340 Piston and Valve Question

Small Block Mopar Engine

  1. Scubakuda19

    Scubakuda19 FABO Gold Member FABO Gold Member

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    Hello, I have a 340 .030 over and the deck was milled .010 and the pistons are now .025 above deck. Does this present a issue for piston integrity? I'm running a set of factory "0" heads with 2.02 and 1.60 valves, what would be the recommended head gasket to achieve proper clearances? Thanks in advance for any help.
     
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    • 4spdragtop

      4spdragtop FABO Gold Member FABO Gold Member

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      I would imagine rocker setup would come into play? Cam lift also? I'm still learning... thus the question marks lol
       
    • yellow rose

      yellow rose Master Of Insanity

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      That’s better than most are. They SHOULD be above the deck. Gasket thickness will be determined by what compression ratio you need or want.

      The only way to know for sure what the CR is is to do a 1/2 down fill, CC the heads. And then don’t let me do the math! You need to measure everything and not guess.
       
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      • Scubakuda19

        Scubakuda19 FABO Gold Member FABO Gold Member

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        Thanks. I just got the heads back from the machine shop and they CC'd at 66. I know the pistons from the factory were .014 out of the hole and the deck was milled .010 so theres the .025 out. May I ask procedure for 1/2 down fill? I believe that the pistons are 10.5 to 1 but that will be determined exactly by the gasket thickness? Thanks
         
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        • RustyRatRod

          RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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          You'd have to have a good bit of duration to have trouble with piston to valve clearance.
           
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          • yellow rose

            yellow rose Master Of Insanity

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            And a pretty slow lobe with that duration.
             
          • Scubakuda19

            Scubakuda19 FABO Gold Member FABO Gold Member

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            Cam 284-484 it's not too radical. I appreciate the input, this is my first build and learning everyday. Thanks
             
          • RustyRatRod

            RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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            You're in fine shape.
             
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            • Scubakuda19

              Scubakuda19 FABO Gold Member FABO Gold Member

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              Thanks. The pistons extending out of the hole .025 doesnt compromise the piston in any way?
               
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              • RustyRatRod

                RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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                No. They were like that right from the factory.
                 
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                • yellow rose

                  yellow rose Master Of Insanity

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                  It’s easier said than typed. But I’ll give it a shot.

                  Bore squared times the stroke length will give you the volume of any cylinder. So in your cascade it’s 4.070 times 4.070 times 12.87 (the number to convert cubic inches to CC’s) and take that number and multiply it by your stroke (or the thickness of the gasket or whatever the depth of the cylinder you are measuring is. And that number is the total volume of your cylinder.

                  4.070*4.070*12.87=213.19026*3.313=706.29933
                  That is IF you have a true flat top piston, with no valve reliefs and a zero deck.

                  Now you get the number 1 cylinder at absolute TDC and put your dial indicator on the piston and lower it .600 (hang with me as I know I said .500 but you can use any piston depth you want, as long as any dome is lower than the deck). Take some grease and seal use it to seal around the top of the piston. That’s why I said go an extra .100 deep. Once you get the grease in there (dial indicator is still on the piston) turn the crank clockwise (direction of rotation in the car just like it runs) until you are at exactly .500 down. Use your clear plexiglass plate with a hole in it and your graduated burrette and slowly fill the cylinder until it’s full and read how many CC’s it took to do that. That will tell you your exact dome or dish volume, including valve notches. I can’t say what yours is. You have to measure it.

                  I have a .187 dome on mine, and I know my numbers so I’ll post the, here so you can see how the math works.

                  My cylinder volume is exactly 695.92543 which is less than yours because my bore is a skosh smaller. That’s why you measure. Sooooo, now we need to know what the volume is at .500 down. Same math.
                  4.04*4.04*12.87=*.500=105.02949 and again, that’s with a flat top and no valve notches. My .500 downfill was exactly 99 CC’s, which means my dome displaces exactly 6.02949 CC’s. Damn this takes some typing.

                  Ok, now we have a cylinder volume, and a 1/2 down fill volume so we know exactly what the piston dome (in my case dome...could be a flat top with valve notches or a dish) but we ain’t done yet, unless you have a .0000 deck height. You don’t. Neither do I. But the math is the same. I’m out of the hole a measured .040 so it’s:
                  4.04*4.04*12.87=210.05899* 040=8.4023596 CC’s and that gets added to the dome volume. So that 14.431849 CC total dome volume. And were not done yet. You need to figure the gasket volume. For me that’s 4.185*4.185*12.87=225.40807*.036=8.1146905 CC’s because my gasket has a 4.185 bore and is .036 thick. Hope this is making sense. And my head is 71 CC’s. Now we are down to simple math.

                  To find your actual compression ratio the math is:

                  Cylinder volume at Bottom Dead Center divided by cylinder volume at Top Dead Center! Simple huh? That means we need to add up the actual cylinder volume, plus the head CC, plus the gasket. So...
                  695.92543+71+8.1146905=775.04012 CC’s. That is my actual total cylinder volume at BDC. Now we add up the volume at TDC, which is 71+8.1149605=79.11469 BUT WAIT...we need account for the dome volume and the positive deck height because that displaces volume at TDC. So that means 79.11469-8.1146905=71CC’s. Ok we are now down to the short strokes.

