January 2008 Articles & Features


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Performance Perspectives

By Mike Mavrigian | January 2008

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When building a performance engine that may spend extended periods near redline, properly tightened rod bolts are essential. Rod bolt stretch measurement offers the assurance that all bolts are “tight enough,” and will stay that way.

If you're building a high-performance or race engine and plan to use aftermarket high-performance connecting rod bolts, you may want to consider a tightening process that all major race engine builders use. Instead of tightening the rod bolts by achieving a torque value, the optimum approach for precision-tightening connecting rod bolts in these applications involves measuring bolt stretch during tightening.

Aftermarket connecting rod bolts may vary in terms of their ideal torque by as much as 10 ft.-lbs. from batch to batch due to variations in heat treating and materials. So if the concern is to arrive at both peak bolt strength as well as big-end roundness, the rod bolts should be measured for stretch. Even when using the best rod bolts available, measuring bolt stretch offers a much more accurate method of achieving ideal clamping loads as opposed to the use of a torque wrench.

Granted, when dealing with production engines that utilize torque-plus-angle specifications (for example, 20 ft.-lbs. followed by a 90° additional turn), you'll need to follow the OEM's procedure. Tightening connecting rod bolts using the stretch-monitoring method really applies only to performance engines that feature aftermarket high-performance rod bolts, where the bolt manufacturer has provided a stretch value reference.

To measure bolt stretch, first measure the total rod bolt length (from the head surface to the tip of the shank) in the bolt's relaxed state. Then measure the length again after the nut has been torqued to value. The difference in length indicates the amount of stretch the bolt experiences in its installed state. For the majority of production rod bolts, stretch should be approximately .006 in. If the stretch is less, the bolt is probably experiencing too much friction that's preventing the proper stretch (requiring lubricant on the threads). If stretch is excessive, the bolt may have been pulled beyond its yield point and is no longer serviceable.

While a micrometer may be used to measure rod bolt length, the most accurate method is to use a specialty gauge that's outfitted with a dial indicator. Excellent gauges are available from ARP, Gear Head Tools and Goodson Shop Supplies. Goodson also offers a rod bolt stretch gauge (Part No. RBG-4) that features spherical points for consistent and repeatable readings, and which can also be rotated for right- or left-hand operation. ARP offers its own bolt stretch gauge (Part No. 100-9941), designed with .0005-in. increments, with a heavy spring and ball tips. Both ARP and Gear Head also offer stretch gauges that feature a finger hole in the body, which is extremely handy in terms of holding the gauge steady while taking the reading.

Here's a good way to look at this: All connecting rod bolts should be regarded as high-tensile springs. The bolt must be stretched short of its yield point to produce accurate, and most importantly, repeatable, clamping of the rod cap to the rod. Improper or unequal bolt clamping force can easily result in an out-of-round rod bore.

Stock, or production, rod bolts typically offer a tensile strength of approximately 150,000 to 160,000 psi. However, due to variances in bolt production, tolerances can be quite extreme, with peak bolt stretch travel occurring anywhere from, say .003 to .006 in. If you use only a torque value in attempting to achieve bolt stretch, you run the risk of achieving unequal rod bolt clamping loads. High-performance rod bolts are manufactured to meet much tighter tensile strength tolerances. ARP, for instance, calculates each and every rod bolt for stretch, and the bolt packages include reference data to that effect. The instructions actually recommend that a specific amount of bolt stretch should be achieved on each bolt. (ARP cites 190,000 psi as its nominal, or base, tensile rating, with much higher actual ratings in some applications.)

How can unequal or inadequate rod bolt tightness affect the connecting rod big end bore shape? If a machine shop tech reconditions the connecting rods using torque value alone to tighten the rod bolts, and the technician who's responsible for final assembly uses the bolt stretch method, the final result can produce out-of-round bores. This is due to frictional variances encountered when comparing the two methods. As a result, the assembler using the stretch method may achieve a higher clamping load on one or more bolts compared to the loads imposed when the rod machinist torqued the rod bolt nuts without regard to actual bolt stretch. In addition, when a bolt is tightened with dry threads (that is, no lube), as much as 80% of the torque is used up by friction. As you can see, variances can be caused by how the rod bolts were stretched (basing the results on torque value or by bolt stretch), and by thread lubrication variances (if the bolts were tightened first with one type of lube then tightened down the road with a different type).

