Bolt Stretch/Tension
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Bolt Stretch/Tension

  1. #1
    Sit N' Spin Jetaholic's Avatar
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    Default Bolt Stretch/Tension

    Can someone please explain the theories behind bolt tension and stretch and what the proper procedure is for torquing main cap studs and rod cap screws?



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    Senior Member cyclone's Avatar
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    Quote Originally Posted by Jetaholic View Post
    Can someone please explain the theories behind bolt tension and stretch and what the proper procedure is for torquing main cap studs and rod cap screws?
    when you tighten a bolt, you need to achieve the proper preload for the bolt to do its job. The bolt stretches a certain amount when you tighten it. In the case of a rod bolt, the bolt has to have enough preload/clamping force to overcome the load that's trying to rip the rod apart. the reciprocating motion of the crankshaft and piston is constantly trying to pull the connecting rod apart. the load is greatest at tdc and bdc when the assembly changes direction. Rod bolts are the highest loaded fasteners in an engine.

    measuring the length of the bolt before you torque it, and then afterwards with a bolt stretch gauge will tell you how much the bolt has stretched. Most bolt manufacturers will tell you how much the bolt needs to stretch to work properly

    The reason its better to measure the stretch of a bolt than simply using a torque wrench is because the coefficient of friction between the bolt head and washer or rod, wont be the same for each bolt.

    For example, i just measured the stretch when assembling the rods on my BBC. most of the bolts would stretch the recommend amount (.0050-.0055) with 70 lb-ft of torque, but there were 3 that need another 10 lb-ft of torque to get the proper stretch. Had i not measured the stretch and just torqued the bolts and called it good, i might have had a bolt failure at high rpm.

    arp makes a really nice gauge.

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    Sit N' Spin Jetaholic's Avatar
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    Very good info! Exactly what I was looking for.

    One more question...is it just the rods that need to be tightened with a stretch gauge, or do main bolts/studs need to be also?



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    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

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    Senior Member cyclone's Avatar
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    Quote Originally Posted by Jetaholic View Post
    Very good info! Exactly what I was looking for.

    One more question...is it just the rods that need to be tightened with a stretch gauge, or do main bolts/studs need to be also?
    there's no way to measure the stretch of a main stud/bolt. check out the photo of how the stretch gauge attaches to the rod bolt.


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    Sit N' Spin Jetaholic's Avatar
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    Makes perfect sense. The bolt will stretch in both directions and with a main stud/bolt there's no way to get the gauge onto the other end of it since it's in the block.

    I'll be running ARP main studs. I know with studs you don't bottom them out...there's an actual procedure for proper torquing of studs. Can you explain that procedure for me?

    Again thanks for all the info.



    Quote Originally Posted by HammerDown
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    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

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    E-7 Sheepdog (ret) SmokinLowriderSS's Avatar
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    If you look at ARP's catologue, page 43, they tell you how to install them.

    "Finger-Tight" in clean threaded holes WILL bottom them out.

    They even advise them, if it is to be permenent, to be Lock-tited into place (install and torque the caps before the Loc-tite is set to prevent mis-alignment).

    1- Clean and chase threads, calibrate torque wrench.
    2- clean caps and hardware.
    3- screw studs in untill FINGER TIGHT, Lock-tite if desired.
    4- Install caps, lube threads.
    5- Torque to values with stud instructions 3 times, torque before thread sealer sets up.

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    havn't posted in a while i got the flu, sorry

    smokinlow means this is for main studs/ not rods

    but same arp cat for rod bolts specs

    what some people miss is lube arp moly grease for threads and under fastener head and washers, this will make a big impact on values.. tq and stretch

    i like to cycle each bolt and studs at least three times before i reach final assembly

    clean up excess lube after your done this stuff is not good on rod bearings and filters

    i sure these guys will chime in and show me just how sick i am.

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    E-7 Sheepdog (ret) SmokinLowriderSS's Avatar
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    Sux flat, hope you're better soon.
    "As democracy is perfected, the office of president represents, more and more closely, the inner soul of the people. On some great and glorious day the plain folks of the land will reach their heart's desire at last and the White House will be adorned by a downright moron."
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    OBAMA: Some people deserve this.
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    E-7 Sheepdog (ret) SmokinLowriderSS's Avatar
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    Quote Originally Posted by flat601 View Post
    havn't posted in a while i got the flu, sorry

    smokinlow means this is for main studs/ not rods

    but same arp cat for rod bolts specs

    what some people miss is lube arp moly grease for threads and under fastener head and washers, this will make a big impact on values.. tq and stretch
    Yep, as I told Jetaholic on the horn last night, where I work, aircraft wheels are 2-part, bolted together. Moly greaase goes on all bolt mating surfaces.
    The washers on both sides (bolt head and nut side), and the bottom of the nut, all those friction surfaces..
    This minimizes any affect of that material drag from affecting the torque values to the bolts.
    We don't grease the threads deliberately, but it's hard to keep them dry greasing everything else.

    Self-locking (crimped) nuts anyhow.
    "As democracy is perfected, the office of president represents, more and more closely, the inner soul of the people. On some great and glorious day the plain folks of the land will reach their heart's desire at last and the White House will be adorned by a downright moron."
    H.L. Mencken

    OBAMA: Some people deserve this.
    The rest of us are being dragged along kicking and screaming.

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    B1 Racing cs19's Avatar
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    Quote Originally Posted by flat601 View Post
    i sure these guys will chime in and show me just how sick i am.
    Yep, it wont be long.

    Good post BTW.

    CS
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    gn7
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    Quote Originally Posted by flat601 View Post

    clean up excess lube after your done this stuff is not good on rod bearings and filters

    i sure these guys will chime in and show me just how sick i am.
    Someone will always chime in Hope you are not implying that it's OK for main and cam bearings



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    Quote Originally Posted by gn7 View Post
    Hope you are not implying that it's OK for main and cam bearings
    LOL.

    There it is Flat601.
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    Quote Originally Posted by gn7 View Post
    Someone will always chime in Hope you are not implying that it's OK for main and cam bearings
    im am sick as a dog and you got to be kidding? ya gn7 just in your motors, yes i seen the pics, right back to ya

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    Senior Member Brendellajet's Avatar
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    If you bottom out a main stud in the block couldn't you measure how much it stretches? I realize their may not be a fixture available for the dial indicator(maybe there is), but if it cant go any deeper, it has to stretch where it can be seen,right?

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