Sometimes disaster can breed an interesting way to check your work — or the work of others. Many years ago, we built a mild small-block Chevy to use as a test mule engine. We used a set of ARP rod bolts but merely torqued them to ARPs’ recommended torque spec and did not check the bolt stretch.
Surprisingly, the engine broke on the dyno after about a couple dozen pulls at a mild power output of around 425 horsepower. We discovered one of the rod bolt nuts had loosened and come off – ripping the cap off the rod and destroying the engine. It even broke the cam into three pieces!
Clearly, the rod bolt was not stretched properly to achieve sufficient clamp load. We wanted to know just how much stretch the remaining rod bolts had. So, we used a rod bolt stretch gauge from ARP and zeroed the gauge with the bolt torqued in place. Then we loosened the rod nut to remove all tension and then re-measured the bolt length. The stretch gauge produced a number of 0.0040-inch. This is how much the bolt contracted in length. The ideal stretch number for that small-block rod bolt was supposed to be 0.0063-inch.
This meant that the rod bolt was not stretched anywhere near its optimal length. This reduced the clamp load and allowed the rod bolt nut to eventually work loose. While we had properly set the torque wrench, we took the wrench to a local FAA tool calibration facility and had the clicker torque wrench tested. Not too surprisingly, it tested roughly 20-percent short of the proper torque at the 65 lb-ft setting. This revealed why our rod bolt failed — our torque wrench was way out of calibration.
We spoke with ARP engineer Chris Brown and he told us that the rod bolt stretch figure established by ARP is 75-percent of the yield strength of the bolt. So if the bolt’s yield strength limit is 0.008-inch, then ARP’s stretch figure would be 0.006-inch.
This technique can also be used to check the veracity of an engine that you may not know the history of. If the rod bolts are from ARP, you can look up the part number and then search for the online instructions on the ARP website to find the original stretch numbers and compare these numbers to what you see on the actual bolts. You may be surprised at what you find, since many engine builders do not use rod bolt stretch when torquing the rod bolts.
Our broken engine convinced us that we will not build an engine without ARP rod bolts and certainly will only stretch the bolts and not trust even a calibrated torque wrench. This way we know the bolt is properly stretched and will do its job.