HOME

Torque-to-yield

Head Bolts & Clamp Load

The whole idea of using Torque-to-yield (TTY) heads bolts is to keep a full-time clamp load on the gasket. It is understood that if the engine loses the clamping force, the head gasket will not live for very long. The clamp load is the pressure that holds the heads, gasket, and block together. This load is provided solely by the bolts and the bolt threads.

The new technology in head gasket materials does not require re-torquing, but does require a full time clamp load. Without this load the combustion process is not contained and will destroy the fire ring as well as the body of the gasket. The full time clamp load is accomplished by using Torque-to-yield bolts. TTY bolts operate like springs. They have the ability to stretch when expansion occurs and recover when the engine cools down.

It is critical to tighten TTY bolts to the torque recommended by the manufacturer; in our case ARP. The proper load is the point just before the bolt starts to stretch. Once stretched beyond it’s limit, the bolt will break, as many of us have experienced. Consequently this it true of any bolt, too little torque will result in a poor seal and head gasket failure.

Any bolts stretched beyond 0.050" should be discarded. Head bolt tolerances are very tight so a micrometer or a thread gauge are two methods that can be used. Simply threading it into the block or a nut is not an appropriate test.

TTY bolts are important on all engines but are more critical on bi-metal engines, aluminum heads and iron blocks.

To better understand the importance of torque load, let’s look at where the torque goes. In this example let’s use the torque spec. of 75 ft/lbs. The entire 75 ft/lbs. does not apply to the clamp load as most believe. Only 10% of the total is used for clamping, the rest is wasted on friction between the bolt, the threads and the bearing surface (50% to bearing surface and 40% to the threads). The underside of the bolt head that contacts the head is referred to as the bearing surface. With this in mind; dirty or damaged threads can absorb enough of the total torque value to result in very little or no clamp load. This results in a wide load variation on different areas of the gasket, an in some instances no load.

The best procedure is to run a bottoming tap in each hole. This can be done quickly and will save the gasket or much more, like your engine, in the long run. The Buick Turbo V-6 engine would require a 7/16"-14 thread tap to clean out the bolt holes in the block. While your at it, don’t forget the intake manifold hole in the heads themselves. To further overcome the friction at the bearing surface and the threads, 30W motor oil or the manufacturers own lubricant will assist in extracting maximum performance from your fasten-ers. It is also important not over oil your fastener going into blind holes. A blind hole has a bottom and too much oil at the bottom can cause a hydraulic effect between the bolt and the bottom of the hole. The result would be a falsetorque reading.

There are a few other factors that effect the head gasket’s life, but improper clamp load is one of the major contributors to head gasket failure. Thousands of head gaskets are produced per run and all with the same technique, material, and method. A vast majority last the life of the engine. Turbo Regals are a special segment and so will require even more attention by the installer. Some of you may be familiar with a complaint circulating about Fel-Pro 1000 head gaskets. There is a good chance that the gaskets are fine.

Too many times head gasket failures are blamed on self-destructing gaskets when the responsibility is often insufficient clamp load.


Tech Tips
	Torque-to-yield Head Bolts & Clamp Load The whole idea of using Torque-to-yield (TTY) heads bolts is to keep a full-time clamp load on the gasket. It is understood that if the engine loses the clamping force, the head gasket will not live for very long. The clamp load is the pressure that holds the heads, gasket, and block together. This load is provided solely by the bolts and the bolt threads. The new technology in head gasket materials does not require re-torquing, but does require a full time clamp load. Without this load the combustion process is not contained and will destroy the fire ring as well as the body of the gasket. The full time clamp load is accomplished by using Torque-to-yield bolts. TTY bolts operate like springs. They have the ability to stretch when expansion occurs and recover when the engine cools down. It is critical to tighten TTY bolts to the torque recommended by the manufacturer; in our case ARP. The proper load is the point just before the bolt starts to stretch. Once stretched beyond it’s limit, the bolt will break, as many of us have experienced. Consequently this it true of any bolt, too little torque will result in a poor seal and head gasket failure. Any bolts stretched beyond 0.050" should be discarded. Head bolt tolerances are very tight so a micrometer or a thread gauge are two methods that can be used. Simply threading it into the block or a nut is not an appropriate test. TTY bolts are important on all engines but are more critical on bi-metal engines, aluminum heads and iron blocks. To better understand the importance of torque load, let’s look at where the torque goes. In this example let’s use the torque spec. of 75 ft/lbs. The entire 75 ft/lbs. does not apply to the clamp load as most believe. Only 10% of the total is used for clamping, the rest is wasted on friction between the bolt, the threads and the bearing surface (50% to bearing surface and 40% to the threads). The underside of the bolt head that contacts the head is referred to as the bearing surface. With this in mind; dirty or damaged threads can absorb enough of the total torque value to result in very little or no clamp load. This results in a wide load variation on different areas of the gasket, an in some instances no load. The best procedure is to run a bottoming tap in each hole. This can be done quickly and will save the gasket or much more, like your engine, in the long run. The Buick Turbo V-6 engine would require a 7/16"-14 thread tap to clean out the bolt holes in the block. While your at it, don’t forget the intake manifold hole in the heads themselves. To further overcome the friction at the bearing surface and the threads, 30W motor oil or the manufacturers own lubricant will assist in extracting maximum performance from your fasten-ers. It is also important not over oil your fastener going into blind holes. A blind hole has a bottom and too much oil at the bottom can cause a hydraulic effect between the bolt and the bottom of the hole. The result would be a falsetorque reading. There are a few other factors that effect the head gasket’s life, but improper clamp load is one of the major contributors to head gasket failure. Thousands of head gaskets are produced per run and all with the same technique, material, and method. A vast majority last the life of the engine. Turbo Regals are a special segment and so will require even more attention by the installer. Some of you may be familiar with a complaint circulating about Fel-Pro 1000 head gaskets. There is a good chance that the gaskets are fine. Too many times head gasket failures are blamed on self-destructing gaskets when the responsibility is often insufficient clamp load.