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Old 05-26-2005, 08:59 AM
k-star k-star is offline Moderator

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I have done a bunch of testing on this subject. I spent an entire day and the best part of another working on it. The conclusion i came up with is that i will never ever use a torque wrench on a rod bolt again,,,,, stretch only for me.!!!

I tested a few different size bolts and cap screws in different rods. All arp fasteners..There are so many factors that effect torque it will make you head spin....

I found that the bolts need cycled a few times to settle in, after that they will repeate for the torque to stretch ratio.... Meaning that after stretching them 2 or 3 times they will stretch say your .006 with the same amount of applied torque..Before that they might be off 5/7 ft lbs to get the correct stretch....I checked the overall length in the new relaxed state and then pulled them up and released them and checked it again. They all seamed to grow about .0008/.001 and then take a set and stay there after the cycles....

I had one rod that had a cap with the nut surface damaged. I installed the nuts and pulled it up to a set stretch and checked the torque,,,, I then machined the nut surface on the cap and did the same test everything the exact same.... It took 15 ft lbs less to get the same amount of stretch..... There is just to many things to go wrong to use a torque setting on a rod bolt in my opinion........

One thing i did find interesting.... I tried different lubes on the threads,,, other then the moly the rest all acted about the same. I used oil, assembly lube, loctite, wd-40 etc.......And the arp moly lube was only a tick better....

Something else,,, as a general rule the bigger bolts were futher away from the needed torque to get the correct stretch...

The closest i could get to the catalog torque/stretch spec's was to cycle the bolts, using the arp moly on the threads and under the nuts. And make sure the mounting surface was in perfect shape. But i was still off a good 10 percent from the recommendations. Meaning i needed to go higher with the torque to get the correct stretch....

I just did those scat rods that i talked about in larrys other thread and they have the good 7/16 cap screws in them... I made a special tool to tighten the bolts in the engine with the gauge still attached. It is made from a craftsman 3/8" breaker bar. I needed to put a small section of pipe on it to get them to pull up to the required .0065 stretch...... I can only guess but i'll bet it was 80 or 90 ft lbs to get it.........

As a side note here,,,, If your machinest sizes the big end using a different method then what you use the bore could very well be off. Before i had my rod hone i had another shop do my rods. I could never figure out why most of the time they were on the small side. I then found out he just pulled them up with a torque wrench and i always used the stretch method. Once i stated re-sizing my self the problem went away..... So ask when you get your rods done how he did the bolts.........


Last edited by k-star; 05-26-2005 at 09:04 AM.
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