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Explain to me "Torque to Yield" and how to do it.

Discussion in 'The Hokey Ass Message Board' started by Roothawg, May 14, 2012.

  1. Roothawg
    Joined: Mar 14, 2001
    Posts: 24,746

    Roothawg
    Member

    I knnow that some of the newer styled heads have bolts that are torque to yield. I have never had any experience with a fastener like this. How do you torque...let's say heads on a SBC?
     
  2. Boryca
    Joined: Jul 18, 2011
    Posts: 710

    Boryca
    Member
    from Detroit

    http://www.boltscience.com/pages/faq.htm#12

    Check that out. It's basically a process of torquing until the faster reaches its near-failure point. A common application these days is connecting rod bolts where you measure the stretch rather than the torque value.

    On a SBC I'm fairly certain it would be something along the lines of torquing to a particular value, then turning the bolt another 1/2 turn or some such. It would be called out by the bolt manufacturer.

    Mike
     
  3. Wolfman1
    Joined: Jul 8, 2010
    Posts: 265

    Wolfman1
    Member

    My understanding is that you would torque down to specs in sequence as normal
    but they are supposed to be a one time use fastener
     
  4. squirrel
    Joined: Sep 23, 2004
    Posts: 56,414

    squirrel
    Member

    SBC has enough head bolts that they don't need to do this crap on it.
     
  5. Roger Walling
    Joined: Sep 26, 2010
    Posts: 1,149

    Roger Walling
    Member

    There is one method that grabs the bolt and streches it to a certin tention, and then the nut is turned a certin amount, (1/2 turn? or so)
     
  6. S_Mazza
    Joined: Apr 27, 2011
    Posts: 363

    S_Mazza
    Member

    The head and block design obviously matters, but the fastener can usually be specified either as a torque-to-yield fastener, or a standard one.

    What I mean exactly is that, if you have an engine where you plan to take the heads off regularly, like a race engine, you can often buy ARP head studs that will last many cycles, and replace OEM torque-to-yield bolts.

    The TTY bolts are actually a good idea from some perspectives. The first linked article didn't explain it very well. This one gives a much better idea of what TTY is for.

    http://www.acl.co.nz/Tech/Torque To Yield Headbolts.pdf

    Basically, what it lets you do is extract the maximum clamping force that each bolt can provide. With a fastener designed for reuse, you have to stay within the elastic limit of the fastener. If you torque it farther, you will stretch it, and then it can't always be reused. The TTY fastener is designed to stretch to a certain extent. By checking the increase in torque compared to the extra rotation of the fastener, you can detect the point where the bolt is starting to over-stretch and head toward failure. At that point, you stop torquing. You have the maximum force you can get out of that fastener.

    For head bolts that are planned to be removed either A.) never or B.) once, it's a good idea.
     
  7. aaggie
    Joined: Nov 21, 2009
    Posts: 2,530

    aaggie
    Member

    I ran into this when I built a 4.3 Chevy V6 for a Terraplane project. The book says you have to use new head bolts every time the heads go back on and after reaching the desired torque the bolt is turned another 70 degrees of rotation.

    I ended up making a template marked at the 70 degree mark and it seemed to work OK. Since then I found several digital torque wrenches that read the torque in foot pounds then register the degrees. A new set of head bolts from GM weren't that expensive and they come with thread sealant already applied.
     
  8. Roothawg
    Joined: Mar 14, 2001
    Posts: 24,746

    Roothawg
    Member

    I have a set that came with the vortec heads I did some trading for. I have never used them before and there are a lot of explainations on the theory searching with Google, but no real life applications. I didn't know if I would have to buy a special torque wrench or what.....if so, I would be money ahead to just buy new bolts and move on.
     
  9. 51fordshoebox
    Joined: Aug 17, 2004
    Posts: 927

    51fordshoebox
    Member

    your supposed to have a torque angle wrench. basically you would torque to a set $$ft/lb of torque plus a set number of degrees beyond that. if it were me id just buy standard head bolts. once you use torque to yield bolts you throw them away and replace them.
     
  10. oldolds
    Joined: Oct 18, 2010
    Posts: 3,424

    oldolds
    Member

    No need to buy a new wrench. Just read the shop manual. Torque to spec. Then turn the correct amount of degrees. Full circle 360 degrees, hence 90 degrees is 1/4 turn.
    Basically turn them till they are tight then just a little bit more
     
  11. Fenders
    Joined: Sep 8, 2007
    Posts: 3,921

    Fenders
    Member

    Crank it down til it breaks.... then back off a half turn.
     
  12. Juggalo56
    Joined: Dec 10, 2006
    Posts: 79

    Juggalo56
    Member

    i work at an off topic car dealership and alot of our head bolts are TTY bolts. can only be used once since theyare a stretch boltm designed to stretch to a certain point and not be re-used again since they have already been stretched. like many other have stated a good rule is any head bolt which the specifications say to torque to a specific amount, then turn a number of degree past that point will be a TTY bolt.
    Dave
     
  13. AZbent
    Joined: Nov 26, 2011
    Posts: 280

    AZbent
    Member

    So you are all telling me that my "German" torque wrench, gutten tight, is of no use. Now I actually have to use the correct tools.
     
  14. S_Mazza
    Joined: Apr 27, 2011
    Posts: 363

    S_Mazza
    Member

    You should use the correct tools, but the repair procedure is usually a little different from the factory assembly procedure. The repair procedure is often written in terms of a certain number of degrees to go, after you have reached a certain torque. That's an approximation based on the average properties of the fastener, if the hole and fastener are clean, the right shape, assembled and lubed correctly. But the factory would use a (robotic) tool reading torque vs. angle to make sure it is correct.

    Anyway, check the manual that you are using, and you will see if there is a simplified procedure.

    Gutten tight. Haha. :)
     
  15. RichFox
    Joined: Dec 3, 2006
    Posts: 10,020

    RichFox
    Member Emeritus

    I can tell when I have arrived at the yield point. The bolt head is in the socket and the rest is in the block.
     
  16. It sucks when the book you have is written for the old style bolts, and the parts store sells you torque to yield, but the paper in the box says "In accordance with manufacturers specifications", and nothing on the box says torque to yield.
     
  17. Roothawg
    Joined: Mar 14, 2001
    Posts: 24,746

    Roothawg
    Member

    I am thinking I should just replace the bolts.
     
  18. If they are used, they need to be replaced anyway. Single use.
    If they are new, use them and buy new ones when the gotta come off if ever.
     

  19. Head down this road & take the left turn @ 3.5 miles before the dead end. Don't pass it up because there's no place to turn around, once you do.
     
  20. stealthcruiser
    Joined: Dec 24, 2002
    Posts: 3,748

    stealthcruiser
    Member


    Root, the tool companies, (Mac, Strap-On), offer a small degree wheel that fits over the drive, ( 1/2), on your torque wrench, then your socket or extension slips on to keep it in place.
     
  21. NoSurf
    Joined: Jul 26, 2002
    Posts: 4,508

    NoSurf
    Member

    Here ya go Root:

    [​IMG]
     

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