ARP Rod Bolt Torque Values

[DonE]

Sorry but I checked the archieves with no luck.

Does anyone know the torque values for 9mm ARP rod bolts for a 3.3 turbo? I can not measure stretch, so I need torque.

Thanks

[ekessel]

With the ARP lube, it's 35 ft/lbs, (they recommend tighten/loosen three times).

-Eric
90 C2

[Jeff Alton]

I just put these on my rods and used a stretch gauge. They took a fair amount more than 35lbs to reach proper stretch. I am not sure how much stretch gets retained in the bolt after it is loosened prior to retorqueing, but I the bolts do have some elasticity.

But eric is right, that is the spec ARP uses if you do the torque method.

Cheers

[DonE]

Thank you

[tadd]

I would consider torquing up a rod or two in a vise measuring the stretch to get 'torque values' for when you put them on the crank. Almost best of both worlds.

tadd

[Rob 930]

Don,

My 930 ARP rod bolts were torqued using both a torque wrench and a stretch gauge. We considered them to be "torqued" when the prescribed stretch value was reached -- the torque wrench was just for reference. When they reached the proper stretch value, the torque wrench read 41 ft-lb. I wouldn't say that you should necessarily torque yours to 41 ft-lb absent the input from a stretch gauge, but you can add this to your mix of incoming information. It seems to confirm what Jeff said above.

Rob

[Jeff Alton]

Rob, that is what I was trying to say but couldn't see to!! When playing around with the gauge I noted that it yoy put up to say 15-18lb of torque on the bolt and then undid it, it would return to its origianl length. My machine shop, which has used 1000's of ARP rod bolts for different applications, also confirms that the two methods don't yeild the same torque on the nut.

All that said, I *think* the majority of people use the torque method and not a stretch gauge and I have never heard of a problem from it.

Cheers

[starlifter1]

Dont you use an dial caliper to measure stretch?

[ekessel]

I found the threaded end of the ARP rod bolt was not ground flat, so I couldn't get a reliable reading with a micrometer (I had a micrometer, but not stretch gauge). Maybe a micrometer, or caliper with a point to fit in the center "divot" on the bolt........

-Eric

[Jeff Alton]

I used the ARP stretch gauge that incorporates a dial indicator, here it is.....





Cheers

[Mike Juzenas]

Note that that gauge is calibrated in .0005" increments unlike most other cheaper guages that are in 0.001" accuracy. Its works great and I had the same results as Jeff where the torque to get the proper stretch is definitely higher than what Arp says.

I bought the gauge at the local auto parts center that has a performance shop.

I seem to remember the 8mm bolts for my 993 were .017" stretch...would have to look up my notes to confirm.

Cheers,

Mike

[tadd]

The extra torque is put into over coming the thread friction. As soon as you stop turning the bolt, that 'needed extra' just disappears. Angular measurments can be much better, but since there is uncertiantily in the starting torque... Best is a stretch measurment.

That said, I have had KICK ASS results with a torque wrench that uses a piezo element and cacluates the rate of force change. That sucker will nail the very, very begining of the plastic range every time for tension applications regardless of the bolt type, material type or size.

tadd

[Jeff Alton]

I not sure if I am saying the same thing here or not, but it is my understanding when you tighten a fastener what you are really trying to achieve is a specific tension. Stretch measures it very accurately, torque puts friction into the equation. That said, it would seem to me that the manufacturer of a given fastener takes that into account when they provide a torque spec.

Cheers

[Rob 930]

Jeff,

You're exactly correct. Because torque is an indirect (but convenient) way to measure bolt tension, manufacturer's torque specs are designed to acheive that desired tension, and have already accounted for frictional losses and other uncertainties inherent in the torquing process. But those uncertainties can be significant, so it was not surprising (to me) to see that the torque spec for these ARP bolts (35 ft-lb) was 17% lower than the torque needed to achieve the specified stretch (in my case). The good news is that there's usually a pretty wide range of acceptable torque levels, and I haven't heard of any rod bolt failures that resulted from assembly when they were torqued with a calibrated torque wrench to 35 ft-lb.

Rob

[tadd]

Catca:
Your insight is spot on, measuring the stretch of the bolt is a direct, accruate way of determining the installed preload on the bolt. That preload is the basis of the friction between the threads that keeps the bolt from 'coming undone'. You want to add enough tension that the bolt won't come undone in operation, but not enough to put the bolt material in its plastic range where it perminately deformes. You want to hit the sweetspot where the bolt is elastic (can stretch and deform) from operational loads and not unloosen, but not over do it and stretch into the plastic region where it does not 'snap back'. Typically this is about 60% of its ultimate yield strength (where it breaks).

Unfortunally, when you install using a 'torque wrench' that installation friction from rotating the threads of the bolt in the hole can vary widely thanks to the metal types, metal finish (surface roughness, platings), lube (dry,oil, moly), and bearing area. So the typical torque tables are only a guide, not gospel. Until the derivative torque wrenches which actually measures the rate of change in applied torque, so the wrench knows when you start into the plastic range, the only accurate way to get the correct preload was to measure stretch directly. For blind tension applications (where you can't get to both sides of the bolt) they came up with these way cool washers that have a thin plastic coating that changes color to let you know when you have the correct preload.

This is also why 'lock washers' are an abomination. Anything that is soft enought to deform while tightening will be soft enough to deform under load. When that happens all the wonderful friction holding your bolt in the hole evaporates and the bolt can unloosen in that fraction of a second. Thread locking compounds avoid this althogether.

Finally, it is always better to overtighten than undertighten from a fatigue standpoint, hence the 'sort of' in the sig line...

tadd

[cstreit]

Originally posted by ekessel
With the ARP lube, it's 35 ft/lbs, (they recommend tighten/loosen three times).

-Eric
90 C2

I'm pretty sure this is for the first use only, not successive uses...

[cstreit]

Wanted to add that you should really use a torque/stretch method. Obviously stretch is the most accurate, HOWEVER I have seen a few bad ARP rod bolts that stretched to the proper value but had VERY little torque ont hem because the bolt was faulty. Would have resulted in a dropped rod.

That said, use stretch but verify that your torque value used is close to their recommended value for torque method.

[Jeff Alton]

Good point Chris. That, in essence, is what I did.

Cheers

[DonE]

Originally posted by cstreit
I'm pretty sure this is for the first use only, not successive uses...

Nope - I checked with ARP tech support yestreday and you can re-use as long as the threads are in perfect shape and you should keep the bolts paired with the nuts. And always use the moly lube for rod bolts.

[cstreit]

Originally posted by DonE
Nope - I checked with ARP tech support yestreday and you can re-use as long as the threads are in perfect shape and you should keep the bolts paired with the nuts. And always use the moly lube for rod bolts.

Don,

I was speaking of the "torque/untorque three times" procedure. You only do this the first time you torque new bolts. When you re-use them later you don't need to torque/untorque them 3 times...

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