Fasteners - Engine Bolt Engineering

At 7,500 rpm, the pistons in your race engine reach either top-dead-center or bottom-dead-center and change directions 25 times per second. That means the piston goes from traveling at top piston speed at mid-cylinder to a dead stop and gets snatched in the opposite direction in 0.04 second. At that speed, there is enough force generated to cause minor plastic deformation of just about any connecting rod.

Now consider that the only thing holding your expensive piston/rod combination on the crank are the two bolts connecting the rod cap to the rod. In a V-8, that's 16 rod bolts bearing the load of the pistons and rods, which weigh a couple of pounds each. A single failure among any of the 16 means you not only won't finish your night of racing, you also will be in the market to replace the block, crank, rods, pistons, and cylinder heads that were probably ruined.

This is only a single example of the stresses that many fasteners inside the engine endure. The same goes for the main cap and cylinder head studs or bolts. Fortunately, top manufacturers are capable of designing and producing specialty fasteners for use in racing engines that far exceed your typical Grade-8 bolt. They will rarely fail when used in the correct application and installed correctly.

When installed correctly, any threaded fastener stretches and provides its clamping force from the fastener's tensile strength trying to pull the fastener back to its original length. If the fastener is stretched too far, the clamping forces are inadequate. If it is stretched too much, the fastener weakens and is much more likely to fail. Unlike most applications in which the bolt is overbuilt for the job, the line between "correctly installed" and "catastrophic failure" is painfully thin in a race engine.

We won't get into metallurgy here, but there are a few important things to note when it comes to fastener design. The first is how the threads are actually put on the fastener, whether it's a bolt or a stud. The most common process for all bolts is to cut the threads out of the material on a lathe and then harden the bolt. These are simply called cut threads.

A fastener with rolled threads has the potential to be much stronger. For this reason, it is more commonly seen on high-stress fasteners. The procedure is similar to the forging process in that the fastener is hardened and the threads are pressed or rolled to the desired depth and length. The threads on the final product are capable of handling much higher clamping loads because the material it is made of has been pressed into shape and none of it is cut away. According to Automotive Racing Products (ARP), one of the top fastener manufacturers in the racing industry, fasteners with rolled threads can display fatigue strength up to 10 times greater than fasteners threaded prior to heat treating.

Another factor to consider is the actual shape of the fastener. In certain cases, the shank of the fastener will vary in diameter. In the case of a bolt, it will get thicker just underneath the head of the bolt. According to ARP, one of the reasons for this is the area of greatest stress on a bolt is the last engaged thread. To help increase the strength of this area, ARP designs the thread to stop at the point of maximum engagement, and the last thread has a fillet radius that transitions to the shank of the bolt. Because the bolt needs to stretch equally, this diameter is maintained until just below the head of the bolt.

When using cylinder head studs, you may notice that the longer studs have a consistent diameter while the shorter studs (typically along the bottom edge of the head below the exhaust ports on a Chevrolet) are undercut with a smaller diameter in the center. This allows the shorter studs to stretch more easily and equalizes the clamping load with the longer studs at the same torque rating. This creates more equalized clamping across the cylinder heads.

One of the first questions many new racers will ask is, "Which should I use, bolts or studs?" Bolts make it easier to remove a cylinder head with the engine still in the car, but it is best to use head studs whenever possible.

The biggest reason for this is cylinder head studs simply provide better and more consistent cylinder head torque loading. That's a fancy way of saying there'll be fewer blown head gaskets. When installed, a bolt must be torqued into place. This creates two forces on the bolt: a twisting motion and a vertical clamping load. But a stud is threaded into place with no clamping load-that comes when the nut is tightened down. Ideally, you only thread the studs finger tight in a relaxed state, install the cylinder head, and torque the nuts into place to provide the clamping load. This removes the twisting force from the equation, and the stud will only stretch in a vertical axis instead of stretching and twisting, in the case of a bolt. This provides more accurate and even clamping forces across all the studs. This is also true, in the case of studs over bolts, when securing the main caps.

There are also secondary benefits to using studs. First, it makes it easier to properly align the head gasket. It is also easier on the threaded holes in the block because you aren't using the threads in the block to pull the stud into place. If you are using an older block or an aluminum block, this is definitely preferable.