If you’re putting together a rotating assembly the first time or just doing some engine maintenance you have to make sure the connecting rods are fastened together correctly, so this means you have to account for rod bolt stretch. Understanding what rod bolt stretch is and how to address it will prevent you from causing any damage to your connecting rods and keep your engine running strong.
The biggest enemies of any fastener are not tightening it down enough, not reaching the correct level of preload, or tightening it past its designed level of preload. When you’re assembling connecting rods, the fastener has a stretch rating you need to tighten them to, so they will have the correct amount of clamping force. The actual clamping force comes from the rebound of the fastener itself. As you tighten the bolt it’s vital that you don’t bypass its yield point — this point is when a fastener has been stretched too much and won’t return to its original length.
When you tighten a rod bolt it’s important to measure the amount of stretch with a stretch gauge — this is the best method of making sure you’re reaching the designated level of stretch for the fastener. Every rod bolt you will ever use has a predetermined amount of stretch that it needs to be tightened to during assembly and how much stretch the rod bolt has needs to be tracked. ARP recommends that you document rod bolt length before assembly and after a rod bolt has been removed — if you see more than .0005-inch increase in length, the rod bolt should be thrown away.
Check out this quick video from ARP that explains connecting rod bolt stretch.