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Cast Iron Exhaust Manifold - !CRACKED!

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Greg72, Nov 28, 2001.

  1. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    I just noticed that the drivers side exhaust manifold has a couple hairline cracks running vertically between the middle two exhaust ports.....couldn't really see them until I got mud all over the engine....now for some reason they are easier to see.....

    So what's the symptom from cracks like this? Is it a slightly rough idle....whistling on deceleration? I know intake leaks can cause all sorts of "surging" problems at idle, but I'm not sure what exhaust leaks cause...

    I had what I thought was a bad PCV valve but I took it out and gave it the "rattle test" and it seemed to be OK....I am constantly hearing a whistle whenever I coast down in the truck until the RPMs get to something really low (like 800RPM)...then the whistle goes away.

    Next question: Is it time to consider a set of quality headers for this Big Block? The block is a 396, and the existing cast iron manifolds seem tight against the framerails already....I find it hard to believe that headers will fit withouth notching the frame. Someone here once mentioned something about Chevelle 396 headers being a "direct fit", but I can't remember the source of that info anymore.



    -Greg72

    '72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
    <font color=blue>See it here: </font color=blue><a target="_blank" href=http://coloradok5.com/gallery/albun38>http://coloradok5.com/gallery/albun38</a>
     
  2. GRINCH

    GRINCH 1/2 ton status

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    Por15 makes a coating to seal them back up . Check them out at <a target="_blank" href=http://www.por15.com>www.por15.com</a>

    The Few, The Proud, The K5 Blazer
     
  3. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Are you talking about the "Fire Seal 2000" product...?

    The cracks are very small, so I'm not sure I could get anything 'inside' the cracks but if that stuff could work as a top coat, I may give it a try....

    Thought for sure someone would "encourage" me to get some headers....not try to fix what I have!! [​IMG]



    -Greg72

    '72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
    <font color=blue>See it here: </font color=blue><a target="_blank" href=http://coloradok5.com/gallery/albun38>http://coloradok5.com/gallery/albun38</a>
     
  4. HarryH3

    HarryH3 1 ton status Author

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    The instructions for their manifold repair kit say to use a grinder to slightly open up the size of the crack, then apply the Fire Seal 2000. [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
    <a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
    It's a great day to be alive...
     
  5. JIM88K5

    JIM88K5 1/2 ton status

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    Get the headers. Try www.jbaheaders.com in addition to
    hooker,hedman an so on... Jim

    88 5.7 5in lift 35"procomp at's
    700-208-456-3in exh w/flowlowder 10 bolts & some ORD stuff
     
  6. Gold Rush

    Gold Rush 1/2 ton status

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    Well first off cracked exhaust manifolds are a common thing on Chevy's. I see them all the time at my shop. The damage that can come from a cracked manifold is a cracked or messed up valve. The crack MAY allow cold outside air into the exhaust port and hot metal (valve) and cold air do not mix, therefore cracking a valve.

    Sounds like it's time for a set of block hugger headers and some dual 3 inch exhaust!!!!!!!!


    David

    <a target="_blank" href=http://www.extremeblazer.com>http://www.extremeblazer.com</a>
     
  7. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    GoldRush....

    Hey, let's not stop there.....if there COULD be a broken valve....I better be safe and get a set of AFR Heads....you know those really gnarly new ones for Chevy Big Blocks!! [​IMG]

    Seriously though....any specific recommendations on a high quality header that will actually fit?



    -Greg72

    '72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
    <font color=blue>See it here: </font color=blue><a target="_blank" href=http://coloradok5.com/gallery/albun38>http://coloradok5.com/gallery/albun38</a>
     
  8. Gold Rush

    Gold Rush 1/2 ton status

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    I don't know of a specific brand or part number that will work with your big block. I have had hedman and dyno max on my 75 before but with a small block. I really like the dyno max headers that are on there now.


    David

    <a target="_blank" href=http://www.extremeblazer.com>http://www.extremeblazer.com</a>
     

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