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Installing A Bolt - Rod Bolt Stretch

What Is Rod Bolt Stretch And Why Do You Need To Do It?

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For decades, engine-builders have been tightening rod bolts with a torque wrench and getting away with it. But in today's high-performance world where we push engines to make a ton more power and spin at higher engine speeds, merely tightening rod bolts with a torque wrench is no longer the best way. In order to know why rod bolt stretch is a more accurate way to install a bolt, we have to get into a little bit of fastener basics.

Bolt BasicsA fastener works like a coil spring. As you tighten a bolt, it will stretch and generate a clamp load. The key to proper tightening of a fastener is to torque it until just slightly less than the bolt's elastic limit. A bolt will stretch slightly as it is tightened. If you tighten the bolt too much, it will stretch beyond its elastic limit. If you measure a bolt's overall length before you overtighten it, then again when you release the preload, the bolt will be slightly longer. This is similar to overstretching a coil spring. It does not return to its tightly packed position because the steel has been stretched and over-yielded. This is the bolt's yield point, where it is permanently deformed. It's the point just before it comes apart. The bolt's ultimate tensile strength is the maximum stress imparted on the bolt before it breaks.

This applies to all fasteners, but it's especially critical with rod bolts because they're the most highly stressed fasteners in an internal combustion engine. With every revolution, the crankshaft yanks on the piston and rod assembly to pull it away from top dead center (TDC). The rod journal pulls on the rod cap, which tries to stretch the rod bolts. This stress becomes greater as engine speed increases since this load increases geometrically with rpm and forces the rod out of round, bending and fatiguing the bolt.

The key to keeping the rod cap on the rod is the amount of load created with the rod bolts. If the load created by the bolts is greater than the tension created by the crankshaft rod journal trying to pry the cap off the rod, then the engine will stay together. If the bolt is not properly preloaded (understretched) then the high-rpm tension is enough to stretch the rod bolts a very tiny amount with each revolution. This high-speed cycling of the bolt is similar to bending a paper clip back and forth until it breaks. That's obviously something you want to avoid.

Torque vs. StretchThe torque spec applied to any particular fastener is merely an estimate of the twisting force required to achieve the correct amount of preload or clamp load. Many times this is the only way to apply fastener load because the bolt threads into a blind hole like in the cylinder block. One advantage to the rod bolt is that both ends of the bolt can be accessed. This allows you to use a rod bolt stretch gauge. This is a specialty tool sold through companies like ARP that will accurately measure the amount of bolt stretch.

The procedure is actually quite simple. Once the connecting rod and cap are installed on the crank, start a nut on the rod bolt, slip on the appropriate-size box-end wrench, and then install the stretch gauge. All ARP connecting rod bolts have a small dimple placed on both ends of the bolt that accurately position the rod bolt gauge pins on the bolt. Next, zero the gauge on the relaxed bolt. Then you carefully tighten the rod bolt until the gauge reads the appropriate stretch amount. For example, a standard 111/432-inch ARP small-block Chevy specs out at 0.0063 inch.

ARP generally sets the stretch figure at 75 percent of the bolt's yield or elastic limit. This creates the most load with still plenty of safe "headroom" between the stretch figure and the bolt's limit. One advantage to measuring bolt stretch is that this figure is unaffected by whatever lubrication you place on the threads. This is not the case when using a torque wrench.

Stretch vs. LubeWhen you tighten a rod bolt nut using a torque wrench set to a specific spec, the wrench is measuring a resistance to movement. This resistance is partially the result of bolt stretch, but the majority of effort required to torque the bolt comes from friction. This friction results from the threads moving against one another as well as the rod bolt nut against the rod cap. This friction changes based on the type of lubricant used. For example, friction will be greatest with no lube and slightly less if oil is applied to the threads and the bottom of the nut. It will be reduced even more if you apply some of ARP's bolt lube to the threads and the face of the nut. The lube's fricton "index" is referred to as its fricton coefficient.

Perhaps you can see where we're headed with this little exercise. If you were to apply the same torque to a fastener with no lube and then use different types of lube, the stretch on the bolt would increase with the ability of the lube to reduce friction. This is why it is extremely difficult for a fastener company to give an accurate torque spec that will properly stretch the bolt because there are so many lubrication friction coefficient variables. If you must torque a rod bolt, ARP offers a torque spec with its lube.

Better BoltsARP offers three different styles of rod bolts for most applications with increasing tensile strength for each version. The standard ARP high-performance 8740 alloy chrome-moly steel bolts offer a tensile strength of roughly 200,000 psi and are good for most all street-oriented applications. The Wave-Loc ARP bolt is the next level up using the same heat-treated 8740 steel but has patented symmetrical waves incorporated into the bolt shank that fit snugly into the rod without the knurling that can cause sharp edges and a stress riser that can cause a crack to eventually form.

ARP's best bolt is the Pro series Wave-Loc that uses higher-strength ARP2000 material with a tensile strength of approximately 220,000 psi. This higher-tensile-strength bolt is offered as an upgrade for ultimate competition engines that experience ultra-high rpm use such as Pro Stock or endurance road racing.

Now you know a little more about the stresses imposed on a connecting rod. For example, a 434ci small-block with a 4.00-inch stroke will generate a massive 3,200 g's at TDC at 6,500 rpm. Multiply the weight of the piston and the small end of the rod by this g-force and you can get an idea of the tension exerted on a connecting rod and its two bolts. Think about that the next time you want to just torque your rod bolts instead of stretching them the way you should.

Why Torque Doesn't WorkWe decided to do a little test to illustrate how rod bolt stretch functions independently of torque depending upon the condition of the bolt, nut, and the lube employed. We decided to torque the same bolt and nut in several different conditions and measure the amount of bolt stretch to evaluate the difference.

We started with a brand-new ARP rod bolt and nut installed on a small-block Chevy connecting rod. The completely dry nut and bolt were first torqued to 50 lb-ft. Next we torqued them four more times and measured the amount of stretch after the fifth sequence. Then we coated the threads and the nut with regular 10W40 engine oil, torqued the nut again, and measured the stretch. Finally, we cleaned the oil and then recoated the same bolt and nut with ARP's special thread lube.

As you can see, the bolt stretch changed with almost all these situations, increasing the bolt stretch with each change by reducing the amount of friction between the nut an the rod and between the threads. Note that even with the ARP lube this combination did not create the ideal 0.0063-inch bolt stretch until we increased the torque to 76 lb-ft. Finally, we kept adding torque to both the oiled and ARP lube configurations to see how much torque they required. Note that the oil required 83 lb-ft to finally reach the proper stretch.

Treatment Torque Applied Rod Bolt Stretch
(lb-ft) (inches)
New bolt and nut, dry 50 0.0025
Same after 5 torquings 50 0.0020
Bolt w/10W40 oil 50 0.0030
Bolt w/ARP lube 50 0.0035
Bolt w/10W40 oil 83 0.0063
Bolt w/ARP lube 76 0.0063
Parts List
Component Manufacturer Part Number
Rod bolt-stretch gauge ARP 100-9941
Assembly lube, 1.69 oz ARP 100-9903
Rod vise Bill Mitchell 216-35090-1
SOURCES
Automotive Racing Products (ARP)
531 Spectrum Circle
Oxnard
CA  93030
805-278-7223
Bill Mitchell Hard Core Parts
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