How much decking is to much?

Small Block Mopar Engine

  1. Cope

    Cope Fusing with fire

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    Just for fun, let' say I can cut as much as I want off the deck and or heads.

    How much can I take?

    Why I ask is I want to raise my comp ratio and I have accss to a big mill.

    What are the draw backs?

    1976 360, W factory heads
     
  2. RustyRatRod

    RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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    Till you hit the water jackets.
     
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    • harrisonm

      harrisonm Well-Known Member

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      The question was "How much is too much?" I'd say about an inch off the block and a half inch off the heads would be too much.
      Seriously, I don't know the answer to that, but you will have to accurately mill the intake as well.
       
    • 273

      273 Well-Known Member

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      I never hear of people going more than 0.060". Some probably have. Better off buying the right pistons.
       
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      • yellow rose

        yellow rose Doctor of Thinkology.

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        On passenger car blocks I stop at .060 because the decks ain't real thick. The X blocks can take an easy .125 and IIRC the R blocks can take a bit more than that.

        The problem is, once you cut it that far, you ain't getting it back. When cutting cylinder heads I'm even more anal about it.

        The decks on the heads are even more thin that the block deck.so I do all my chamber work and finalize size and the cut maybe .030-.040 off and that's it. The more you take off the head the harder it is to get it to hold a head gasket.

        If you can buy the correct piston I'd do that. Then you cut the decks to square them up and surface the head to get the chamber volume when you want it.

        Personally, I have about a .200 high dome on my piston. I also have the flat of the piston .042 out of the deck. So with a 72 cc chamber I'm at 11.01:1 and with a quick gasket change I can be at 11.25:1 and if that isn't enough, I can still take .060 off the head. That will make me 11.8:1 or so. And I run on cast iron heads and pump gas.
         
      • jos51700

        jos51700 Well-Known Member

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        The max you can take off is equal to the bare minimum required to get it flat.

        Then you do some machine work with your mill, make some money, and buy high compression pistons.

        Remember, thick n' rigid is always better than thin n' flexible.
         
      • RustyRatRod

        RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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        You can usually get by with .030-.040 on the heads before you need to mill the intake. If it was me, I would not mill for compression.....although I have done it. Milling to gain compression means removing a LOT of material. Usually around .070" equals one compression point.

        Normally, small blocks blueprint (actual measured compression ratio) very low. Oftentimes in the high 7s, like 7.8 or 7.9. Yes, they were rated at 8.2 and 8.5. Even the early 340 suffered this. They generally blueprinted down around 9.2 to 9.5, yet were rated at 10.2.

        So, that means you'll have to mill about .140" off heads and or block on the average small block to get compression around 9.5. Then, you'll have to mill the intake to match. What you end up with is an engine that will only work with its own upper half. That is, unless you mill the block too, then you have a whole bastard engine.

        What I recommend is blueprinting the deck height to spec. What is the spec? 9.6 ain't it? They are almost always high and crooked. The heads are much the same. The gasket surfaces on the heads is usually long. That means the combustion chambers are big.

        By correcting all of this, you assure yourself that you have the deck of the block and heads where they are supposed to be. You also correct things like combustion chamber size and lifter preload, since you are straightening up the block deck surface. Some say it's splitting hairs, but I say it;s the difference between a good build and a not so good one.

        Once you get those things (literally) squared away, you simply choose a piston to achieve the compression ratio you are looking for.
         
      • 318willrun

        318willrun Stomper 4x4... we kept energizer in business

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        Assuming your talking about a factory family motor (LA small block so to speak), my opinion is if you want to deck it square (flat), great. IF you decking it for compression, my question is why? Pistons will get ya where you want to be for compression. And compression is not the tale-all of the story of performance, flow is....
        • to answer the question, I don't know how much you can take off the block and be safe. Each block would be different. (if I understand the question).
         
      • dibbons

        dibbons Well-Known Member

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        Recently re-surfaced deck on two 340 blocks, they both were so far off on the passenger side that it took two cuts just to square that side off. Machine shop told me to order different Cometic gaskets because the passenger side pistons are now .005" popped out further out than the driver's side pistons (.022" vs .017" above deck).
         
