Connecting Rod Bolts – Stretch vs. Torque

Today I’d like to discuss the proper method for performance connecting rod bolt installation. There are a few ways to tighten connecting rod bolts, including the basic torque method, the stretch method, and the torque to yield method. Without a doubt, bolt stretch is the only truly accurate way to establish proper bolt preload.  To better understand why, we need to look at how a bolt works.

Understanding Stretch

A bolt functions similar to a spring, in that it needs to be stretched beyond it’s static length in order to apply a clamping force. In the case of a connecting rod bolt, the clamping force must exceed the tensile load imparted by the attached components during high rpm operation. This is accomplished through a combination of bolt material selection and stretch dimension.

A common method for determining ideal stretch, is by incrementally increasing the amount of stretch until the bolt becomes permanently elongated. This dimension is termed yield, and a typical target stretch range is then determined to be 75-80 % of yield, with the min/max provided to the end user at +/- 5 % of  this target. As an example:

• Yield = .007″ stretch
• Target stretch is 75-80 % of yield = .0052″ – .0056″
• +/- 5 % of target = Min/Max of .0049″ – .0059″ (recommended stretch range)

Yield varies, and is affected by a variety of factors, such as material, bolt length, under-head register length, and shank diameter.

 Proper Use of Torque

As explained above, what we really want to do is install the rod bolt at a near optimal percentage of yield in order to apply as much clamping force as possible without putting the integrity of the bolt at risk. Using a torque wrench to tighten the rod bolts in your high performance engine is a risky proposition, unless you are willing to invest some effort into quantifying the readings.

Torque in regards to bolt tightening is just the amount of force required to overcome incremental increases in friction. Torque readings (friction) are affected by thread quality, composition of lubricant, and surface finish of the bolt spot-face machined on the cap of the rod. Beyond the friction variables, not all torque wrenches – or users for that matter, are created equal.

I have used nearly every type of torque wrench available, and have a very high quality calibration unit at my disposal. The only torque wrenches that are consistently correct throughout the scale of readings, are high quality dial type. The ever-popular “clicker” type wrenches rarely apply the torque that the user has dialed in, and the higher the torque, the more it’s off. With that being said, they will torque consistently, just not the proper amount of torque.

Torque vs. Stretch

Now that we understand that the stretch value is what needs to be achieved, it’s necessary to verify what amount of torque on the wrench being used is going to stretch the bolt within spec. All rod manufacturers provide an information sheet that will list the recommended torque spec and bolt stretch range. Here’s the process I use to match my torque wrench reading the proper stretch value:

• Install the cap on the rod with the manufacturers recommended lube on the bolts.
• Carefully draw the cap into the rod using a nut-driver alternating from side to side in order to keep everything square until the cap & bolts are properly seated.
• Loosen one bolt just enough to be sure there is no load on it.
• Install the bolt stretch gauge on the loose bolt.
• Set the dial to “0” – remove and re-install gauge to verify it still reads “0”
• Remove the gauge.
• Finger tighten the loose, just measured bolt.
• Torque the opposing bolt to 25 ft. lbs. – this assures it has a small amount of preload on it.
• Torque the measured bolt to the manufacturer’s recommended value.
• Install the stretch gauge, and see what the bolt stretched to.
• If the bolt isn’t within specs, loosen and adjust torque up or down until you reach the desired stretch.
• If the bolt is stretched within the specified range, loosen the opposing bolt and repeat the measure/torque/verify procedure.
• Repeat these steps to 4 bolts to assure that the same torque reading provides the same amount of stretch.

At this point you should be able to use the torque wrench reading determined in the outlined process to torque the rest of the bolts, or continue to stretch each bolt individually. I should also be clear that you must zero the dial on the stretch gauge for each bolt. The center dimples used for the gauge are not a critical dimension, and will vary from bolt to bolt.

That’s all for today. I hope I’ve cleared up any of the mystery surrounding proper rod bolt installation for you. As always, I look forward to your feedback, questions, or comments. You can comment directly to this or any other article on the site or send me an e-mail. Please invite your friends to join us, and thanks for visiting …..