In a high-volume production rebuilding facility, technicians may not have the time to measure for bolt stretch. However, a slower-paced operation attempting to obtain maximum accuracy (for a race engine, as an example) is far better off using the stretch method instead of the torque method.

Consider the
Friction Factor

If a bolt is tightened by addressing the torque value only, you may not necessarily achieve the desired preload, due to the variable of friction. Since we can't predict the frictional loss, measuring rod bolt stretch provides the most accurate method of ensuring both that the clamping loads will be sufficient for the task and that each pair of rod bolts will achieve equal loads.

Bolt stretch is affected by a number of factors, including tensile strength and mass, including the length and diameter of the bolt being stretched. For example, let's consider two 3 /8-in. x 1-in. bolts. One features a 1-in.-long shank, with threads on the full length of the shank. The other bolt features 3 /4 in. of shank length that's full-diameter and smooth and only 1 /4 in. of thread length at the tip. The bolt with partial thread will stretch less, because the shank area between the head and nut engagement area has a thicker cross section. The partial-thread bolt will have a .375-in.-diameter shank, while the all-thread bolt will have only a .324-in.-diameter shank (due to the smaller root diameter inside the thread path).

While we cannot control the reaction of the connecting rod base material, at least consider the potential compression of the connecting rod material itself during bolt clamping. As the bolt is tightened, its head will tend to embed itself into the rod, slightly compressing the stock material of the connecting rod. Production rods are typically softer; after a bolt head sinks in, the material beneath it “work hardens” under compression.

ARP recommends that the bolt stretch be based on the bolt itself, and not on the compression of the rod, since we can't accurately predict what the rod does in this state. Too many variables exist in terms of rod bolts and connecting rods, so we can't draw any general conclusions regarding ideal connecting rod bolt stretch. However, to use the Chevy small-block 350 as an example, ARP typically looks for an installed bolt stretch of .0063 in. Since each engine/rod/bolt application differs, we cannot assume that ideal bolt stretch would be the same for other applications.

How Much Torque
Is Needed?

We need to modify our thinking in this regard. When a rod bolt is tightened using a torque wrench, it makes more sense to consider what load is required instead. After all, torquing to a specific value is simply a means to an end; the load is the important factor.

When torque is applied to a nut or bolt head, most of the input is spent in friction. At the end of the process, 85% to 95% of the energy transferred through the wrench has been lost. In other words, the clamp load itself may represent only 5% to 15% of your effort. This is why using a stretch gauge to tighten rod bolts makes so much sense.

Because of this frictional loss factor, slight variations in the frictional conditions can result in huge changes in the resulting preload. These variables include surface roughness, surface finish, the lubricant used, load range reached, dimensions of the nut or bolt head, temperature and torquing sequence.

The greater the friction, the higher the torsional stress in the fastener body. Since torsion is a function of the imposed friction, a given material reaches its yield strength sooner when the friction is high rather than low. During tightening, the apparent yield strength drops by 10% to 20% from the yield strength measured in tensile.

As a consequence, when regular preloads are required, quality engine oils are sufficient for thread, washer, under-nut and under-head lubrication. In addition, the relatively higher friction will prevent any loosening under vibration. A black oxide finish may help to ensure consistent preloads (past the mandatory three- to six-step pretightening procedure to break in the mating surfaces).

When higher loads are needed, the inevitable torsion stress can be minimized by choosing slicker threads. Less friction will bring the yield point up a few percent, allowing the clamp to be closer to yield. It will also reduce the residual torsion present when the wrench is released.

Finally, choose lubes carefully. When ultra-slick lubes are used, high preload is required to prevent backing off under vibration. For this reason, be aware of undertorquing and make sure your fastener is not losing clamping force during service, which can result from compression of the mating part or stretching of the fastener. Also, be absolutely confident in your torquing procedure. Slick lubes will emphasize the precision of any torque wrench.