STRETCHING FOR LIFE
tid-bits from the bench
with excerpts from ARP's resources
OUR GOAL
ARP rod bolts...the de facto standard in fastening technology. It is with no surprise that a large number of rod makers (including us) use ARP as their "go-to" fastener to keep that cap firmly attached to the rod...as arguably, the most important fasteners in any engine are the connecting rod bolts. They are the key to the entire rotating assembly and the weakest link in the connecting rod itself. A broken bolt will lead to a grenaded engine...and will end a happy day real quick.
So *why* do we need to use a stretch gauge? Why can't we just swing the bolts to a pre-determined value with a torque wrench and call it a day? Below we will attempt to explain the importance of having the most accurate clamping load possible so that you can keep racing...not keep repairing.
FRICTION...OR LACK OF
In addition to using a rod bolt with sufficient strength to withstand the tremendous cyclical strains placed upon it, it is absolutely mandatory that the bolts be properly tightened. The preferred method of monitoring the correct amount of tension is through the use of a STRETCH GAUGE.
Why is it the preferred method?
One word...friction. Maybe two...frictional loss.
Friction is a challenging problem because it varies so much, and is extremely difficult to attain repeatability by using commonly known lubricants. The best way to avoid the known variables associated with different lubricants is by using the stretch method. By using the Stretch Gauge you are removing the friction variable, and the clamp load can be controlled AND repeated. The type of lubricant is actually irrelevant.
To make matters even hairier, we have also found that each time a new bolt is torqued and loosened IN AN ALUMINUM ROD, the friction value will decrease with each torque cycle. This is because of "thread burnishing". You are essentially decreasing the friction by "polishing" the threads with repeated cycles of tightening and loosening the bolt (torquing).
When this cycle happens an excessive number of times, the torque value recommended for the bolt will spread farther and farther away from the stretch value recommended for the same bolt. In other words, if your torque value is a recommended 50 ft/pnds (or .006" stretch), you could very well achieve that .006" stretch with only 35 ft/pnds! We've seen this happen many times. The relationship between torque and stretch values *should* be very similar...if not the same, and NOT spread apart to such a degree as in the above example.
What happened? Those repeated torque cycles were the culprit. Hmm...
Just think...if you didn't have a Stretch Gauge to give you such information, I would predict a guaranteed rod punt in your future. This is a SERIOUS factor to consider...and the Stretch Gauge protects you from such erroneous readings created by excessive "thread burnishing".
How do you avoid that scenario in the first place? Simple. Don't burnish the rod's threads excessively. Don't torque/loosen/torque/loosen any more than necessary to achieve your target journal clearance.
We can't speak for other rod makers, but thankfully with our rods, the rod bolts come "seasoned" or "preset" to have the bolt give a very accurate torque wrench value to stretch value...provided you are using the specified weight of motor oil and your torque wrench is calibrated. Basically, torque the bolt to 90 ft/pnds, and you're already pretty darn close to the recommended stretch value...if not, spot on.
This is more work for us, yes...but we want to make sure things go as smoothly as possible for you at assembly time...as we realize, not everyone owns a Stretch Gauge. We do what we do.
Regardless, stretch is stretch and the ONE value in a connecting rod that needs to be closely monitored...NOT torque values. Because essentially, torque is merely measuring friction...not the true clamping load that is given by the bolt.
