point. Having the correct clamping load at the engine block/head interface will provide proper sealing of the head gasket without creating cylinder bore distortion that can diminish piston ring seal.
This high amount of expansion can take a conventional bolt to and beyond its yield failure point which will produce gaps between the head and block and compromise the head gasket’s ability to seal properly. It should be noted that mixed material iron-block engines also experience some expansion as the different metallurgical makeup of the head and block and the use of multi-layered metal head gaskets must be accounted for. Suspension components are also a popular place to find TTY fasteners. Extensive use of aluminum in ball joints, tie rod ends, and steering knuckles typically make TTY fasteners the go-to in these areas.
Following manufacturer guidelines during removal and installation is critical to realize the expected performance of TTY fasteners. This relates to both the sequence or order in which fasteners are loosened or tightened and the staged torque values at each step. The engine should be fully cool before work begins.
Typically, a TTY fastener has an initial seating torque spec, followed by additional steps that each tighten the fastener by a specified angle turn. It should be noted, this amount of effort would snap a conventional fastener because of its smaller yield zone. The initial tightening is performed with a torque wrench and the additional steps are accomplished using a torque angle gauge. This is called the torque-angle method, but some fasteners have different installation procedures, so follow the appropriate directions carefully.
It’s also wise to ensure that threads are free from dirt, corrosion, thread-locking compounds, or anything else that could bind and possibly produce false torque readings. At the same time, be wary about over-oiling the bolt threads; doing so may hydro lock the bolt and give false torque readings. It’s also critical that any bolts passing through a water jacket in the head have a sealer applied to the threads to prevent leaks and corrosion.
The need to replace a TTY fastener usually arises from removing the parts that the fasteners join together – a head swap, front-end rebuild, or work done on any component system that uses TTY hardware. In some cases, modifications can push the stock TTY hardware beyond its load capacity. There are replacement TTY bolts, hybrid TTY fasteners, and conventional-type fasteners made from super strong material better suited to handle expansion. Hybrid TTY designs can be reused because they have a safety margin engineered into them, a range within the yield zone that can be reached in subsequent uses.
Mr. Gasket and Earl’s Racing Products offer a wide array of head-bolt kits and related products, available in hex-head or 12-point-head configuration. These kits take a different approach to the yield zone. Thanks to their use of exotic materials, the bolts are strong enough to withstand the rigors of expansion and are reusable. These kits incorporate head-bolt washers to produce a more equal distribution of torque load across the bolt while protecting the mounting surface, which is usually aluminum, from galling or marring.
For example, Earl’s head bolts are constructed from top-quality 170,000 psi chromoly which is approximately 15% stronger than Grade-8-rated hardware. These bolts have a reduced wrench size for extra socket clearance, which is especially useful for working around rocker arms when re-torquing heads. The bolt’s wide-flanged head provides more support for the kit’s extra thick chromoly washers.
Torque-to-yield fasteners are an integral part of modern automotive systems. Knowing the correct way to choose and install them will give you the results you’re looking for. By understanding these sophisticated components, your car can deliver the performance you crave, with the reliability you need.