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Ho Hum 74 03-21-2004 01:31 PM

ARP Rod bolt won't torque!
Hi guys,

Building my crank with new ARP hardware and my #1 rod was a little stiff when you started to turn it but when it got moving seemed fine. I measured the length of all the bolts and they range from 71.60 to 71.21mm BUT the #1 rod bolts were 71.40 and 72.00mm!! Soemthing didn't smell right.

I undid the nuts on #1 and re-torqued. The one side measures 71.35mm (within range) and when I went to torque the other side it just would not reach the 45 lb.ft I set on the wrench! I stopped somewhere before 45 lb.ft and now it is stretched to well over 72mm. I think it has gone through plastic deformation.

I'll call ARP in the morning but I guess I need a new bolt and nut. I was re-lubricating at each re-torque.

Oh, I'm so ready to put this thing this. Sigh. Can you purchase individual bolts?


Wayne 962 03-21-2004 02:34 PM

In order to measure stretch, you have to measure the bolt when it's unstretched first.

If you're rod is stiff, then there is definitely something wrong...


viejopatron 03-21-2004 08:33 PM

Jeez Wayne,

I know there's nutin wrong with my stiff rod....

Tristan - $hitcan those two bolts, however talk to ARP first. Hardware is processed in batches and the processing is not foolproof. ARP may want the parts back for a hardness test. Ask them if they'll comp you two new ones.

Ho Hum 74 03-22-2004 03:10 AM


Thanks for the input as usual.

I pulled the rod and it exhibits necking typically associated with plastic tensile deformation. It has also elongated about a mm more than the other.

I'll follow up with ARP today.

Big question is: What is their designed safety factor for the elongation of these bolts? If I was torquing and accidently went to 55 lbft instead of 45 am I still within the safety factor?


vichang4 03-22-2004 04:14 AM

That is the same question I asked myself as I was torquing my 9mm ARP bolts, what is the safety factor for these bolts? The 9 mm bolts go to 35 ft/lbs, does 45 ft/lbs ruin them?

Are your bolts new? I wonder how heat cycling and multiple torquing effects them.

snowman 03-22-2004 10:40 PM

Why in the heck are you measuring torque?? These bolts are designed to be tigntened to a pre determined STRETCH , NOT TORQUE. Why?? because torque can vary plus or minus over 100%. Torque is the LEAST accurate method of measuring the tightness of a bolt. IT changes EACH time the same bolt is tightened! A LOT. Why? because the threads are burnished each time they are used, making them more slippery. IN OTHER WORDS, NEVER use torque to measure STREARCH! just use a micrometer. By the way you measure stretch with a dial indicator, not a torque wrench. First you hook up the streatch indicator, a u shaped thing the clamps around the rod bolt. Zero it out, and then start turning the wrench. You stop when you see 0.006" or whatever the mfg says you should see. That simple, NO torque wrench involved.

Ho Hum 74 03-23-2004 05:02 AM


Look, I understand that the stretch method is the best way of getting close to the true tensile stress/strain curve of a ductile material - I went to engineering school. I also know that the 45 lb.ft of torque that ARP has written on their instructions for those of us who don't have a stretch gauge, has to have some margin of safety. All I'm asking is what that safety factor is. I know that the coefficient of friction is variable for every situation but again my argument is that repeated applications of this technique HAVE to yield results that lie within a suitable safety margin. ARP would NOT have documented this alternative method if they didn't think the results would lie within that safety factor EVERY time it was executed.

KTL 03-23-2004 10:05 AM

I'm no expert by any means whatsoever. All I can add is that I have little faith in torque wrenches when it comes to critical stuff. Lug nuts? Who cares. Rod bolts? They're kinda important.

Ease your mind by putting down the torque wrench (who knows, it might not be well-calibrated?) and get a stretch gauge.

An economical stretch gauge like this one (scroll down a bit)

***If the cheapness factor bothers you, Tavia seems to make a nice one for around $100-$150 depending on where you shop. Try for the Tavia or the ARP stretch gauge***

is still going to be much more accurate than the torque wrench for all the reasons stated by Jack and others.

I agree that since ARP provides a torque spec, it should be an acceptable way of installing the fastener. Maybe they have a lot of faith in their ARP moly lube? Why ARP doesn't strictly recommend the stretch method is strange to me. But i'm just an average dope, so I do what they tell me. Then they throw this in the catalog:

Friction is an extremely challenging problem because it is so variable and difficult to control. The best way to avoid the pitfalls of friction is by using the stretch method. This way preload is controlled and independent of friction. Each time the bolt is torqued and loosened, the friction factor gets smaller. Eventually the friction levels out and becomes constant for all following repetitions. Therefore, when installing a new bolt where the stretch method can not be used, the bolt should be tightened and loosened several times before final torque. The number of cycles depends on the lubricant. When using ARP® recommended lubes, five loosening and tightening cycles is enough. This will “break in” the threads sufficiently.

ARP kinda gets off the hook if you use a different lubricant than their moly lube? Because different lubricants are going to produce different torque values.

vichang4 03-23-2004 03:59 PM

In my situation, I am replacing the rod bolts during a top end rebuild. I am not spliting the case, and therefore cannot use a stretch gauge. I did calibrate my torque wrench just before I torqued the rod bolts. By the way the torque wrench is probably 10 years old, gets used a lot, and was dead on.

snowman 03-23-2004 07:20 PM

Sorry, I did not mean to sound condesending, wasn't intended.

