Just Purchased Virgin 340 Short - What to do with Pistons

Small Block Mopar Engine

  1. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    Got a picture of the top of the pistons?
    I'm kindof ignorant here, but shouldn't stock '71 pistons be above the deck about .045?
    And 65 cc is close to the min, would that suggest the heads have been milled?
     
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    • nm9stheham

      nm9stheham Well-Known Member

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      Mmmmm... you deck height is nowhere near to typical for the early high compression pistons; those stick .018" nominal out of the bores. If they are truly .070" down in the bore at TDC, then IDK what they are; they may be the later low compression 340 factory pistons, but I did not think they were down in the hole that much. The Silvolite 1267 mid-compression replacements don't sit low like that.

      So is this 'in the bore by .07" ' for real or did you mean .007" or ???? Do the pistons have 2 or 4 eyebrows? What actual heads do you have?

      If you were to put in newer Icon or KB or other newer pistons, you would save considerable weight. The Icon's have a light ring pack. But not sure any of that is of value to you for your use. And the crank would need re-balanced as noted.

      Of course we know nothing about the condition of the present pistons, bores, rings, ring grooves. So it is a guess as to what it will do.
       
    • pittsburghracer

      pittsburghracer FABO Gold Member FABO Gold Member

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      Edelbrock even has a special part number for those pistons with an open chamber
       
    • 512Stroker

      512Stroker We are all here because we are not all there.

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      OP if you run into piston issues I have a complete set of stock pistons from a 69 340 that are in good shape. You could have them for the freight.
       
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      • davescuda

        davescuda FABO Gold Member FABO Gold Member

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        what is the vin on the block? My car came from your area.
         
      • cudaracer

        cudaracer Well-Known Member

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        No offense taken, it's just that many people have info at their fingertips, and just tweak a value or two. I would have to research what the cc's in the piston are for stock, as well as typical deck height, which is never accurate. And so on. Yes, the calculations are easy peasy
         
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        • cudaracer

          cudaracer Well-Known Member

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          The measurement was 0.07" and you could visually see it into the bore, clearly, where 0.007" would look almost flush.
          So yeah, I will have to tear this down to see exactly what I am working with. Virgin bore does not equate to original pistons, as I believe I have found out.

          I am using 906 heads, slightly decked and cc'd at 65. They were on my 318 that I will swap shorts with, and yes, my CR went way down with these heads. The pistons do have 4 eyebrows and the top does look identical to original factory piston tops for 340 that I have seen.

          I plan on using the pistons, assuming in good shape, but will probably replace rings. Can't see putting old rings back in, but I will leave that up to my machinist to determine if they are good, as I have no way of knowing.
           
        • Marcohotrod

          Marcohotrod Well-Known Member

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          1968-71 340 pistons = comp distance - 1.840" - 2 valve notches - 725 grams plus pin = heavy , above deck around .020". 1972-3 340 = comp d - 1.740" - 4 notches - 725 grams plus pin = below deck .080". if rebalancing is in the budget, you want to keep it and rev high for years, go lighter pistons and pins. a longer stroke = shorter/lighter pistons, and more friction. a J E piston for 4.060" X 3.480" weighs 513 grams and revs quick. 906 big block heads?
           
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          • nm9stheham

            nm9stheham Well-Known Member

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            New rings are not expensive. But as mentioned above, wear in the ring grooves is just as critical. If you have worn ring grooves, the rings will flex and twist constantly, and will wear out quickly. Back-in-the-day, you could reuse a set of pistons with worn ring grooves, by having the grooves machined wider and then inserting a ring spacer to take up the slack and get back to original groove width. A ring spacer looks like an oil control scraper ring. Dunno if many machine shops will do that anymore, or if they can do it economically.

            Good deal on the heads; tnx for rthe info.

            If the piston tops truly are .070" down in the hole, then even with these milled heads at 65 cc, and with a .039" head gasket, you are looking at around 8.0 static CR. Yep... typical smog era stuff! It'll only go up to around 8.25 with the .028" head gasket, but the heads and deck need to be good and flat to use those, per reports here of them tending to not hold at times.

            So if the measurements we are discussing are right, then you sure as heck don't want to cam this thing up an use it as a cruiser.....your dynamic CR is gonna be in the toilet. Think: more sluggish than a stock smog era 318 ......ooooo-ow, that hurts to even think about LOL

            Maybe it is time to PM to 512stroker on his hi-comp 340 pistons.... that is a generous-sounding offer IMHO.
             
