We checked the rod bolts on one rod and got much lower stretch than I expected. At 75 ftlbs, the chart showed .006. When we checked with a gauge, we were getting .004. When we ran the torque to 85 ftlbs, we only got to .0045. We were using the ARP lube per the instructions.

I thought the stretch numbers would be lower, but not that low. When we torque everything down, I am wondering if it is reasonable to chase the .006 figure, as that seems like it will be over 100 ftlbs per the torque wrench. I may try a different torque wrench, but the one we were using was a quality piece from Snap-On. Do these numbers seem out of line?

 

had you pre-torqued ?? i think it calls for it

 

Fatigue is what makes many parts fail and it's an elusive swine. If you can pre-stress the bolt to 60% of it's ultimate tensile strength it's fatigue life goes way up. Tightening with a torque wrench is just an approximate way to measure the residual stress in the bolt and can vary dramatically depending on the condition of the parts and their lubrication and even what lube is being used. Since the torque wrench measures the friction too it makes it tough to isolate just the stress on the bolt and get it right.

Many people are anxious about using a stretch guage the first time, because they're comfortable using torque wrenches for every tightening job on the engine. Since rod bolts are subjected to the greatest cyclic stress in the engine, and we can measure the actual bolt installed length, (try that on a main bearing or head bolt), using stretch is much more accurate in getting close to the yeild strength without going over. Bottom line - trust the actual measured stretch more than any torque readings.

If these are new bolts though you need to tighten each at least 3 times to burnish the threads and to seat the rod bolt in the cap before the torque readings start to get consistent. ARP recommends 5 times for their bolts. The first just overcomes all the friction between the various parts, the second time begins the burnishing, and the third time willl begin to yield consistent results. This should be done with the rod in a vise (with protectors on the jaws) using a box end wrench on the nut/cap screw head and your stretch guage.

Most people don't realize that undertightening is much worse for a bolt's fatigue strength than overtightening - as long as you stay below the yield strength of the bolt. Getting close without going over is much easier when the bolt is measured.

 

Good stuff here. I bought the rods new (Eagle I beams) with ARP bolts installed and tight, around 50-60 ftlbs. We measured stretch when loosening them the first time and had stretch of around .003.

Four of the rods have been torqued to 50 ftlbs and are on the crank so we can check for clearances on the stroker crank. From reading the last message, I need to cycle the fasteners a few times before going after true readings.

 

i couldn't remember the exact #