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.

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.

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.

SOURCE
Automotive Racing Products (ARP)
531 Spectrum Circle
Oxnard
CA  93030
805-278-7223