408 Cam on the way

Small Block Mopar Engine

  1. Wyrmrider

    Wyrmrider FABO Gold Member FABO Gold Member

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    without re-reding it looks like LA pistons in a magnum block _+.020
    so a .050 gasket should work with not loose piston to wall
    good block finish get some cometics maybe .055
    but measure all four corners first

    on your cam
    that'S why EVERY cam has to be checked
    right SlantSix64
    they have to move the lobe centers for best fit on the core
    glad you found it instad of later posters are finding out
     
  2. Idaho

    Idaho Well-Known Member

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    Edit

    1B7854C5-5EC8-4A4C-9C99-A243297C7D38.jpeg

    3899E8D9-8A8C-4149-8B30-EFBB1DF33EE9.jpeg

    5BF167C7-85BB-454F-B87A-19C8C6A9485E.jpeg

    99BE5C87-31C2-4CF3-A1B1-35DE7BB10C76.jpeg

    SCR 0.070.png

    So I did some measuring of things.
    Piston to deck stats:
    Step up is 0.020 above deck.
    Piston rim is 0.070 below deck.
    Dish is 0.150 below deck.

    I figured out KB uses the step up portion as the compression height by measuring from the pin hole, adding half the pin diameter.
    Pin hole to rim ranged 0.868 to 0.880. Half the pin is 0.492. Step up is 0.090 above the rim.
    0.880 + 0.492 +0.090 = 1.462. The spec is 1.465
    Oddly every piston had one side around 0.010 higher than the other. Not sure if that's by design.

    I had to scratch my head a little to decide how to do the CC. I decided to start with the step up reference point at zero deck.
    CC of the cylinders with piston 0.5" down from there is 131 cc.
    Using the formula to get 1" of cylinder volume (d/2)squared x 51.48
    (4.030/2) squared x 51.48 =209
    Divide by 2 for volume of a half inch is 104.5.
    131 - 104.5 =26.5 This is my total dish volume below the step up.
    Spec is 25 cc. I think its worth doing to have the SCR nailed down.

    I ran the calculation with a 0.070 gasket because KB calls for a minimum 0.040 clearance from piston to head. I'm fairly sure I read that cometic makes a 0.070. At 0.020 this gives me 0.050. I'll for sure be checking all 4 corners.

    So much for quench. Had I known about this quirky piston design I'd have found something else. Too much faith in the machinist who picked it out I suppose. Since I've used one for mock up I doubt I can return them.

    If I can't find at least a 0.060 gasket I'll be asking for a piston exchange anyway, or the machinist can shave them a little at his cost. Then we may have an issue of the balance.
     
    Last edited: May 16, 2020
  3. yellow rose

    yellow rose Overnight Sensation FABO Gold Member

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  4. Idaho

    Idaho Well-Known Member

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    Half inch down from where the step up is flush (0.520" down from TDC) is 131 cc.
     
  5. Idaho

    Idaho Well-Known Member

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    I've done the 4 corners. The block tapers 0.005 front to back. The result is the number 7 cylinder has a protrusion of 0.025. That's after swapping pistons, putting the shortest in that position.
    Cometic makes a 0.070 gasket. Looks like I'll need it. Not cheap.
    Anyone know what that will do to the fit of the intake?
     
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    • Cal Tonsley

      Cal Tonsley Well-Known Member

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      Probably need Cometic intake gaskets or similar as well, I’d guess something like .090”-.120”
       
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      • j par

        j par Well-hung Member

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        What did the others protrude at?
         
      • Idaho

        Idaho Well-Known Member

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        1 = 0.020
        2 = 0.019
        7 = 0.025
        8 = 0.024

        Those are the averages with rocking the pistons.
        I may take 7 and 8 down by 0.005 with sandpaper and a glass plate so I can use a 0.060 gasket. I'd like my compression a little higher.
        First I'm going to clay the valves in case that dictates the gasket.
         
      • Wyrmrider

        Wyrmrider FABO Gold Member FABO Gold Member

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        .050 is too much
        .035 0n worst eben .030
        thanks for checking all 4 corners abd swapping parts
        it's attention to that kind of detail that makes the difference
        pity the guy that gets the long piston/ rod combo in the low hole without checking
        block not squared?
         
      • Idaho

        Idaho Well-Known Member

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        I appreciate your input. I've got all the books but the help here makes a big difference. I enjoy taking time doing the measuring.
        I was only considering 0.005, not 0.050. Just enough to even them up with minimal effect on balance or compression.
        The block was supposed to be squared. I don't know if 0.005 off is common?
         
      • j par

        j par Well-hung Member

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        Is this all new stuff??..
         
      • j par

        j par Well-hung Member

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        One of these days I'm going to have to wrap my mind around a piston with a dome and a dish...
         
      • beanhead

        beanhead Well-Known Member

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        The 440 I just got back from my lmachinist was worse, .010" variance lowest to highest between all four corners. I gave him the rotating assembly for mock-up before any cutting was done, all nice and flat now.
         
      • Wyrmrider

        Wyrmrider FABO Gold Member FABO Gold Member

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        was looking at .050 quench
        ,020 positive deck with the first ,070 gasket straw man
        .005 is a bunch
        we got them dead nuts on with the Bridgeport even before BHJ tooling
        measure twice cut once (usually :)
         
      • Idaho

        Idaho Well-Known Member

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        Yes sir. Kinda goofy pistons. Hindsight is 20 20.

