What is weight of stock 340 piston

Discussion in 'Small Block Mopar Engine' started by 340doc, Dec 5, 2018.

  1. 340doc

    340doc Well-Known Member

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    Would anyone know the weight of a factory 10.5 to 1 piston?
     
  2. nm9stheham

    nm9stheham Well-Known Member

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    The data point I got a while back is 719 grams and an old one here (that is a bit corroded) is right around that number. Pin is 154 grams (for a couple of clean ones here) and that is what I found listed.

    Are you doing comparisons, or seeing how much you can save in weight, or what?
     
  3. 340doc

    340doc Well-Known Member

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    For comparison. Looks like I might have to bore my block and figured might as well go lighter if I can. Thanks for the info nm.
     
  4. nm9stheham

    nm9stheham Well-Known Member

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    Well that is about as easy as it gets, considering how heavy the stock 340 pistons are LOL. My 'stock' bobweight computations for the 340 is just over 2300 grams..... change over to KB pistons and SCAT I-beam rods and voila, you are suddenly at 1900 grams or under...! It'll be similar for any other aftermarket light piston, forged or hyper.

    We did the KB's and SCAT I-beam rods on my son's 340 and bobweight ended up at 1893 grams, even with a set of SCAT rods that was a tad above average SCAT weight. About 45% of the bobweight reduction was from the pistons, and about 55% was from the rod change. Took 81-82 grams off each end counterweight of a cast 273 crank to balance. You would take more grams than that off of a 340 crank to balance it to a <1900 gram bobweight.
     
  5. 340doc

    340doc Well-Known Member

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    That's some difference. I was gonna look into the Scat or Eagle rods. Hard to beat for the price. I guess your happy with the Scat rods? Quality piece?
     
  6. nm9stheham

    nm9stheham Well-Known Member

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    I can only say SCAT I-beams look good and have a good reputation; we did not subject them to any destructive testing (so far) LOL. One or more of the professional engine builders here have posted that, if you re-torque the rod ends multiple times on some of the foreign cast rods, you may eventually see some distortion in the big end dimensions. We just torqued ours 2x (plastigage check for bearing clearance and then the final assembly torque), so we presumably avoided any such dimensional change.

    I think some of the guys like 70aarcuda have been hammering on a number of the SCAT rods for quite a while on the strip; don't know if those are the I-beams or H-beams. Those are the guys who have been seriously 'testing' them for some time.

    The rod weights are advertised to be matched within +/- 2 grams, and our 1st set weighed out within a +/-1 grams total weight variation. With a close weight tolerance on the good aftermarket pistons, it makes balancing a lot less pricey for general hot rod use, where matching to within a couple of grams is quite adequate. I just weighed up the pistons and pins and ring and locks and bearings, combined that with the rod end weights listed on the SCAT box, used a standard bobweight formula from the Eagle site, handed the resulting bobweight number and the crank to the machine shop, and asked them to balance to that bobweight. The $$$ savings in doing it that way (plus going lower in bobweight, so you just take weight off of the crank) paid for almost half of the rods' cost plus an inexpensive but surprisingly accurate scale. Hard to beat that IMHO.....and I enjoy doing that type of work.
     
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    • 340doc

      340doc Well-Known Member

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      I've never had a stock rod fail, grind beams and a good set of bolts. That includes many years of hard use on a street/strip 4 spd car. The prices of a new set of rods are very attractive though. Appreciate your insight.
       
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      • pishta

        pishta I know I'm right....

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        trick is to get a good big end weight with any DIY jig. The varience is unbelievable with the smallest bit of pivot movement on either end. I bet that pro jig in all the videos would give a different weight with the slightest amount of skew in the horizontal plane. I ended up using a ball bearing in the little ends rod bushing so it would find its highest point horizontally and using 3 ball bearings under a metal plate to support the big end at a perfect 90 degree on the scales plate. That way the lateral position was always the same with no x-axis force, and still could only get 2-3g repeatablilty. Cheap scales dont handle side to side force well when looking for repeatablilty. straight weight is eazy, highly repeatable on even the HF .1g scale. 4 of 5 weigh ins on a piston will give me the same .1g result with 1 being .1g off. sometimes just tapping the table will get it to fall in line with the others.
         
      • MOPAROFFICIAL

        MOPAROFFICIAL Well-Known Member

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        I keep remembering 740 grams when I think back.

        Maybe that's the 340 rod I'm remembering
         
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        • nm9stheham

          nm9stheham Well-Known Member

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          The heavy rods are around 758.... I got an average of 760.6 grams on a set of 340 rods within the last month. Light rods are 726 grams nominal. Not sure where I picked up the 719 grams but the old ones that I have are close. (They are so corroded from being left in a wet box by the PO that I can't take their weight as gospel. I just got them for the rods.)
           
