14 bolt head torque specs...

[STP]

Plan to bolt my GN1R's on this weekend and wanted to see what the consensus was on torque specs. Don't want to over torque them....Torque them all the same? Or less on the bottom rail? Using studs... Thx Scott

 

[mayosmechanic]

Short bolts...65 lb ft Long bolts...75 lb ft Torque in three steps 45,55,65.Then long bolts to 75

 

[ChrisCairns]

I've heard people saying for years to torque long bolts different amounts than short bolts.

Why would anyone want different clamping forces being exerted on different sections of the head??

 

[mayosmechanic]

That is the sequence Champion has come up with.Can any gasketing experts out there answer that one?

 

[ijames]

I'm pretty sure the longer bolt/stud will stretch more at the same torque, so you have to torque to a higher value to get the same clamping force. (99% sure and too lazy to go look it up for that last 1% .)

 

[ChrisCairns]

I for one would really appreciate it (please oh please ) if you could go look it up. I've often wondered the reason people kept saying torque higher on longer.

I understand that the longer bolt would stretch more but once it's stretched shouldn't the torque still be the same on all bolts??

 

[ijames]

[Warning, the short answer is I was kind of wrong . Skip to next paragraph.] First, when tightening a normal bolt (not a torque to yield bolt), the stress in the bolt is kept below the yield point so that the bolt acts as a spring. As you tighten it, it will stretch but not permanently deform, so when you loosen it, it returns to its original length. Under these conditions the elongation is equal to the load in pounds times the length divided by the cross-sectional area times Young's modulus [P*L/(A*Y)]. Young's modulus is basically the spring constant and is 29,000,000 psi for basically all steels. When you tighten a bolt, a portion of the torque goes into overcoming friction on the threads and under the head of the bolt, and the rest is transformed by the inclined plane of the threads into an axial force which provides the clamping force. That's why uniformly lubricating the head and threads is so important, so the fraction that goes into clamping is constant for all the bolts, so the clamping force is uniform. That's also why there are different torque specs depending on what lube you use, because that changes the ratio of friction to clamping force. ARP's website has some info on clamping force versus torque. Anyway, to make up some numbers, let us say that the cross sectional area of the bolt is 0.15 sq. in. (a 7/16-14 bolt), the length is 4 inches, and that for a stress limit of 60,000 psi (a factor of three safety margin with a 180,000 psi tensile strength bolt) the clamping force is 6355 lbs (calculations not shown ). Now, take (6355 lbs * 4 in) / (0.15 in^2 * 29000000 lbs/in^2) = .0058". So our 4" long bolt elongated 5.8 thousandths of an inch under these conditions.

However, while a longer bolt will elongate more under the same stress, under the same stress the clamping force will be the same no matter how long the bolt is. You just need to turn the bolt farther to get the same torque and thus the same stress. In other words, a 1" bolt will elongate .0014" at the above stress level, while the 4" bolt elongates .0058" at the same torque and the same stress level. You just have to turn the 4" bolt an extra 20 degrees or so to get to the same torque. So there really isn't any reason I can see based on internal stress and resulting clamping force to torque longer bolts to a higher value than the shorter bolts. However, as the bolt warms up the amount its length increases is proportional to length, so the longer bolts will grow more and thus their clamping force will drop further than the shorter bolts. Maybe this is the reason? Except that steel expands a little less than cast iron so the bolts will grow less than the head so the clamping force actually rises with iron heads and rises much more with aluminum heads. However, the long bolts will always have less clamping force than the short bolts since they expanded more, so I bet this is the real reason since you can't torque the bolts at engine operating temperature.

 

[ChrisCairns]

Wow Carl. Nice writeup !!

It makes sense now....I always believed that the same torque applied to different length bolts would apply the same clamping pressure...yet the bolt mfg. always said to use different values based on bolt length.

You've now explained that under heat the bolts would change lengths and the longer would "lose" it's clamping force more than the shorter.

As you also pointed out....now it's simply a matter of final torquing while the engine is hot and running. That should be fun while the headers and valve covers are off.

Thanks for taking the time.

 

[DMan]

Outstanding writeup Carl! A true engineering reply from an engineer (or someone with Machinery's Handbook at his bedside!)

One question..

Originally posted by ijames ...since you can't torque the bolts at engine operating temperature. [/B]

You probably could throw the bolts in an oven for a couple hours and then torque them..on second thought neither the head nor block will be at operating temp so your critical dimensions will be off..scratch that.

Respectfully yours, from a fellow 'rocket scientist',

 

[ijames]

No, I'm no engineer even though I'm playing one at work these days. Theoretically, I'm a chemist, but I'm a pretty promiscous reader .