When you build your engines, you put measured thought into every component you use. From the intake system to the exhaust manifold, everything has its place and nothing is installed without careful consideration.
But what about the fasteners? Some of the most critical components are held together with relatively low-cost fasteners including screws, nuts and bolts, and studs. No matter whether you’re building a restoration showpiece, a high horsepower street machine or a stump-pulling torque monster, the fasteners you use are more critical than you may believe.
“Fasteners are typically the last thing anyone thinks of, but that’s changing,” says Mike Mowins, president of Global Licensing for the Phillips Screw Company. “You’ve obviously got some ‘cookbook standards’ that you can go back to, but quite often guys who are building custom engines – to get ultra-high performance – do stuff similar to what you see in the aerospace world.”
Consider this: the head bolts have to withstand tremendous loads to keep the cylinder head sealed tightly against the head gasket and block. In an engine with four-inch cylinder bores and peak combustion pressures of around 1,100 pounds per square inch, each cylinder exerts about 13,827 lbs. of pressure against the cylinder head at full throttle. In fact, head bolts may have to handle loads of more than five tons per bolt at wide-open throttle.
The clamp load that is typically required to keep the head gasket sealed under these operating conditions is about three times the peak pressure exerted against the head (this is called the “lift-off” force). As a result, the bolts around the combustion chamber have to exert a combined force of about 41,500 lbs. to hold the head in place. If each cylinder has four head bolts around it, each bolt has to exert a clamping load of 10,375 lbs. If there are five head bolts per cylinder, the load needs to be 8,300 lbs. per bolt. If there are six bolts per hole, then the load required drops to 6,916 lbs. per bolt.
In a performance engine or diesel the loads are even higher. With peak combustion pressures of up to 1,400 psi or higher in a performance engine, or 2,400 psi in a diesel, the load on the head bolts is far greater. Consequently, the clamping force on the head bolts is even more critical than those in a stock engine.
A leak-free seal may be the most obvious goal for a customer, but engine builders should think beyond this, says Mowins.
“Not leaking (lubricant, coolant or fuel) is, of course, critical because if you lose one liquid seal you lose the engine, but effective containment doesn’t usually improve performance,” he explains. “If you can make components lighter, smaller and more robust you can create additive performance benefits. Lowered mass of con-rod bolts lowers inertial resistance and increases output. Lower head heights on casing bolts allows closer packing of components and lighter components, again boosting performance. More robust bolt and screw drive systems provide improved assembly and removal minimizing time to assemble and dis-assemble. High performance thread forming screws can tap their own threads in cast holes in the block eliminating nuts. We’re seeing some very high strength, very great capability and exotic materials being discussed in the automotive world.”
Different materials and designs have different advantages in different applications, and selecting the right fastener for the job may be difficult at best when choosing from such a wide array of materials. Variables such as strength, temperature, movement, vibration, and fatigue all come into play when deciding on the right bolt or stud. Not all applications are the same and not all fasteners are appropriate just because they may fit.
Here are some terms you may be (or should be) familiar with.
Tensile strength is the most common mechanical property that is referred to when talking about fastener strength. It is the maximum tension-applied load the fastener can support before it fractures.
Fatigue in a fastener can cause sudden, unexpected failures. A fatigued fastener can fail even when loads are below the strength of the material due to operating under constant cyclic loads. Fatigue strength is often defined as the maximum stress a fastener can withstand for a specified number of repeated cycles before it fails.
Some of the most important fasteners in the engine such as cylinder head bolts, connecting rod bolts and main bearing and cap bolts are subject to fatigue forces. It is important to use fasteners with high fatigue strength, as well as the tensile strength, to hold the joint together under high-pressure forces in these applications.
Torsion strength is typically the amount of torque or friction a fastener can safely handle before it breaks. One thing to remember is that when torque is applied to a fastener, most of the input is spent in overcoming friction. Roughly 85-95 percent of the energy you have spent tightening the fastener is lost, leaving only about 5-15 percent in actual clampload. Because of this, any slight variation in friction can lead to significant changes in resulting preload conditions.
These variables include surface roughness, surface finish, lube, load-range, dimensions, temperature and torque sequence. This is why it’s so important to achieve consistent friction conditions and to use the methods that allow the most consistent torquing. So the preload target will depend on the lube you use (most use a Moly lube or 30 weight oil) and the tightening sequence.
Each material has a certain amount of elastic range, meaning the fastener can be stretched to a certain point but when the load is released it can return to the original shape. But if the load applied exceeds the elastic range and therefore causes the fastener to go past the yield point, it then reaches what is called the plastic range of the fastener. The fastener material is no longer able to return to original size.
The proof load is an applied tensile load that can be applied before permanent deformation. It represents the useable range of a fastener before it goes into its “plastic range” where it cannot return to its original size and shape. At this point of yield, permanent elongation of the fastener sets in. If you continued to load the fastener, it will reach its ultimate tensile strength in which “necking” or elongation occurs until it is stretched to the point of breakage.
There is definitely a relationship between torque and preload, but there is some confusion as to the difference. With connecting rods it is not too difficult to use the stretch method and to measure the preload by measuring stretch. But in head bolts it is much more difficult to measure and you’re basically reliant on the torque wrench to stretch the bolt. A torque wrench needs to be recalibrated often and you need very clean threads that have been burnished in so there is very little friction. The preload is the force on the bolt that clamps the joint together. Torque, however, is just the mechanism used to get the desired preload.
