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Fastener Tech 101

Posted by 99gst_racer, Oct 1, 2010

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  1. 99gst_racer

    99gst_racer Moderator

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    I find myself reiterating often about fastener functions, torquing, yield strengths, etc, and I've finally decided to put it all on paper and in one thread. :) Enjoy.



    What is a fastener?
    A fastener is a threaded bolt or stud (and nut) used to clamp (or fasten) and hold two or more objects together. Generally, all automotive fasteners are made of some sort of steel or alloy of varying strengths.


    How exactly does a fastener work?
    A fastener works by using a measure clamping force between two or more objects. All fastener materials have an advertised yield strength and tensile strength. Simply put, yield strength is the point when the fastener will begin to permanently deform and become "failed" and unusable. Tensile strength refers to the point in which the fastener's diameter begins to "neck down" due to far exceeding it's yield strength. We typically only use a fasteners yield strength in the automotive world, as any fastener that is pushed beyond it's yield strength becomes questionable.

    We use a fasteners advertised yield strength and diameter to calculate it's maximum torque value. A good rule of thumb is to torque a fastener to 75-85% of it's yield strength per given diameter. This is usually a safe and conservative amount that will allow the fastener to function to the best of it's ability and without resulting in failure. A properly torqued fastener will actually stretch a small amount when installed since no material is incompressible. This force occurs when a fastener is tighten beyond "finger tight" and is commonly called a preload or clamp load. In a sense, a preloaded fastener acts much like a spring. We torque it down and stretch the bolt/stud to a measured amount, and it's natural reaction to want to rebound is what creates the clamping force. When the fastener is removed, it should completely rebound back to it's initial free-standing length. If a fastener does not completely rebound, then it's considered to be compromised and is no longer fit for use. Even if the difference in length is only +.001", that's enough to deem the fastener as weakened and failed. This happens when a fastener is torqued to a value beyond it's yield strength. This is why a greater torque value does not always equate to more clamping force. That only stands true if you're still working within the limitations of the materials yield strength. If you find yourself in a situation where you've maxed out a particular fastener and you are in need of more clamping force, then it's time to step up to a stronger material. NEVER TORQUE BEYOND IT'S RATED YIELD STRENGTH!


    Strengths of commonly used materials:

    Grade 5 - YS: 90,000 psi, TS: 120,000 psi
    Grade 8 - YS: 120,000 psi, TS: 150,000 psi
    8740 chrome moly - YS: 180,000 psi, TS: 200,000 psi
    L19/H11 tool steel - YS: 200-230,000 psi, TS: 260,000 psi


    Proper Installation?
    To achieve an accurate torque measurement, we need to overcome as much friction as possible using the least amount of "extra" force. The prime area for friction occurrence are the threads (more specifically, the top side of the threads) and the 'flat' underneath the nut or bolt head. To combat against friction, we use a lubricant on these areas. Without a lubricant, the threads will drag and create friction and that will "inflate" the torque value, which will result in less clamp load than you intended or measured. Not all lubricants were created equal either. For example, moly lube will relieve much more friction that when using motor oil. In a sense, 120 ft/lbs with motor oil is basically 80 ft/lbs of actually fastener torque and 40 ft/lbs of friction drag. Using moly lube will help overcome the greatest amount of friction, and will yield a much more accurate clamp force across the board. Different materials and surface coatings will also have an affect on friction, and most manufacturers will account for this when they recommend a torque vale for their fastener, so it's always important to take their recommendation into account rather than only use a rule of thumb every time. It's also important to periodically have your torque wrench check for accuracy. It's not uncommon for even the best wrenches on the market to lose a small amount of accuracy.


    Measuring stretch?
    This won't apply to most of us. I can personally attest that I've never bothered with measuring any of my fasteners. I've never felt that I was at the level where it becomes important enough to be deemed mandatory. However, if you're picky and have access to a stretch gauge, then it definitely doesn't hurt.

    Basically, you just need to measure and record the free-standing lengths of each fastener. And when the fastener is removed at any point, re-measure, record, and compare your findings with previous measurements. As previously mentioned, the fastener should ALWAYS rebound. If it doesn't completely rebound, then it's junk and will need to be replaced.