        I read all different methods to CC. The only thing that made sense to me was to start with the highest surface at zero deck. Down 0.5". Subtract measured volume from calculated 0.5" cylinder volume. It treats everything below the upper step as a "dish", including the valve reliefs, the actual dish, and the space between the rim and the raised step. The protrusion is entered as a minus number where the "below deck" entry would go. The "dish" is subtracted.

        My machinist purchased the rotating assembly for me so he could balance and set it up. The rod bearings were still in sealed packages so I know he did not check piston to deck.

        Sorry, can you explain the straw man comment?
        KB says this piston does not have a quench pad. Perhaps the step up area is too small to be considered? I have no idea.
        Were you milling blocks on the Bridgeport?
         
        Last edited: May 17, 2020
      • Wyrmrider

        Wyrmrider FABO Gold Member FABO Gold Member

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        look up "straw "man
        a hypoetical test
        some kb do not consider the quench dome for compression ht
        they used to show the height of the quench dome in the catakog
        step dish is fun
        Bridgeport- yes
        much easier now with the bhj tooling
        line up on the cam and crank and cut then flip 90 degrees
        comes out real close and takes care of twist
         
      • beanhead

        beanhead Well-Known Member

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        Dang, that's a bummer. Well, like you're doing, much of what I've learned about engine work was by having to fix the "professional's" oversight. You'll gain some knowledge not a lot of others have when it's all done...good luck!
         
      • Idaho

        Idaho Well-Known Member

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        I'm still not clear what you're trying to tell me about quench, piston clearance and head gasket thickness. I'm getting old. Brain is creaky.

        Thanks. You're right about that.
        Next time, if there is one, I'll be prepared.
         
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        • yellow rose

          yellow rose Overnight Sensation FABO Gold Member

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          You have a 23 cc dish, but I’m not sure I trust that 100%. When you do the down fill, its .500 down form TDC, you don’t add in the dome or quench pad.

          Did you figure you compression ratio yet?
           
        • lead69

          lead69 hopeless car junkie

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          Just curious if you checked piston rock?
          Piston rocking in bore VS deck height / compression ratio - Don Terrill’s Speed-Talk
           
        • Idaho

          Idaho Well-Known Member

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          I put a screen shot of the compression calculator in post 102. How I did the CC accounting for the step and dish is in post 115.
          A 0.070 gasket puts it at 9.5.


          All my numbers are the averages taken from rocking the pistons. Thanks for asking.
           
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          • yellow rose

            yellow rose Overnight Sensation FABO Gold Member

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            Not beating a dead horse but my math is your CR is 8.53:1 not the 9.5:1 your showing.

            Here are my numbers using what you’ve posted.

            836 cc’s for the full cylinder
            .520 down fill is 109 cc’s
            Your down fill is 131 cc’s
            Your dish volume is 22 cc’s
            Your chamber is 61 cc’s
            Your gasket is 15 cc’s

            I’m making a bit of a guess on your gasket volume because you’ve posted the gasket thickness but not the bore diameter. I used a bore diameter of 4.080 as a guess. It may be slightly smaller, or it could be bigger.

            When you add up the actual dish of 22 and the gasket of 15 and the 61 of the chamber you get 98 cc’s.

            Then it’s the division of the swept volume of the cylinder which is 836 by your total volume of the chamber of 98 and you get 8.5306:1 and no doubt my math could be wrong.

            I know from the years I have of doing this the number one killer of engine performance is the mismatch of CR and cam timing. If the CR is lower than you think and you cam for the higher CR it’s lazy and under performs.

            If you miss the other way and the CR higher than you think and cam accordingly it will rattle its brains out and be miserable to tune.

            Before doing anything I hope several members check my math to see if I have gotten sideways somewhere. If my math is correct you have a full point lower CR than you have posted.

            I hope I’m wrong.

            EDIT: I AM WRONG AND YOU ARE CORRECT. I woke up at 3 this morning and realized I forgot to add in the chamber volume, gasket volume and the dish volume to get the total swept volume. When I do it correctly I get 9.53:1 and this is the perfect example of why I shouldn’t do math late at night after not getting any sleep for the 3 days prior.

            So...my bad and I apologize.

            YR
             
            Last edited: May 18, 2020
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            • Idaho

              Idaho Well-Known Member

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              No apologies needed. I appreciate your taking time to help out.
              I spoke to the machinist today. He also went over the CR. He was apologetic about the 0.005 error on the decks and is going to shave the pistons steps on the lathe by 0.020 or so at no charge. I'll be measuring the height above deck for each one so they can all be cut to come to the same final height, likely zero deck. With that I can use a regular gasket and bring the compression up a little.
               
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              • Idaho

                Idaho Well-Known Member

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                I adjusted the dish volume for 0.020 shaved off from the 26.5 I measured to 25 cc.
                This is the CR I had in mind.

                compression.png
                 
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                • Idaho

                  Idaho Well-Known Member

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                  The pistons are at the machine shop.
                  Meanwhile I'm working on the valve train, in particular the pushrod length.
                  I'm going to transition the conversation back to my build thread, so please check that out and offer any helpful input that you can:
                  Project Frog Princess
                   
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