Concerning dimensional specifications, GM Performance Parts maintains strict numbers on their connecting rods, e.g., the wrist pin bushing is honed to + or - 0.0002-inch. Meanwhile, the journal ends are honed to + or - 0.0002-inch.  The taper, bell mouth and barrel is machined to less than 0.0002-inch while the bend and twist limit in these rods is 0.004-inch over an 8.00-inch bar.

The Amount Of Friction That Must Be Overcome…

Another consideration is torque.  Torque is defined as the amount of friction that must be overcome to cause a nut or bolt to turn.  Manufacturers of most quality fasteners, designed for racing or other extreme uses will supply some tightening specifications, torque

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reference numbers, or stretch data.  It's no secret that this information is usually listed in foot-pounds of torque.  GMPP states that of all the effort applied to a given fastener, 50 percent of the torque is used to overcome the friction under the head of the bolt and 40 percent is required to overcome the friction between the mating threads. This means that of all the torque applied, 90 percent is required to overcome forces, which have absolutely nothing to do with the job at hand.  Tom Molnar, former head of Oliver Racing Parts sums it up this way:  "You can look at this as a torque wrench being only 10 percent accurate."  That’s something to ponder.

It's easy to see that friction is an extremely challenging problem because it has so many variables and of course, is difficult to control. Molnar also states that many fastener manufacturers recommend the stretch method of measurement since the pre-load is closely controlled and is obviously independent of friction.

Molnar offers more insight into friction: "The friction for a particular installation can change from one application of torque to the next.  That is, when a bolt is torqued for the first time, the friction is usually at its highest value. Each additional time the fastener is torqued and then loosened, the friction factor becomes smaller."

Thread lubricants are absolutely critical since they are the primary element when determining friction. In many shops it's common to use motor oil because it’s readily available.  Who doesn’t have a partially full bottle of his or her favorite engine oil on the shelf?  Unfortunately, there's a catch.  When you use specially formulated, low friction lubricants designed for a specific task, the required torque can be reduced as much as 20 to 30 percent.  The reverse is also true.  ARP points out that if the recommended tightening specifications are based on the use of a special lubricant, the use of motor oil or other non-specified lubricant will result in insufficient pre-load.  Engine oil is a good hydraulic bearing material, but it is a poor extreme pressure lubricant.  If it is used on bolts, the torque required would actually increase, due to galling which makes the surfaces rougher.  Essentially, the torque must be increased to compensate for the added friction induced by the non-specified lube. On a side note here, the GMPP connecting rods shown in the accompany photos have fasteners (from ARP) that are designed to be torqued or stretched using 30 wt oil.  The bottom line her is be sure to check the type of lube mandated by the rod and/or fastener manufacturer before you tighten anything down.

Here’s another consideration: The surface finish of the fastener is also a contributor to the friction factor. A black oxide finish on a bolt behaves differently than a fastener that is polished, so it’s very important to observe the tightening recommendations supplied with each type of bolt.

It's easy to see that without a method of accurately measuring the stretch, it is a simple process to exceed the yield of the material and essentially fail the fastener--long before the engine has been fired. ARP adds this advice:  "Is there a predictable way to accurately follow the manufacturer's instructions and end up with correctly installed parts?  Absolutely -- if you follow the principles of stretch.  Because it is bolt stretch that provides clamping force, we primarily recommend the use of the stretch method.  The second choice is the Torque/Angle method.  Remember, check it twice."

In truth, the high performance racing industry cannot take credit for inventing the Torque/Angle method of measuring fastener installation. All of the major car manufacturers in the world as well as the majority of diesel engine manufacturers follow this regimen, not because it is easier, but because it more accurately pre-loads the fasteners.

Remember: Torque does not measure bolt stretch. It measures friction. For any fastener to supply clamp loads high enough to keep the parts bolted together, the fastener must be stretched the proper amount. That's why stretch is often preferred instead of torque when it comes to critical fasteners. And that folks, is no stretch of the imagination.  Besides, it’s a whole bunch better than “doing windows”. 

Sources
ARP Inc. 805-339-2200 arp-bolts.com	GM Performance Parts gmperformanceparts.com

 

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