Torque is GROSSLY inaccurate, and yes you can ruin a bolt using torque, especially using repeated tighening techniques. I don't know where ARP gets it process for torque. I do not know of any peer reviewed process like what they recommend. I kinda suspect that a bunch (or one or two) marketing types came up with it, for whatever reasons marketing people do things. And marketing people do a lot of really stupid, indefensable, things, and you end up reading about them when they get sued. Bottom line, money, lack of scrouples, whatever. Just because a company reccommends something, dosen't mean its a good idea, that they have properly researched it or whatever, especially in the aftermarket performance parts business, its more like "buyer beware".

KTL 03-24-2004 04:59 AM


Originally posted by vichang4
I did calibrate my torque wrench just before I torqued the rod bolts. By the way the torque wrench is probably 10 years old, gets used a lot, and was dead on.
I think you're missing the point?

Just because the torque wrench was dead on, doesn't mean the rod bolts were properly preloaded. The torque value is not what's important. What's important is inducing the proper preload on the bolt. Hence the strong recommendation by Jack to use a stretch gauge, because the proper amount of stretch is what you are ultimately trying to attain!

rick-l 03-24-2004 07:35 AM

But where do you find out how much it is supposed to be streched?? Seems like it would depend on the alloy cross section etc.


Originally posted by KTL
Ease your mind by putting down the torque wrench (who knows, it might not be well-calibrated?) and get a stretch gauge.

Ho Hum 74 03-24-2004 08:12 AM


But where do you find out how much it is supposed to be streched?? Seems like it would depend on the alloy cross section etc.
They tell you the exact amount when you buy the bolts.

The amount of stretch (actually called 'strain') is determined by measuring the change in length of the bolt. Then the load is calculated from the stress vs. strain diagram which is linear for tensile load on a ductile, homogenous material. By using this relationship of stress vs. strain you can work out the stress for a given amount of strain. Stress is a pressure and therefore you can work out the force (pre-load) on the bolt if you know the cross sectional area.


KTL 03-24-2004 08:14 AM

The install and stretch info. should be included with the bolts.

You can also go to ARP's website and download the complete catalog.

Gotta love the internet!

The chart (from the complete catalog) on Page 25 says the stretch for the 10mm ARP 911 rod bolts, ARP p/n 204-6001 Pro Wave ARP® 2000 bolts, should be 0.0117 in.

The same chart says the torque spec with ARP® Lube is 45 lb-ft.......................... :D

1fastredsc 03-24-2004 08:19 AM

I'm curious, that picture that shows the stretch gauge looks like a c clamp with a dial indicator that's spring loaded. I wonder if there is a way to spring load my dial indicator that i use for cam timing and then only have to purchase the c clamp to measure the stretch?

snowman 03-24-2004 07:32 PM

The dial indicator isn't spring loaded, other than that built into the indicator itself. The C clamp arrangement is exactly what it looks like. What you are doing is holding the dial indicator in position so that it will indicate any change in length of the bolt. The clamping arrangement must be robust enough to hold everything in place, and not change with the bumping and abuse necessary in tightening the bolt. If you have never seen a real one, buy or borrow one until you are confident in how it works.

cstreit 03-26-2004 05:57 AM

I know builders who have been torqueing rod bolts for years without a failure... While I understand the technical reasons for stretch vs. torque, I'm not convinced that the tolerances are so narrow that the torque method is that bad.

Yes the stretch method is more accurate. we realy know the Mean Time Before Failure (that was for you Tristan) for a torqued rod bolt vs. a stretch method rod bolt? Is there a practical difference? (ie 1/50,000 stretch measured bolts have an issue vs. 1/39,000 torqued bolts do...)

I talked to Tristan and clearly his bolt was defective.

Having said that, I submit THIS:

Had Tristan used the stretch method, with a defective rod-bolt, he would have measured the effective stretch and the bolt would have been under-torqued! He more than likely would have paid the price with catostrophic engine failure when it let go.

I think I'll keep using the torque method.

snowman 03-26-2004 10:29 PM

HUH??? never seen any scientific info to back up this viewpoint. Aircraft people are paranoid about any possible failure of anything, due to the costly results of failures in their business. All the fancy stuff on fasteners was developed by and for aircraft, so I would listen to what they say as most likely the best way to do things.

nreed 03-27-2004 05:06 AM

Ho Hum,

You have me a little worried now...

I have new ARP bolts and will use them in the next couple of days. My part number is 204-6005; are these the parts you are talking about?

If so, did ARP happen to mention anything about manufacturing dates or batch numbers? I am wondering if there is a way to know if mine are from the same batch as yours.

Ho Hum 74 03-27-2004 10:16 AM


Aircraft people are paranoid about any possible failure of anything, due to the costly results of failures in their business
Snowman - one thing you're forgetting is that design safety factors on aircraft usually run from about 1.2 to 1.5 as opposed to higher multiples for other mechanically engineered applicaitons like automobiles. Therefore you have to use methods that will reliably keep you within this range. Hence the stretch method. Secondly, all critical aircraft fasteners are lockwired which add additional safety.

I stand by my opinion that this method MUST CONSISTENTLY give results that are within the engineered safety factor for the bolt (which are higher than the aerospace industry). ARP have verified this with me.


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