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Engine Fasteners - Keep It Tight!

Properly Selecting & Installing Engine Fasteners

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 installation is 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.

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.

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.

Because a rod bolt or other engine fastener must give and take to a degree, it acts much like a spring when correctly selected and installed. Just as a spring can be overstretched, a rod bolt can permanently deform if installed or tightened incorrectly. The point at which a bolt will overstretch and deform is called the yield strength. Most performance-bolt manufacturers, like ARP, provide torque and stretch values equal to 75 percent of the fastener's yield strength. So when a particular bolt is tightened to spec, it still has another 25 percent of stretch to its yield strength. This allows the fastener to rebound and maintain a clamping load. For a rod bolt, the 25 percent remaining yield strength allows it to withstand added loads at high rpm. If the fastener length exceeds the yield strength, it is ruined and must be replaced.

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