We’ve covered the topic of bolt stretch versus fastener torque quite a bit over the past near-decade of EngineLabs’ existence, but that’s because it’s an incredibly important topic that really needs to be understood. ARP doesn’t just pound out fasteners for engines, they truly understand the needs of fasteners and engine builders. As such, they have a line of bolt stretch gauges designed to fit almost any connecting rod bolt configuration you can imagine.
Stretch Vs. Torque
First, we should dive into the subject of stretch vs. torque, just so you are coming into this with a complete understanding of what is being measured. When a bolt is properly fastened, it is essentially a spring, providing a clamp load between two surfaces. Because a bolt’s range of stretch is incredibly small when compared to that of an actual spring — we are dealing with thousandths of an inch range, instead of inches — this is an incredibly precise measurement.
Generally speaking, the ideal rod bolt stretch is in the .0060 to .0065-inch range to achieve desired clamp load, but will be specified by the connecting rod manufacturer. There are calculations that can be performed by people who really love numbers to determine how much rotational effort it takes to achieve that much stretch in the fastener, and that number becomes your torque spec.
However, we’ve seen in previous testing, where a number of external variables can change that calculated torque value significantly. For example, a bolt torqued to spec with no lubricant at all can be as much as 30-percent different from the same bolt torqued to the same value, with a proper lubricant. And even then, torquing to the proper spec might leave you 15-percent shy of the actual stretch target.
The only way to ever know for sure that you are (or aren’t) getting the full clamp load from a fastener is to physically measure the fastener itself. While this can be challenging in most engine fastener applications, rod bolts are in a unique position where there is ready access to both ends of the fastener. This is convenient, as the rod bolt is one of the most critical components in an engine, and one subjected to some of the highest forces in the entire rotating assembly.
Measuring Rod Bolts
Measuring rod bolt stretch is comparative as opposed to absolute. That means the rod stretch gauge must be zeroed on the fastener prior to torquing, and then the gauge will show the variance from zero, as opposed to the absolute length of the fastener.
There are several methods that can be used in order to measure the stretch of the fasteners in an engine build. Since we’re measuring to half a thousandth of an inch, there is likely to be some slight variation in unloaded fastener length throughout the 16 rod bolts present in a standard V8 engine. That means there can be some error if we were to set zero on one bolt and apply it to the other 15.
The easiest method is to set zero on one fastener and then tighten the rod bolts of that connecting to proper spec and remeasure that reference bolt. You can then adjust torque on that bolt to achieve your specified stretch, and apply that torque value to the remaining 15 bolts. Conversely, if you aren’t afraid of a lot of work, you could measure each bolt’s free length and record the differences, so that you can accurately measure the amount of stretch of each individual bolt.
In addition to being an assembly tool, measuring rod bolt stretch can also be a maintenance item. ARP recommends that you record your measurements upon initial assembly and if you find that the free length of the bolt exceeds .0005 inch of growth, it be discarded and a new bolt used.
With how critical of a component the connecting rod bolt is in a high-performance engine, being able to actually measure your fastener stretch is a definite advantage over having to trust the torque value alone.