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            • Nat

              Nat Well-Known Member

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              you sure it is a 1971 ? sounds like the pistons were changed 4 brow sound like replacement pistons . . with that low compression throw a supercharger on it
               
            • Wyrmrider

              Wyrmrider FABO Gold Member FABO Gold Member

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              cast rebuilder pistons and boost do not mix well
              actually not evea NA if it detonates
               
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              • toolmanmike

                toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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                Let's not talk about turbos. Man are they fun but they can wreck shit. (cars,engines,and even lives) A good running 340 doesn't need a turbo. There's a reason why the manufacturers put them on 2 liter 4 bangers.
                 
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                • cudaracer

                  cudaracer Well-Known Member

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                  OK, just measured comp height, and it's only 1.75". Yikes, that proves that it's 0.07" in bore as well. I calculate 8.23:1 with 0.047 gasket. Did they offer a low CR 340 in 71? So I need new pistons, any reason why I should shop someone other than KB?

                  Block number is 2780930-340-7
                   
                  Last edited: May 13, 2020
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                  • yellow rose

                    yellow rose Overnight Sensation FABO Gold Member

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                    72-73 340’s were all low compression like that. Nothing wrong with a KB piston.
                     
                  • gm1236

                    gm1236 Well-Known Member

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                    Could be an early ‘72 motor.
                     
                  • Mattax

                    Mattax Just the facts, ma'am FABO Gold Member

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                    That's the casting number.
                    upload_2020-5-13_21-5-19.png
                    On the other side of the Block, will be the casting date and time.

                    Engine number, including VIN or partial VIN was stamped onto a pads.
                    Look on the front, below the heads.
                    upload_2020-5-13_21-8-40.png
                    and above the pan rail on the right.
                    upload_2020-5-13_21-9-48.png


                    Some examples with decoding comments here:
                    Mopar Lost and Found
                     
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                    • 512Stroker

                      512Stroker We are all here because we are not all there.

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                      Well that's a 69 block
                      But I do not believe that is factory paint and it appears that those freeze plugs have been replaced.
                      My guess is that that engine has been apart before. Hence the different pistons.
                       
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                      • Mattax

                        Mattax Just the facts, ma'am FABO Gold Member

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                        Yes! That's my latest engine. Bought it a couple years ago from another member here. I wanted another backup, one that could meet SCCA Street Prepared rules without a major a hassle. It's sitting on the stand now. I need to clear a few projects that became higher priorities before returning it.
                        My decode on it is here: 1969 built 340, unknown car.

                        IIRC I used a couple books (Taylor and the FSMs) and this
                        How to ID a Small Block Engine Block:
                         
                      • crackedback

                        crackedback FABO Gold Member FABO Gold Member

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                        71 casting date could easily be model year 72 with the lower compression in the hole pistons. Never seen a 68-71 340 with factory pistons not at least at deck height or above.
                         
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                        • Mattax

                          Mattax Just the facts, ma'am FABO Gold Member

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                          Just showing him where to look. Yea. The engine I pictured is not virgin! LOL. .030 over.
                          I think we can safely assume that the engine gm1236 has in front of him has been apart even if its not been bored over.
                           
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                          • Mattax

                            Mattax Just the facts, ma'am FABO Gold Member

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                            Factory ever use a piston with 4 valve clearence 'eyebrows'?

                            That's strictly aftermarket, isn't it?
                             
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                            • davescuda

                              davescuda FABO Gold Member FABO Gold Member

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                              So what is the vin on the block?
                               
                            • pt70

                              pt70 Well-Known Member

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                              1972 and 1973 340's came with 4 valve reliefs in the pistons. Around 8.5 compression ratio.
                              1968 thru 1971 340's came with 2 valve reliefs in the pistons. Around 10.5 compression ratio.
                               
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                              • cudaracer

                                cudaracer Well-Known Member

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                                Casting date is 6 - 18 -71.
                                I truly thank everyone for the contribututions, as I feel we all learn something. Certainly myself.
                                 
                                Last edited: May 14, 2020
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                                • Wyrmrider

                                  Wyrmrider FABO Gold Member FABO Gold Member

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                                  get some quench with those new pistons
                                  easier with closed chamber heads
                                  reproducing 70 gas is pricey
                                   
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