    I hope most of you find this to be informational and useful. Hopefully this will help reduce the common comments that I see around here like "bolts aren't supposed to stretch" and "bolts that stretch are trashed". As always, feel free to post questions and I'll do my best to answer them. :dsm:
     

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  2. yepes281

    yepes281 Proven Member

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    Thank you for clarifying what a fastener is. Every time i tell someone i work at fastenal their like, OMG whats a fastener!?!?!? OMG
     

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  3. 99gst_racer

    99gst_racer Moderator

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    Commonly used head stud maximum torque values:
    These ratings are all with moly lube, as that is the only lubricant I will ever personally recommend.

    7-bolt (11mm) standard ARPs: 75-80 ft/lbs
    6-bolt (12mm) standard ARPs: 85-88 ft/lbs

    7-bolt (11mm) ARP L19 or A1 H11: 95-100 ft/lbs
    6-bolt (12mm) ARP L19 or A1 H11: 100-105 ft/lbs
     
    Last edited: Sep 21, 2011

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  4. iugrad92turbo

    iugrad92turbo Proven Member

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    Interesting thread nice reading.
     

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  5. ricktb

    ricktb Proven Member

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    i would love to get all scientific on every nut and bolt on my car, but some places are hard enough to get a normal box end wrench in there, nevermind a socket or even a torque wrench, so I go by feel when I can't put a torque wrench on it. Some interesting info here, tho thanks. Should I be lubricating all my bolts as I torque them to spec? I thought the friction in the threads is what holds the bolt from backing out under vibration, heat cold, etc.
     

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  6. 99gst_racer

    99gst_racer Moderator

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    I don't use a torque wrench on 90% of the fasteners on my car. Many of them aren't critical enough to require a specific clamp load - they just need to be tight enough to not come loose.

    I would only worry about using a thread lubricant on high torque applications. The lesser the clamp load, then lesser the torque value can be, and that means less friction on the threads. Moly is important where there is a great amount of friction present and a very accurate clamp load is required - like on a cylinder head or main girdle.

    The clamp load is what holds the fastener from coming loose. There will always be some amount of friction on the threads even when a lubricant is used. Lube helps overcome most of it, but not all of it. So the small amount of friction that is there probably plays a small part in fastener retention, but preload is the primary retaining force.


    While we're on the topic, it's also worth noting that the level of friction of the threads changes. Friction is at it's highest level the first time that fastener is tightened. As you cycle torquing and loosening, the level of friction will continue to reduce itself and eventually level out at a constant. ARP recommend 5 cycles to reduce their fasteners to a lesser and constant friction value using their moly lube.
     

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  7. 123bobby123

    123bobby123 Supporting Member

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    Paul, good write up. I just wanted to add something as I'm sure others have run into this as well. On my 1g I have come across some instances where a threaded through hole had to be drilled out and "converted" from a joint where a bolt is threaded into a piece of the subframe to a joint where a bolt and nut is used. The inboard bolts for the half circle brackets on the front LCA's is an example of this. When you do something like this it is often tempting to get the strongest bolt you can to avoid failure, but you have to take two things into account here.

    1.) The stronger a bolt you get, the more torque will be required to get the proper bolt stretch. But you have to make sure that the nut you use is of equal or higher grade/class then the bolt you use. If the nut you use is of a lower class or grade then you will just strip the nut out. Think about an example using a steel bolt and a plastic nut (as silly as this may seem).

    2.) Take into account the material you are clamping between the two washers or washer and bolt/nut. Is it a solid piece of metal or stamped metal? Stamped metal may collapse when you tighten the fastener if it is not strong enough.

    Bill
     
  8. GRAVEDIGGER

    GRAVEDIGGER Proven Member

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    I never knew ARP recommended 5 cycles, but that makes perfect sense. Very interesting and informative read Paul. Thanks for all the great info:thumb:
     

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  9. shane90gsx

    shane90gsx Proven Member

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    Very cool:hellyeah: how about other tq specs?

    arp rod bolts. (say used with egl rods) 2000's?
    arp main studs?
     