                  Volume at BDC is 775.04012 and the volume at TDC is 71. Therefore:

                  775.04012/71=awe crap...going by my memory I missed something. Hang on...gotta find my note book.

                  Ok, let start over with my actual numbers.

                  71+8.1146905-8.4023596-6.02949=64.682841 and that is the volume at TDC. Good thing I have my notes here or I’d be screwed. So...now it’s the total of volume at BDC which is:

                  64.682841+696.422=761.10484 and that’s the total of the volume at BDC. So...now it’s:

                  761.10485/64.682841=11.767619 actual, measured compression ratio. I’m doing that on pump gas and iron heads, so my butt may be sucking buttermilk but I think I can do it. So...the math above was:

                  Head CC plus gasket volume minus dome volume minus positive deck height for the actual volume at TDC and...

                  volume at TDC (64.682841) plus the swept volume (696.422) of the cylinder and that gives you your total volume at BDC and you simply divide the BDC volume by the TDC volume and that’s it.

                  FWIW it’s much easier to do it than to type and read it. Damn, hope I didn’t confuse you. In reality, it the measuring of a few volumes, doing some simple math and you’re done.

                  YR
                   
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                  • rumblefish360

                    rumblefish360 So close, yet so far away FABO Gold Member

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                    I just checked piston to valve clearance on my 340 with the replacement KB slugs (@10.51 measured) with a .017 positive deck dome height & a .546 lift cam and there is enough room for a truck to drive through.

                    Now that I know this, ha ha ha, the next lift used will be much more!
                     
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                    • Garrett Ellison

                      Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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                      A light piston modification to improve piston to valve clearance. It's just beveling the sharp edge from the valve relief trough. Probably not necessary in your case but not a bad idea if you decide to go larger later on. Do you know how much the heads have been milled on that?
                      [​IMG][​IMG]
                       
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                      • nm9stheham

                        nm9stheham Well-Known Member

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                        OP, set it sounds like you have open chambered heads. The pistons ought to stick up into the open area all around and have no clearance issues to the head. You can just put a piston at TDC, sticking out, and set the head on the block with no gasket; the head ought to set flush without hitting the piston. You can put a thin piece of clayon the piston head when yo do that and see what thickness you end up with.

                        Sounds like your heads may have been milled something like .030" based on your cc numbers. But even then the recess in the head will be something like .080" deep, and that, plus the head gasket thickness, will keep the pistons far away from the head.

                        Just curious: What .030" pistons do you have?
                         
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                        • =Max Wedge=

                          =Max Wedge= FABO Gold Member FABO Gold Member

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                          OP, Are the pistons new? What brand or a part number would be great as well. Thanks.
                           
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                          • Scubakuda19

                            Scubakuda19 FABO Gold Member FABO Gold Member

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                            I'm sorry I put you threw all that, I'm not confused at all you did a fantastic job explaining., thank you.
                             
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                            • Scubakuda19

                              Scubakuda19 FABO Gold Member FABO Gold Member

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                              I bought the shortblock full rebuilt. I was told they were 10.5 to 1 Ross pistons. The heads I bought last year and just had them re-done and didnt ask the machinist if they were or had been milled. I guess it would've been easier for me to build from scratch vs buying a short block that was already done and building backwards.
                               
                            • Scubakuda19

                              Scubakuda19 FABO Gold Member FABO Gold Member

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                              Brand new, I was told 10.5 to 1 Ross pistons, I didnt see anything stamped on the pistons, I looked at the bottom sides and they have C45 looks like on the bottom side, I will double check this today. Thanks
                               
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                              • Cudafever

                                Cudafever Well-Known Member

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                                My 340 specced out to be 9.4:1 for a 10.5 piston.
                                You have more piston above deck and a few less cc on head volume..........so you may be closer to 10.1..........CC everything and know what you really have!
                                That requires deciding on the thickness of the head gasket as well.
                                 
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                                • oldkimmer

                                  oldkimmer FABO Gold Member FABO Gold Member

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                                  The factory engines came with a steel shim gasket. .022 compressed. I have ran the 284/484 cam many times and with pistons .045 out of the bore. With the steel shim gasket. Kim.
                                   
                                  Last edited: May 23, 2020
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                                  • Garrett Ellison

                                    Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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                                    • oldkimmer

                                      oldkimmer FABO Gold Member FABO Gold Member

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                                      Sweet. Kim
                                       
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                                      • Scubakuda19

                                        Scubakuda19 FABO Gold Member FABO Gold Member

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                                      • Scubakuda19

                                        Scubakuda19 FABO Gold Member FABO Gold Member

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                                        Thank you!
                                         
                                      • 4spdragtop

                                        4spdragtop FABO Gold Member FABO Gold Member

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