      • RustyRatRod

        RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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        Find a new machine shop.
         
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        • dibbons

          dibbons Well-Known Member

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          What's the machine shop(s) have to do with the block not being square from the factory? I am talking about two 340 blocks that were decked at two different machine shops (about 1,500 miles apart from one another) where they both found the same discrepancy, namely they found the passenger side deck was higher than the driver's side deck (and neither would clean up after the first cut). We measured the piston deck height's before hand and knew the problems going in.
           
        • yellow rose

          yellow rose Doctor of Thinkology.

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          Because unless you are running closed chamber heads the piston should be at least .017 out and another .005 won't kill it.

          In reality, the piston needs to be somewhere near .040 out to get a flat top piston to 10.5:1 without cutting the heads to death.
           
        • yellow rose

          yellow rose Doctor of Thinkology.

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          Rusty, IIRC the blueprint deck height is 9.560 and I've seen them as tall as 9.625. You also need to make sure the pin height in the piston is at blueprint spec and rod length is at spec. If any one of the three (or all three) are off you have to correct it with deck height.
           
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          • 273

            273 Well-Known Member

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            Dont go over board on CR. Everyone wants to run 10:1. But then you got to run a cam that gives away your bottom end. Which is fine if you throw in a set of 4.10 and 3500 stall which most dont. To gain 4% hp over 9:1.

            Now you have a finicky street car? That has to always be in perfect tune.

            To me run only the amount of CR your cam needs to work. Most street cam are only gonna need 8.5 - 9.1.
             
          • dibbons

            dibbons Well-Known Member

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            I wonder what compression ratio would be recommended for the 340 factory profile cam then?
             
          • teringer

            teringer ase master hall of fame

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            I've cut .110 off early 360 blocks back when you couldn't buy pistons ran 12to 1 with no problems
             
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            • mbaird

              mbaird mbaird FABO Gold Member

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              [QUOTE= Remember, thick n' rigid is always better than thin n' flexible.

              Thats what she said!

              I just couldnt help myself....
               
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              • Marcohotrod

                Marcohotrod Well-Known Member

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                we cut an early 360 block back in the day .090" with no problems for a guy that had forged pistons with a 1.591" comp. height. a good machine shop does not just square a block, the equalize both sides. so with good rods and pistons we do not need different head gaskets. I would forget cometics overpriced 2 different gaskets and run 1008 fel pros and cut 1 head .005 more. I think the blueprint on a 273 318 340 360 is 9.600" and a magnum 318 360 is 9.560"
                 
              • SpeedThrills

                SpeedThrills Well-Known Member

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                Sounds like factory casting tolerances were one of the reasons engines didn't last "back in the day". Plus the casting recipe...
                Aftermarket blocks put weight on the wrong end of a drag car, but, I guess, for good reason.
                 
              • RustyRatRod

                RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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                The way you worded it, it appeared that you said the machine shop gave it back totally uneven. That they could not get it straight.
                 
              • RustyRatRod

                RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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                10.2:1 just like they were rated.
                 
              • RustyRatRod

                RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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                Absolutely, but for this discussion, we hadn't gotten there yet. lol And don't forget indexing the crank!
                 
              • RustyRatRod

                RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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                I went back and re-read what you wrote and I stand by what I said. You clearly state the shop gave you back a block that was still uneven side to side. They need to find a new profession cause the one they chose don't suit um too well. They told you to rely on different head gaskets. What a effin joke.
                 
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                • MOPAROFFICIAL

                  MOPAROFFICIAL Well-Known Member

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                  I've milled .060 with no issue.
                  Heads i have on now were milled .048
                   
                • MOPAROFFICIAL

                  MOPAROFFICIAL Well-Known Member

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                  Decking parallel and square decking are two different things on the price sheet ;) . That bank will crank even at least ... so they're half way there. Lol ...but i agree with you, I'd have told them to go mill the other side the same and basically chastise them for being lazy and thoughtless.
                  Amazing.
                   
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