Assembling the bottom end of an engine requires plenty of precision, and if you neglect any details, the consequences could end up in your oil pan. Getting your rod bolts tightened down is the most important step in this process, and that means you really need to measure how you secure these fasteners. The most accurate way to do this is through a stretch gauge.

While using an impact wrench to quickly tighten a rod bolt down before you blast it with a few turns from a torque wrench might seem like a good idea, it’s probably the worst thing you can do. Every fastener used in an engine build has something known as a “yield point”— if you tighten it past this point, it will fail. The stretch gauge is the most important tool you’ll need when it comes time to start assembling the bottom end of an engine so you avoid exceeding a bolt’s yield point.

A stretch gauge is how you measure the preload of a rod bolt and ensure you’re installing it per the manufacturer’s instructions. Rod bolts, like those from ARP, will have dimples on each end where you place the stretch gauge to measure as you tighten the bolt down. Before you turn a wrench on the bolt for the first time, you’ll want to be sure you measure it so you can check its stretch as you tighten it down. From there, you just begin to tighten the rod bolts down per the procedure for your application while making sure you don’t exceed the recommended stretch amount.

Check out ARP’s website more right here to learn more about bolt stretch and how to properly install rod bolts with a stretch gauge.