Valve to piston clearance?

Small Block Mopar Engine

  1. dartfreak75

    dartfreak75 Well-Known Member

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    Ok I have been doing alot of reading and research on this topic and everything I see is how to check it after the engine is already assembled. Is there any formula or calculations to check and make sure you will have adequate clearance before you start buying parts? For example I have a piston a goal compression and a cam chosen that I want to use how can figure out if it will work without buying it and test fiting it?
     
  2. RustyRatRod

    RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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    General rule of thumb is .080" on the intake and .100" on the exhaust. As for the best way to figure, physical set up and measuring with clay.
     
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    • dartfreak75

      dartfreak75 Well-Known Member

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      So there is no way to calculate for it before hand?
       
    • dartfreak75

      dartfreak75 Well-Known Member

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      Thanks I knew the general min and I have found tons of info on how to measure it. Just seems like there has to be a way to calculate it before you start buying parts.
       
    • JBurch

      JBurch Well-Known Member

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      You don't, as already stated you have to test fit and measure........it's part of the fun building process

      A thought........buy a CAD program and do a design analysis
       
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      • dartfreak75

        dartfreak75 Well-Known Member

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        In that case Hopefully I can find someone here with a similar setup as mine and get some info from them.
         
      • sireland67

        sireland67 Well-Known Member

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        You could mock it up with an old cam, that would give you an idea of how much clearance you have.
         
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        • dartfreak75

          dartfreak75 Well-Known Member

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          Does any one here run a 318 with flat top pistons around .03 under the deck??
           
        • stumblinhorse

          stumblinhorse Well-Known Member

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          You are going thru some of the same thought processes as me. I am planning and getting parts for a street 390 build. Trying not to buy the wrong stuff, etc.

          Don’t overthink this part. If you make sure you have the right compression height pistons and you get a reputable cam. You are 99% of the way there. You need to mock it up once you have everything but chances are that you will have more space than you want and you will need to take off some from the deck. Especially with if you are going to use a mopar head. Those are just my thoughts, I am a rookie.
           
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          • 72Dart6pack

            72Dart6pack Harder Better Faster Stronger.

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            Piston manufacturer usually states max valve lift. Go to there web site and look.
             
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            • Marcohotrod

              Marcohotrod Well-Known Member

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              flat top pistons with or without valve notches? have the heads been cut? head gasket thickness? valve sizes? there are too many variables involved-that is why we do not have a formula for small block mopar engines.
               
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              • 70aarcuda

                70aarcuda Master Hoader of SBM FABO Gold Member

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                It is not max lift that cause problems...the piston is way down in the bore then...it is during the overlap period where the exhaust closing and intake is opening...
                duration...LSA and retarding or advancing the cam all change the valve to piston clearance..
                 
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                • 72Dart6pack

                  72Dart6pack Harder Better Faster Stronger.

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                  Squeeze as much horsepower as you can out of it.
                  :thumbsup:
                   
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                  • nm9stheham

                    nm9stheham Well-Known Member

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                    OP, maybe describe the pistons, milling, decking, and that cam you have in mind, and someone might be able to relate their measurements for a similar setup.

                    I have a spreadsheet for piston position versus crank angle. But the details of cam lift versus crank angle would be needed to work this on paper, plus a direct measurements off of the head for how far the valve sits above the head surface when closed.
                     
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                    • dartfreak75

                      dartfreak75 Well-Known Member

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                      The build combo in mind is a 318 bored .040 over with h814cp speed pro piston it's a true flat top no relief cuts the block will be cut .037 to get the piston. 030 in the hole. I will be using a regular felpro blue gasket (.04-.045) the heads will be cut around .005-.01 just enough to clean them and make sure they are flat.
                      The heads are 302 heads with std sized valves and this cam. And the cam will be set straight up dot to dot. No additional advance

                      Screenshot_20181122-053907_Samsung Internet.jpg
                       
                    • stumblinhorse

                      stumblinhorse Well-Known Member

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                      Haven’t seen that in a spec for a piston. Not sure how they would know?

                      Specs for my pistons are here. 1 Set IC846.030/1 Set 3910BD8.030

                      I see valve relief depth and size... not max lift.
                       
                    • dartfreak75

                      dartfreak75 Well-Known Member

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                      Are you building a 318 stroker?
                       
                    • stumblinhorse

                      stumblinhorse Well-Known Member

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                      Yup
                       
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                      • dartfreak75

                        dartfreak75 Well-Known Member

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                        I didn't know they made 318 pistons that short!! Whatcha doing 4 inch stroke?
                         
                      • stumblinhorse

                        stumblinhorse Well-Known Member

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                        Yes, scat 4” crank.
                         
                      • RustyRatRod

                        RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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                        I would put that together and not even think twice. The duration is what you have to worry about, not the lift when concerned with interference issues and that cam doesn't have anywhere near enough duration to worry about.
                         
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                        • dartfreak75

                          dartfreak75 Well-Known Member

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                          That is the goal!! Lol
                           
                        • crackedback

                          crackedback FABO Gold Member FABO Gold Member

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                          It should clear by miles with that dinky duration camshaft.

                          Best to verify....
                           
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                          • RustyRatRod

                            RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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                            My goal would simply be to have fun.
                             
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                            • dartfreak75

                              dartfreak75 Well-Known Member

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                              Thanks rusty!! I appreciate your help that is a huge relief. Iv been up reading and studying about this since about 5 am lol
                              Do you think that will be a pretty combo?
                               
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