When your foot is blue and your motor is reaching for the moon, we know what you're thinking: It's a good thing this engine's rock solid. At least we hope that's true. And as important as good-quality engine fasteners are for today's performance engines, proper installationis equally critical. Make a mistake here by installing a mediocre bolt or overtightening it and all of your hard work, cool parts, and budget may take a hit.
There are many aspects of selecting and installing engine fasteners the right way, including proper torque and bolt stretch where applicable. Your engine is packed with a variety of head bolts, intake manifold bolts, main cap bolts, rod bolts, and more. By selecting the best fasteners for your application and installing them the right way, you can run your motor hard for years to come. Follow along as we explain how and why.
Make it Strong
In the world of mundane engines and humdrum performance, standard-issue engine hardware in good condition works reasonably well. In the world of high-performance street or full-race setups, much greater demands are placed on engine fasteners as power levels and engine speeds are increased. Of all the fasteners in your engine, the rod bolts are the most critical because they are the most stressed from a reciprocating load. Rod bolts must withstand all the dynamic force caused by the moving piston and connecting rod.
Constructing a rod bolt that will provide outstanding reliability starts with determining the load it will carry. This is accomplished by calculating the dynamic force generated by the moving piston and rod, which is dependent on a portion of the weight of the reciprocating parts and the engine speed. This dynamic force is at its maximum value at TDC and BDC.
The reciprocating load, however, is not the only cause of stress on a rod bolt. A secondary effect arises due to the flexibility of the big end of the connecting rod. This also causes flexing at the bolted joint, and as engine speed increases so does the flexing. This load additionally works to stretch the rod bolts. With varying tension loads and the flexing of the bolts, rod bolts must be able to apply a clamping force greater than the load placed on the connecting rod's big end. With this information collected, a safety factor can be calculated for the level of stress the particular rod bolt (properly installed) will endure.
On With It
Engine bolts require a specified tension to install. Three methods are used to determine how much tension is placed on a fastener: torque wrench, stretch gauge, and torque angle. Of these three, the stretch gauge is the most accurate, but not always a possibility due to physical constraints. The best way to install and tighten rod bolts is by preloading the bolt to meet the manufacturer's stretch value. If a rod bolt is tightened using the torque method, there may exist too little or too much clamping force.
Here are two different rod-bolt...
Here are two different rod-bolt styles. The one on the left screws into the rod to secure the cap, whereas the one on the right secures the cap with nuts; this style works well for most street applications. To minimize friction, ARP recommends using its assembly lubricant on most threads prior to assembly.
Rod bolts are best tightened...
Rod bolts are best tightened using the stretch method. The rod bolt is first measured at its free (static) length. Then a stretch gauge is mounted with two pinpoint spots above and below the rod bolt. As the bolt is tightened, the amount of stretch that occurs is monitored on a dial indicator. This particular rod bolt is designed to stretch 0.0063 inch.