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Engine head stud torque/angle

  1. Jun 19, 2012 #1
    Hi, cant believe I haven't discovered this forum until now - I would post in an introductions section, however I couldn't find one!

    This question is loosely based around mechanical engineering, however may be applied to automotive, so apologies for posting in the wrong section.

    I have had a set of hardened head studs made for my vehicle (for upgraded power/boost etc), the stock ones are grade 10.9, and the never ones should *in theory* be above 12.9 grade, by the by, they are stronger due to them being hardened.

    My understanding is that in this situation, the newer hardened studs should stretch the same amount as the old studs under the same amount of loads, however due to their hardness and bearing hookes law in mind, they should purely be able to stretch further without yielding?, am I correct in my assumption?. Therefore would be able to be torqued to a higher torque angle than the stock bolts, applying a greater force on its respective load.

    Stock torque angle is 220°, and with the newer bolts then a greater torque angle should be able to be acheived, without damage and yield of the bolts (assuming same conditions, bolt pitch, friction etc)

    Or am I totally barking up the wrong tree here, and due to them being heat treated and hardened, this would reduce the malleability, therefore increasing the load on the bolts before yield point, although lower the malleability and ability for them to stretch as much before the yield point? so a higher clamping force will be achieved with less of a torque angle?

    Any help on this subject would be greatly appreciated, heres and a photo of the new products.


    Many thanks, dave
  2. jcsd
  3. Jun 20, 2012 #2
    Don't care how hard they are. Care about their yield strength. Once you look that up, the ideal is to load them to 95% of yield if you want them to be reusable. Some engineering applications call for them to go past yield, but then they are one time use only.

    If you use a torque wrench, then you must settle for a load much less than yield because of the great uncertainty in converting torque to load.

    But if you are using the angle of turn method, then you can at least scale that up by the ratio of yield strengths of the old and new studs. If you learn the math required for the task, you may be able to go well beyond that.

    All your assumptions based on Hook's Law are valid.
  4. Jun 20, 2012 #3
    It is a good thing that your studs are so long. That increases your joint stiffness ratio, which does all sorts of good things for you. One of your best references is Machinery's Handbook. You should be able to look up everything you need in that.
  5. Jun 20, 2012 #4
    Before just wanging up the torque, I'd have a look at the relative strengths of the threads.
    By just upping the bolt strength, you can get to a point where you start winding out the female thread.

    It's unlikely but something to bear in mind.
  6. Jun 20, 2012 #5
    As Chris says, need to check but the female threads are probably good. The standard fix would be a helicoil, which would be much stronger. Look to see how much thread engagement you have. If it is at least 2D, I would not worry about it. It is always best if the female threads are cut into softer material so that they will yield and then more threads carry the load.
    Last edited: Jun 20, 2012
  7. Jun 23, 2012 #6

    jack action

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