Your race engine is only as strong as the fasteners holding it together, and the fasteners
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.
One interesting design characteristic ARP uses on its cylinder head studs is an undercut o
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.
Studs are preferable to head bolts because the stud can be threaded into the block in a re
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.
When applying moly-lube to a bolt, don't forget all the friction surfaces, which include u
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.
When using aftermarket head bolts or fasteners, follow the engine manufacturer's recommend
Finally, you should be aware of the exact torque specifications your fastener manufacturer recommends for your application. The best torque specs for engines that at first seem similar are not always the same. For example, all things being equal, you should always use less torque on bolts or studs when using aluminum cylinder heads than when using cast iron. This is because aluminum expands more with heat than iron. If you use torque specs for iron with a motor using aluminum cylinder heads, the extra stretch caused by a hot aluminum cylinder head can actually cause the cylinder head bolts or studs to fail.
We've already discussed the importance of obtaining proper preload, or stretch, on a fastener. The most common way of doing this is by measuring the torque, or force, applied when tightening a bolt into a threaded hole or a nut on a stud. The other method is to physically measure how much a fastener stretches when it is installed.
The problem with measuring torque is that it often can be a bit inexact. A torque wrench doesn't measure bolt stretch, just the force required to turn the bolt. Some of the required force is bolt stretch, or preload, but some of that is also friction between the threads caused by such variables as the type of lubricant used, the slickness of the metal, and even debris or damaged threads. Measuring stretch removes all those variables and is therefore much more consistent when it comes to measuring preload.
If you do not have access to a bolt stretch gauge and prefer to depend on torque, ARP reco
Of course, the only time you can measure bolt stretch is when you can reach both sides of the bolt. This won't work when threading a fastener into a blind hole (e.g., main caps and cylinder head bolts), so a torque wrench must be used. But you can get to both sides of the rod bolts when they are installed in the engine, which is fortunate because they are under the most stress of any fastener of any kind in the engine.
Measuring stretch is relatively straightforward. You can use any of a number of calipers as long as they are accurate within 0.005 inch, but a dedicated rod bolt stretch gauge makes the entire process much easier. Fortunately, measuring the stretch on every rod bolt isn't necessary. As long as you are consistent in your methods and use quality products, you can determine how much torque is required to stretch a bolt the desired amount and use a torque wrench on the rest of the bolts.
Begin by installing the rod bolt in your gauge in a relaxed state and calibrate your gauge. Next, install the bolt in the rod and begin torquing the bolt in 5-pound increments, measuring the stretch as you go. Note when you reach the desired amount of stretch (usually 0.006 to 0.007, but check with your fastener manufacturer), and note the amount of torque required to get it there. Now when you assemble one connecting rod on the crank in the engine, you can check the bolt stretch against your torque number. If it remains consistent, you can torque the rest of your rod bolts to that number and be confident that the bolt stretch is correct on all the rods.
This is the best way to measure fastener preload with a bolt stretch gauge. In most cases
Be aware, however, that every time you stretch the rod bolts on a new rod the torque required will be reduced. This is because every time you tighten the bolt you are polishing the threads a bit, especially if you are using a moly-based lubricant, which has a metallic base. Over the course of a build you will pre-fit the rods three or more times, and if you use the same amount of torque each time, you may stretch the bolts too far by the time you are doing final assembly. For that reason, you should always use your stretch gauge on at least one set of rod bolts every time you assemble the rods. You can also use the stretch gauge to monitor the health of the rod bolts. Check a bolt or two every time you take the rods apart. If the bolt is permanently stretched over 0.001 from its original length, it has failed and should be discarded. You can also do this check during the teardown process before an engine rebuild to see if the rod bolts are good for another cycle.
It may seem odd at first that the type of lubricant you use can play such a significant role in the proper installation of engine fasteners, but it is true. A lubricant won't cause a properly stretched fastener to wiggle loose, but it will ensure that you achieve the proper clamping load to allow the fastener to do its job.
Also, using a lubricant on fasteners prevents galling between the metals and aids disassembly. Many engine builders prefer motor oil because it is readily available, which is fine, but just be aware that the difference in lubricity between the oil you have on hand and the lubricant your fastener manufacturer recommends may be quite different. For example, ARP provides torque recommendations for its fasteners for use with motor oil and its recommended moly lubricant. This is because the moly-lube is much slicker and won't wipe off as easily as conventional motor oil, which means the torque required to properly preload a bolt will be less than if motor oil was used. Also, be sure to apply your lubricant to all the friction surfaces. In the case of a bolt, this would be the threads, under the head of the bolt, and the washer.
If you do not have access to a bolt stretch gauge and prefer to depend on torque, ARP recommends torquing all your rod bolts at least three times, with moly-lube applied, before pre-fitting and assembly. This is because each time the nuts are torqued into place, the threads on the bolt and nut are polished a little bit. Each time the rod is assembled, the reduction in friction between the threads indicates the same amount of torque will stretch the bolt a little more. After three times, the drop in friction will level out. This is the point at which ARP's torque recommendations are ideal.