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Exhaust Flange Spring Tension?

Discussion in 'Technical Q&A' started by BrandSB, Mar 13, 2010.

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  1. BrandSB

    BrandSB Karting

    Jun 11, 2008
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    Canton, Michigan
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    Steve
    #1 BrandSB, Mar 13, 2010
    Last edited by a moderator: Sep 7, 2017
    I'm completing the installation of new Fabspeed headers and Quicksilver test pipes ('95 355) and have a very basic question. When I removed the original flange bolts from the flange couplings, the springs were not completely compressed, presumably allowing some room for thermal expansion in the system. When re-installing with new gaskets at the interfaces, the bolts feel somewhat loose unless they are tightened until the springs are coil-bound. Questions:

    1. How loose should the springs be to allow for thermal expansion?
    2. It's assumed that the system will seal when warmed up, even though the bolts are not tight?
    3. What will happen if the bolts are completely tightened with no provision for expansion?

    I'd appreciate feedback before starting the engine and possibly creating a sealing issue.
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  2. ronrob

    ronrob Formula Junior

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    Ronald Brown
    #2 ronrob, Mar 13, 2010
    The bolts are OE and the exhaust system is not - so no surprise that spring compression on fitting is not the same. You have to add some spacers on the threaded side so that you have at least some space between the coils when the pipes are snug. IMO torque does not come into it. You can feel when it's right.
     
  3. eric355

    eric355 Formula 3
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    Eric DECOUX
    #3 eric355, Mar 13, 2010
    Last edited by a moderator: Sep 7, 2017
    In addition to what Ron wrote, you should have a spacer between the bolt and the spring (item 58 on the picture) which defines its minimum length. When cold, you should torque until the spring is the same length than the spacer (anyway you cannot go further ...), when hot, the long bolt expands and the spring maintains the contact and the effort between the flanges.
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  4. ronrob

    ronrob Formula Junior

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    #4 ronrob, Mar 14, 2010
    Nice one Eric!
     
  5. mwr4440

    mwr4440 Five Time F1 World Champ
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    #5 mwr4440, Mar 14, 2010
    Just some useless FYI.

    If you want new bolts (on a 308GTSi) they are M8 Hardness 8.8, 70mm.

    If you get softer bolts and put on ANY CAR, you can imagine what might happen.

    Look at the origional bolt head for the numbers before you replace them with any ole (similar) thing from Home Depot.

    YMMV.
     
  6. Motob

    Motob Formula 3
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    Nov 11, 2003
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    #6 Motob, Mar 14, 2010
    Eric is right, the spacer inside the spring defines its length when tight. You just tighten the nut until you cannot tighten it anymore (the two washers on either side of the spring are now tight against the tube, if there is any expansion in the bolt length hot, the spring keeps everything tight.
     
  7. BrandSB

    BrandSB Karting

    Jun 11, 2008
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    Steve
    #7 BrandSB, Mar 15, 2010
    Thanks guys, got it. I thought the spring was 'floating' and wasn't aware of the spacer inside. Makes sense.
     

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