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  10. 97gst_spyder

    97gst_spyder Proven Member

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    All I had with my ARP L19's was 10w-30 Mobile One... I torqued them to 100ft/lbs. Should I have went higher?
     
  11. BigBeans

    BigBeans Proven Member

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    Yes.

    I just did mine @ 105 with Ultra-Torque lube, and some people go higher than that. You should probably be around 135-140 with motor oil.

    You could always pick up a tube of Ultra Tq lube for a few bucks. Once you use it you'll never go back to motor oil. It's so much smoother, easier and more accurate.
     
  12. 97gst_spyder

    97gst_spyder Proven Member

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    Oh, wow. That might be why my compression is 182-180-184-200...

    What store has that lube?
     
  13. BigBeans

    BigBeans Proven Member

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    Check ARPs website for a local ARP dealer.

    I got mine at a speed shop locally, but it's not a chain store.
     
  14. 99gst_racer

    99gst_racer Moderator

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  15. Stew Pidaso

    Stew Pidaso Proven Member

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    Very interesting read but just want to clarify real quick on the heat cycles. ARP recommends 5 heat cycles so is that torquing the studs to 100-105 (l19 in my application), going through heat cycle, then re-torquing back to 100-105. Could you go through the procedures for torquing and heat cycling please. I've done many searches for l19 torque specs and what others have done and it seems everyones method is different.
     

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  16. AwD4g63TsI

    AwD4g63TsI Proven Member

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    He never mentioned heat cycles. When referring to "cycles", he's referring to tightening to full torque, loosening, tightening, loosening, etc...5 times total. This will wear down the microscopic rough edges on your threads and make them smooth, thus giving you more accurate torque readings and the higher clamping force you want from your fastener.
     

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  17. 99gst_racer

    99gst_racer Moderator

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    ARP recommends 5 torque cycles, which has nothing to do with heat cycles.
     

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  18. Stew Pidaso

    Stew Pidaso Proven Member

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    Oh ok my bad on that mistake. What should the cycles be for l19's? I heard that the last cycle is the most important and should be the biggest jump. Is that correct? Also are heat cycles of any importance with head studs. There are threads on here about it but all of the responses are mixed opinions.
     

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  19. 99gst_racer

    99gst_racer Moderator

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    The reason for torquing is steps is so that there isn't ever a large deviation of torqued value from one stud to the next. The rule of thumb is to divide the final value by three, as to do 3 roughly-even steps. This isn't set in stone though, so just make sure you aren't ever taking any big leaps. Assuming you have a 6-bolt (12mm L19's), I'd do 35-35-30 or 35-35-35, depending if your final value is 100 or 105 ft/lbs.

    I haven't found any evidence that indicates re-torquing after heat cycling is necessary.
     

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  20. Stew Pidaso

    Stew Pidaso Proven Member

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    I have a problem with this statement because clamp load can be achieved when torquing a bolt and the clamp load stays stagnant, due to friction, when removing the wrench. However when removing bolts, torque decreases, by roughly 25% in most cases if not more.

    I did a study on this. I applied torque via a torque wrench through a transducer and to the bolt, the transducer was reading the torque being supplied to the system via the torque wrench. The bolt was being threaded into a piece of a hub and between the bolt head and hub piece was a load cell to measure and display clamp load between the bolt and hub. What I saw was when we torqued the bolt to spec (260 ft-lbs), we received a stagnant clamp load (roughly 30,389 lbf) but when loosening the bolt, the torque required to loosen was only 175 ft-lbs. This was a "dry" setup meaning no lubricants were used and we haven't done any testing with lubricants in bolts to know what it does with respect to clamp load or torque.

    What I'm getting to is that even though torque was decreased in order to get the bolt free from the "nut" clamp load was not lost. Also to keep in mind this was in a matter of a few seconds, I torqued the bolt to spec, immediately flipped the wrench into "removal" and loosened the bolt. Studies have been done where the bolt is left over night or over the weekend and I've seen bolts lose up to 40% of their torque upon removal but clamp load remains stagnant after being tightened.
     

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