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Technical Exhaust Header Knock

Discussion in 'Traditional Hot Rods' started by iafraser39, Feb 5, 2020.

  1. iafraser39
    Joined: Aug 2, 2009
    Posts: 156

    from Warmland

    Is there such thing as a Header Knock? I recently 401 nailhead out for a 425, using the headers the were on the 401. When I get on it when the engine is not up to temperature,
    there is a knock or rattle that comes from the exhaust, it goes away once the engine is up
    to temperature. I never experienced this on the 401. Cam anybody explain this.
  2. fastcar1953
    Joined: Oct 23, 2009
    Posts: 2,161


    Something loose or broke? When heated up it expands and gets less noisy , not gone.
  3. donsz
    Joined: Nov 23, 2010
    Posts: 211


    Does the header have a heat riser?
  4. Could it be better exhaust port scavenging causing a lean condition?

    Sent from my iPad using The H.A.M.B. mobile app

  5. BJR
    Joined: Mar 11, 2005
    Posts: 6,459


    Piston skirt knock, goes away when warm, SBC are famous for this.
    Elcohaulic and olscrounger like this.
  6. Lloyd's paint & glass
    Joined: Nov 16, 2019
    Posts: 2,482

    Lloyd's paint & glass

  7. 19Eddy30
    Joined: Mar 27, 2011
    Posts: 1,007

    from VA

    I had 1 Exhaust valve Guide go bad that made a knocking sound , the sound was the Valve centering its self on Valve seat when closing.
  8. realsteeler
    Joined: Mar 30, 2012
    Posts: 38


    Not sure if this applies to any of you. But my hearing has deteriorated with age. A couple of years ago, I could hear this god awful clatter coming out of my sbc. Sounded to me like I'd done a big end bearing, although oil pressure was still good.
    Not having time to mess with it, I quickly fitted a new crate engine.
    Fired it up. Much to my horror, the noise still there:(.
    So I asked Mrs to listen while I revved it up.
    She thought the noise was coming from further back towards the middle of the truck.
    Turned out to be a loose baffle in one of the mufflers:rolleyes:.
    Never did tell her the whole story!!
    Lloyd's paint & glass likes this.
  9. iafraser39
    Joined: Aug 2, 2009
    Posts: 156

    from Warmland

    This s a brand new rebuild by Russ Martin of Centerville Auto. I swapped all the bling and headers off my 401. The headers are long tube and have no loose ends in them. Could it be that there is a different cam in the 425, which incidentally is bored out to 435 cubes. The noise didn't happen with the 401, and only happens when I put my foot into it and its not up to temperature.
  10. Smokeybear
    Joined: Apr 20, 2011
    Posts: 325


    I've had a small exhaust leak that sounded like a knock that went away when it warmed up. The metal expands and sealed up the leak, warm it up and put a wrench on each bolt, let it cool and wrench them again. That's how I got mine to seal.
  11. triumph 1
    Joined: Feb 9, 2011
    Posts: 457

    triumph 1

    It’s never a good idea to put your foot into it before the engine has some oil temp. If it has forged pistons it could definitely be the noise until they warm up & expand a little.

    Sent from my iPhone using H.A.M.B.

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