          Last edited: Dec 5, 2018
        • nm9stheham

          nm9stheham Well-Known Member

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          Yep, I tried all sorts of techniques to get the side load error out of my $50 scale.....I tried a sub-plate on the scale on oil of various weights to relieve the side load error... 75W90 gear oil was the best. But it still had several grams of variance in repeatability. Finally, finding the area to tap on the machinist's flat that supported everything to make thing 'settle in' did the trick. Ended up with a +/-1 gram consistency on the big end. Even with that, the sum of the small and big end weights average about 1 gram below the total rod weight by itself so there is probably a small uneven-ness somewhere.

          My setup is on a machinist's flat, and is carefully leveled in both axes. I took a lot of care to get the measurement plate even and level, and symmetrical in weight, but it it probably still off a hair. It is the hardest part of a DIY balance process. I just checked a fresh batch of SCAT rods on my setup and got their numbers within +/- 1 gram.. at that point, I could not say if it was the setup of the normal rod variance.

          This is a balance machine mfr's series of intro/training videos and when they do the big end weights, they either have a really repeatable setup, or they are more casual than expected in measuring big ends weights. Their vertical suspension rod weight scale is interesting; it is in part 3 or 4.
           
        • nm9stheham

          nm9stheham Well-Known Member

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          Hey Pishta....
          I tried one of the hanging rod weight setups.... holy crap! LOL I see why you say what you said with it being a problem with a cheap scale. My hang length was nice and long, about 2', and everything was plumb and square... not good at all. It seemed like the slightest tilt of the rod would change the reading drastically. I would gently squeeze the parallel threads together hanging the hook supporting the small end of the rod, so it would very slightly lift the rod's small end while the big end is on the scale; this should make the rod's big end weight go up slightly.... nope, went DOWN.... a LOT. Seems like any tilting force on the cheap scale's surface is the issue.

          Went back to my flat plate jig.....got a 1 gram variance putting the same rod on a coupla times.
           
        • pishta

          pishta I know I'm right....

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          Well. Your pretty darn close! The shiny ones are a 'balanced 'set. All taken off the little end pad...? The rusty one is untouched. All the balanced caps weigh the same so the must have hit the big end with a flat belt sander
          20181205_120843.jpg 20181205_120922.jpg 20181205_120944.jpg 20181205_120901.jpg
           
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          • pishta

            pishta I know I'm right....

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            Im thinking anyones jig is going to give a different reading and that includes shop to shop. We have both seen the slightest variance move the scale 10g (~1/3 oz) in either direction. Im starting to believe that the balance you get with the shops bobweight (1X4) is gonna get you close, but there is nothing short of a dynamic balance in the block that's gonna get you nuts on (at one RPM!!) ...and with under and overbalance practices and moving balance numbers throughout the RPM range...its just needs to be close. For example , there is an added 'oil assumption' value, "...Eagle uses 5g for small block assemblies and 15g for big block assemblies. (all balance shops use an estimated oil weight) Since it is impossible to accurately represent this value, it is just an estimate.." so that right there is saying 10g out of balance isnt going to make a noticeable difference. Road kill garage had a 360 assembly 'balanced' at a machine shop and they said it 'shook like a paint shaker' on the dyno. I think those guys know the difference between a neutral balance and a Detroit balance in their parts.
             
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          • nm9stheham

            nm9stheham Well-Known Member

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            I think that the problem we're having here scale's quality and how it is constructed, and how any external side load or tilt on the scale's surface can throw it off. And IMHO, pretty repeatable and accurate numbers can be had. It is all in the test setup, equipment, and calibration. So I think a job within a few grams can be had with a consistent home-shop setup, and that any decent professional shop should be able to easily hit that mark and better.

            I think I now see why my flat plate jig is consistent with this particular cheap scale: the weight on the scale rests on a single point; with the weight passing through that single point, it isn't able to tilt the scale's surface. (The jig's stability improved quite a bit when I changed it from 2 points resting on the scale to just one. I'll get some pix of it.) But, for the vertical hanging setup, with the rod's side or big end resting on the scale, it can tilt the scale's surface with any slight movement of the far end of the rod and thus change the cheap scale's readings.

            But, to get to 1/2 inch-ounce error (I've read that productions limits were up to +/-2 inch-ounces), then, I agree, somewhere in the 5-10 grams range would stay within that limit. Of course the more accurately the piston-rod part is done, then the better the crank can be made to match.
             
          • pishta

            pishta I know I'm right....

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            back to the drawing board, looks like he was hanging far more than I was. Will give me something to do tomorrow in the garage. My issue will be making the big end spindle as my drill press is OOS at the moment. