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High Altitiude Head

Discussion in 'Early CJ5 and CJ6 Tech' started by Howard Eisenhauer, Sep 1, 2022.

  1. Sep 1, 2022
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

    Tantallon, Nova...
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    Nov 22, 2003
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    8,118
    Seeing a 7.8 compression ratio F4 head in the classifieds got me thinking- What's the exact difference between this & the standard head? Is there a difference in the combustion chamber or did the factory just grind a bit extra off the shiny side?

    Enquiring Minds Want to Know...
     
    Howard Eisenhauer, Sep 1, 2022
    #1
  2. Sep 1, 2022
    mickeykelley

    mickeykelley Well-Known Member

    Republic of Texas
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    My guess is just sliced a little off. That’s what they did the the wagons L226.
     
    mickeykelley, Sep 1, 2022
    #2
    ziv likes this.
  3. Sep 1, 2022
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Hills of NH
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    But... wasn't the compression ratio number cast into the heads? If so, they weren't merely taken from the regular production run and milled.


    R.9b8f79ed913e29238a07cc6b499fcced.jpeg
     
    Last edited: Sep 1, 2022
    PeteL, Sep 1, 2022
    #3
  4. Sep 2, 2022
    danielbuck

    danielbuck Uncle Buck

    USA
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    maybe a certain percentage was run through with modified casting text, and then machined farther than the others.
     
    danielbuck, Sep 2, 2022
    #4
    PeteL likes this.
  5. Sep 5, 2022
    oldtime

    oldtime oldtime

    St. Charles,...
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    The head Pete shows you can clearly see the casting number 648830. That is an early casting number.
    So apparently that head removed from an early SW or PU. You can also clearly see a CR number cast, not stamped into the head of 7.4 /1 ratio. Note it has an early vented valve cover.

    Scotties head shows the same early casting number of 648830 but it also shows a stamped finished part
    number of 800395. 800395 is a post 1953 part number.
    [​IMG]
    This head has a stamped CR of 7.8/1 ratio.
    [​IMG]
    Note it has a late non vented valve cover.
    [​IMG]

    On my own Hurricane I have a late casting numbered head with a 7.4/1 CR stamping.
    7.4 indicates the head was made for 5’000 to 10’000 ft operations.
    That said I shaved the head flat so now it’s much closer to being a 7.8/1 ratio head.
     
    Last edited: Sep 5, 2022
    oldtime, Sep 5, 2022
    #5
    Arnold Layne and melvinm like this.
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