Shear strength is the maximum load a fastener can take before it fails when it is applied at a right angle to its axis. A load occurring in one transverse plane is called single shear, while a load that is applied in two planes, where a fastener may be sliced in three pieces, is called double shear.
Racing applications require high quality, precision tolerance fasteners to achieve the clampload and fatigue strength that is needed in a harsh environment where there are extreme forces placed against them. Materials used to make fasteners vary with the application and load carrying needs.
The most common high-grade material is medium carbon alloy steel that is used in making SAE J429 Grade 8 bolts. These bolts are often used by OEMs in high stress applications and in some racing applications and are rated at 150,000 psi tensile and 130,000 psi yield with a proof load of 120,000 psi.
To the Future and Beyond
“I think the things you’re going to be seeing in the near term – and long term as well – will revolve around the big automotive manufacturers looking to CAFE standards going down over the next few years,” says Mowins. “They’re looking at weight as a key driver on what they’re doing in new development. And a lot of that hinges on being able to take mass out of the bolt. Overall, using a higher strength material allows you to use fewer bolts per part.”
Many engineers nowadays are looking to minimize the number of parts as well as ensuring that those parts that are used are effectively seated to the right levels. Mowins says taking math out of that rotating number, allows them to decrease the rotating inertia, which will subsequently give start up and greater efficiency capability in the engine.
“Dealing with lighter weight, aluminum blocks and how you thread form into that is a key challenge for any of the new developers,” Mowins says. “Obviously how to get clamp load effectively through the joint and the drive system is always a concern.”
This is a concern not only for vehicle OEMs, but manufacturers of fasteners and tools. Phillips Screw Company, Mowins says, develops head designs and new fastener innovations and then licenses those designs to bolt manufacturers and socket makers.
“We’re the original Phillips screw that invented the Phillips screw and screwdriver back in the 1930s. Our founder was Henry Phillips, and all we’ve ever done has been innovation and research, and then licensing to a network of global companies around the world to make the ideas that we dream up,” Mowins says. “We design a better mousetrap, and then companies make our sockets and our bolt designs.”
Mowins sees long term trends toward increased use of lightweight materials like magnesium and aluminum. Bimetallic expansion rates between cast iron and aluminum components have been a sealing challenge for years, but in the future the interaction between the fastener and the component won’t only be physical, but chemical.
“We’ll continue to see more parts made out of exotic materials,” says Mowins. “How do you mitigate the galvanic corrosion affects that are all going to roll in with that? Magnesium parts are going to give us the worst headaches, because it is such a galvanic attractant. It’s a true anodic material, so you’ve got to really isolate it, especially when you’re doing any kind of fastening on it. And then when you look at the carbon fiber parts, more galvanic corrosion, plus impact issues to consider. It’s very high strength, but it’s also prone to impact areas.”
You’ve got to take a look at fastening technology in those areas to mitigate any fiber fracture and compression issues, with popular carbon fiber applications. “You’re seeing those come into play now. And things like pulleys that are in turbocharging lines, and other types of things that are made out of carbon fiber are popular. How can you get the ultra lightweight performance but still maintain the rigidity that you need?” Mowins asks rhetorically.
Measuring will continue to gain in importance, both during the installation and after. “There’s a bunch of different things out there,” Mowins says. “It depends on how critical your engine application is. You’ve now got electronic bolts that have electronics inside that can measure the stretch of the bolt during installation. Of course you need the proper tool that goes along with that. You can do some really great things ensuring the correct torque capability, and getting the correct load out of your bolt in the application.”
Mowins says these intriguing technologies will only become commonplace. And they’re all being developed with the goal of consistency and repeatability.
“When it comes to the tools themselves, you’re looking more at repeatability and reproducibility studies that are going into the tool. How you’re actually doing that on a case-by-case basis, making sure that the torque you’re actually imparting is consistent across all of the joints is critical,” he says. “In addition, there are new load-indicating films that can be put into a joint, that will then allow an analysis on how that load is actually spreading over the joint.”
What will these fastener advancements mean for engine builders? Custom engine builders may have to go much further to adapt than production engine remanufacturers.
“Obviously, the tools that we’re dealing with today are becoming more and more advanced. In the old days, it came down to, is my air pressure at line four the same as my air pressure at one? And if it’s not, what’s that doing to the torque coming out of my pneumatic tool? And now you’re into torque monitored electronic tools that really are giving you a much more precise tighten pattern than you’ve ever had before,” Mowins says. “Networked factories utilize a central dashboard to collect the torque information for all of the tools on the shop floor. The CEO or the plant manager can quickly be able to tell where he’s got a problem in his tightening sequences or torques where he may have bad production.”
Reproducibility isn’t as critical to the typical CER of course, but the need for accuracy remains. “New tools have been developed with Bluetooth that will actually record your time of turn, run down, time of run, angle of turn, and give you the best guess that you actually reach the torque in the load that you’re actually going to require,” says Mowins. “There are a number of other intriguing technologies out there as well, all designed to make engine component fastening easier and more accurate.”