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engineczar 05-26-2005 08:16 AM

rod bolt stretch (poll)
What's the highest torque you've had to use to get the proper stretch?

I've got a 2.2 mopar mini stock motor I'm building and I'm using ARP 3/8 220kPSI rod bolts. The spec is 43ft/lbs with ARP lube or .0055 stretch with oil.

To get the proper stretch I eventually had to go to 70 ft/lbs. I'm just curious how high others have had to go because even though the literature is somewhat clear, for someone not stretching them it seems like telling them 43ft/lbs is kinda asking for a failure.

coldknock 05-26-2005 08:25 AM

I'l tell ya in a few weeks. I just bought a stretch gauge and I haven't used it yet.

I've always torqued the bolts 10ft/lb over spec and let them sit like that until I build the engine anyway. I couldn't tell you if it made a difference, that's just what my gramps told me to do and we never lost one of his engines from a bolt failure.


k-star 05-26-2005 08:59 AM

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.........


NDNslicks4me 05-26-2005 09:05 AM

It seems to be pretty unanimous. I wonder why the manufactures recommendations are so far away from the real world examples? Could the Tq # be a "theoretical" measurement(ie no friction)?

machine shop tom 05-26-2005 09:07 AM

We use the moly lube. It takes around 55 ft. lbs. on a 350 with the ARP bolts. You have to be careful, though, sometimes after a couple of torquings, the bolts will stretch a little easier. Quite some time ago, I had to take a set of BBC wave-lock bolts to 85 ft lbs with SAE 30 oil. That's when I started using the moly lube.


k-star 05-26-2005 09:17 AM


Originally Posted by NDNslicks4me
It seems to be pretty unanimous. I wonder why the manufactures recommendations are so far away from the real world examples? Could the Tq # be a "theoretical" measurement(ie no friction)?

What many people do not realize about fasteners is how they actually stay tight.... They are really a spring in a sense. The stretch of the bolt it what keeps the nuts on. The stretch keeps the pressure on the nut, keeping it from backing off...not the rotational torque. I am sure there is a relations ship between the two that is exact in controlled conditions...... But engine building is seldom a controlled condidtion....L.O.L.

I am sure the torque is a acceptable way with good results but being a engineer i am a little more anal about things.... I never had a rod bolt failure when i did torque them and didn't check the stretch, but now that i have seen what happens i cannot bring myself to go back......

I have seen a few engines built with the nuts laying in the pan, both from proffesional and non professional engine builders, and i am sure that the reason was because the bolt was not stretched correct causing the nuts to come off and well you know the rest..........


johnsongrass1 05-26-2005 10:00 AM

If you have 800 in a stock refresh, torque is fine. Look how long it's been working, lok and see what high volume(GM, Ford, ect...)assembly plant's are doing.

When building something that cost's 10 to 30 grand, You can't afford not to.

crazy larry 05-26-2005 10:44 AM

I torqued mine, arp's. and have to say that i had major misgivings for not getting a stretch gage. but so far, so good. over one year and 11,000 miles and no cato. yet. ;)

I did go fifteen pounds heavier on torque, as long as the crank was able to spin freely. just out of fear. :sweat:

TurboS10 05-26-2005 10:46 AM

I am curious how much torque you put on those new rods K-star. I have the same ARP bolts in my rods. I called ARP and they recommended 62lb/ft with moly lube, which I used. Mine came torqued in the rods and were torqued another two times between the engine rebuild and my machinist checking them at that time. If the trend is that stretch gets easier after a few cycles, I feel better. 80-90 lb/ft is a big difference over 62....makes me a tad nervous.


k-star 05-26-2005 11:21 AM

Sorry Chris i can't help you out there..... The engine is in the car!!!! If not i would check them for you......

I didn't even get close to them with a torque wrench so i am guessing at the torque but i am sure it was over 62 ft lbs,,,,

The other issue that comes into play is the calibration of the torque wrench you are using....

You had your truck out a few times and leaned on it pretty good didn't you????? If it made it through a few pulls it will be ok.....No worries....


TurboS10 05-26-2005 12:04 PM


Originally Posted by k-star

You had your truck out a few times and leaned on it pretty good didn't you????? If it made it through a few pulls it will be ok.....No worries....


6000RPM with 12PSI many times now......I would have thought that would pop them loose if anything would.


347mustang 05-26-2005 12:45 PM

its all a bunch as comes the same ol grandpa has a 9.90 car..small block chevy...he has never ..never used a stretch gauge..just a torque wrench...he runs the car all year and in the winter tears the motor apart for new gaskets etc..and inspection..he has never lost a motor over a rod bolt..and he has built 100;s of race motors for the locals...

machine shop tom 05-26-2005 12:51 PM


Originally Posted by 347mustang
its all a bunch as b.s................



k-star 05-26-2005 01:00 PM


Originally Posted by 347mustang
he has never lost a motor over a rod bolt..and he has built 100;s of race motors for the locals...

Good for him.................


ratlover 05-26-2005 01:04 PM

you said it Tom.
hey 347, how many motors have YOU built? :confused:

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