Valve Clearance

Big Block A body Tech

  1. Aaron Majcan

    Aaron Majcan FABO Gold Member FABO Gold Member

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    I have this Lunati cam: Lunati Voodoo Cam and Lifter Kits 10230704LK

    I'm hoping someone much smarter than me can help me with valve clearance calculations based on its lift and duration.

    Its for a 383 0.040" over with flat top pistons sitting 0.015" down hole at TDC. I don't know enough about cam rotation to know if I will have enough clearance. I don't have the pistons yet so before I buy them, I need to know if they'll work first. The heads are Edelbrock E-Street 5090.
     
  2. fishmens67

    fishmens67 Well-Known Member

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    flat top with no reliefs and if it indeed is .015 in the hole, I think clearance will be an issue.
    I'm thinking the pistons will be much further in the hole without decking, like .080 or so.
     
  3. RustyRatRod

    RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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    That cam doesn't have enough duration to run the valves into the pistons at zero deck.
     
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    • fishmens67

      fishmens67 Well-Known Member

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      I wouldn't gamble. measure , larger valves will make it close , no room for error.
       
    • '63GT

      '63GT Well-Known Member

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      Mathematically, your intake valve will be approximately .116 (edited) off the seat at TDC. IF you have the cam advanced 4°. give or take.. less by .010 if straight up.
      If you know how much the intake valve is above the cyl heads deck surface, you can subtract that amount, plus gasket thickness, deck height (piston down = .015) and valve relief, if any.

      You're gonna have to get some measurements off the head..
       
      Last edited: Aug 13, 2018
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      • Aaron Majcan

        Aaron Majcan FABO Gold Member FABO Gold Member

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        So if I place my head face down on a table and measure valve travel until it touches the table, add crushed head gasket thickness and subtract .116", that should give me any clearance I have? (if any)
         
      • yellow rose

        yellow rose Doctor of Thinkology.

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        Why don't you just do a mock up and check your P/V with clay? Then you will know what you have.
         
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        • Aaron Majcan

          Aaron Majcan FABO Gold Member FABO Gold Member

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          Because I don't have the pistons yet. I don't want to buy them only to have them not work out and be stuck with pistons I can't use.
           
        • JBurch

          JBurch Well-Known Member

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          As YR stated, why not mock up your assembly and then find out IF you hit or NOT, and WHAT your clearance is, if any.......it's good to ask questions here, but there is no substitute for checking and measuring on your own........some one 1000 miles away does not have a crystal ball clear enough to tell you "YUP", "Nope" and be dead certain...............it's not that hard to do
           
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          • yellow rose

            yellow rose Doctor of Thinkology.

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            You don't know anyone that can notch a piston? Unless you are running a fairly long camshaft you probably won't have issues. It still needs to be checked.

            Buy the correct piston to get the compression ratio and deck height. Then check P/V and if you have to notch the Pistons so be it. It ain't that hard to do.
             
          • JBurch

            JBurch Well-Known Member

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            Here's a question, or 2, or 3...........for the sake of discussion, what are your piston choices IF the valves hit, what would you do IF you only had .010inch clearance with the valve open........loose rule of thumb is .080 intake, .100 exhaust, how would that impact your build..........are you gonna push pistons down in the hole to gain P/V clearance at the sake of loosing compression..........if valves are close, or hit, you can always have eyebrows cut for clearance......done all the time
             
          • '63GT

            '63GT Well-Known Member

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            The clearest cut way, is to do what YellowRose said. It's empirical, no math required, just a little set-up. But I see you don't have all your parts yet...

            You subtract from .116
            Hypothetical example: subtract these measurements. Valve to deck .080"
            Steel shim head gasket .018
            Piston down hole .015
            =.113 or -.003 P/V
            So, IF your measurements are like the hypothetical examples I used, you'll have a problem.

            BUT, IF you order these pistons (a bargin at $389, OR a set like them) all your worries will disappear. KB162-40
            These have .264" worth of valve relief Int/Exh and you'll have enough clearance for more cam than you'll want. AND, you can take RustyRatRod's 2 cents to the bank !
            LKB162.gif
             
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            • 67Dart273

              67Dart273 Well-Known Member

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              Well measure it with what you have, and then reference that figure to the proposed pistons. Laying the heads on a table doesn't do it because minimum clearance does not happen at TDC. It happens as the pistons move, and the valves open, and of course varies with cam "degreeing"
               
            • Aaron Majcan

              Aaron Majcan FABO Gold Member FABO Gold Member

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              The idea here was to get member thoughts on clearances based on the duration of the cam because mathematically, I'm unsure of the calculations. Yes I'm sure someone could notch pistons for me if I needed to but I'd rather not add unnecessary machining costs if I can just get the right pistons the first time.

              Mathematically, it sounds like my duration is not long enough to be a real concern so I can now confidently buy the pistons in mind and check the clearances with clay once mocked up. Thanks folks.
               
            • Aaron Majcan

              Aaron Majcan FABO Gold Member FABO Gold Member

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              What's the compression height of these pistons? I need 1.92"
               
            • JBurch

              JBurch Well-Known Member

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              They are 1.908
               
            • '63GT

              '63GT Well-Known Member

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              I've got a headache now...
               
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              • Aaron Majcan

                Aaron Majcan FABO Gold Member FABO Gold Member

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                I didnt intend to confuse anyone. All sorted now though, thanks everyone.
                 
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                • fishmens67

                  fishmens67 Well-Known Member

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                  #1 why would you choose a cam before the short block is together. you want as close to zero deck as you can get with the 383.
                   
                • Aaron Majcan

                  Aaron Majcan FABO Gold Member FABO Gold Member

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                  I had 1.848" compression height pistons in it already that came with my block when I bought it and it put me 0.090"down hole at TDC when installed them. I found 1.92" flat tops that will get me much closer to zero deck. I didnt think to measure my piston clearance before I pulled them out. Rookie mistake.
                   
                • '63GT

                  '63GT Well-Known Member

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                  No disrespect, but most of us here have found it necessary to 'deck' a block in order to achieve the desired deck height. ..just the way it goes. In fact, you'll end up with a much better product when finished, cause in many cases, the block is out of square or some other crazy anomaly. As well as having a better surface for head gasket bite.
                  BUT, this is only done when you know what pistons, rods, and crank you will be using.
                   
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                  • 440 Mike

                    440 Mike Well-Known Member

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                    If I have this right, you already have your block rough bored .040? Why not buy one cast .040 piston with less compression distance than you want to run and mock it up, see what you have for valve clearance and add compression height to it mathematically. Check for deck squareness at the same time.

                    I'm told that 383's are a little more critical when it comes to PV clearance because of the shorter stroke/longer dwell time of the piston.
                     
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