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Torque vs stretch in rebuilding engines

Discussion in 'Technical Q&A' started by UroTrash, Jun 27, 2004.

  1. UroTrash

    UroTrash Three Time F1 World Champ
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    Jan 20, 2004
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    Let me start by saying that I'm an armchair mechanic and have not done anything bigger than a brake rebuild , a timing chain change and a clutch exchange (all on an ancient BMW 2002), so forgive me my ignorance.

    I saw on speedvision the other day a show where a couple of guys were rebuilding an engine and they were not torqueing the studs but rather were using special studs that they measured lenghtwise to get the proper tightness. they said it was much better than torqueing and I think they said its the way high stress (i.e. drag racing) engines are built.

    I have never hear of that before. Is it new or am I just not in the loop?
     
  2. fatbillybob

    fatbillybob F1 World Champ
    Consultant Owner

    Aug 10, 2002
    19,002
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    on rod bolts you should always try to get a bolt stretch spec.. Some manufactueres do not list a torque spec purposly so that you have to use new bolts. Once a rod bolt is used it should not be reused. Once the metal has yield it is no good. I'm a bit crazy so when I rebuild I do bolt stretch but watch the torque too so that I know the torque is a reasonable number relative to the stretch. You know...measure twice cut once mentality.
     
  3. PSk

    PSk F1 World Champ

    Nov 20, 2002
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    Pete
    The reason for correctly torquing a bolt/stud is to get the stretch the bolt into the correct area or yield.

    As said above in other posts over torquing will over stretch the bolt/stud and thus weaken it. Under torquing simply means it is not going to correct clamp the thing it is holding on to.

    Designers specify the bolt/size size to get the correct clamping force ... and could not give a rats arse about the torque, they are interested in the clamping force the bolt is going to maintain ... which comes from the properties of the bolt/stud and changes as it is stretched.

    Pete's reminding us that you do a bolt up to a particular torque to get a particular stretch out of the bolt which directly relates to the clamping force ... no other reason ;)

    ps: Thus a stretch spec is probably more accurate that using a torque spec ... as the correct stretch is really what you want.
     
  4. Dale

    Dale F1 Veteran

    Oct 7, 2003
    5,211
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    Dale Juan
    There are three ways to tension/stretch a fastener,
    1-torque wrench
    2-measuring the stretch
    3-turning a predetermined amount(angle torque)
    number 2 is the most accurate period,only any good on rod bolts though like to see someone use no 2 method on head bolts-you can't do it,
    so what we going to use on the mains/head bolts,well you have the data in front of you,if no 3 method you will need a base to work from so its no 1 first which will be a low setting of around 10-20 lbs,ft, there are angle torque gauges on the market which you will need for no 3 method,
    torque wrench's have a bit of a problem with FRICTION thats why methods 2 and 3 are now used so much,
    FRICTION-NOT GOOD so use the best lube you can on the fastener,not the oil can you have on the shelf,
    if you use method 1,2,or 3 use the very best anti friction lube you can on major engine fastener components,
    oh,and make sure the work area is clean and enjoy it dont rush it, its not a race,

    cheers
    Dale.
     
  5. yelcab

    yelcab F1 World Champ
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    Nov 29, 2001
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    Mitchell Le
    porsche 911 head studs use a combo of 1 and 3:

    Step 1: torque to a low 15 Nm setting,
    Step 2: turn an additional 90 degrees.

    Lubed only at the base of the head nut with Moly lube.
     
  6. Dale

    Dale F1 Veteran

    Oct 7, 2003
    5,211
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    Dale Juan
    Wayne, been reading about the locktite bit on rod bolts/studs,
    do you use locktite on rod bolts in every type of engine you have built,

    cheers
    Dale.
     

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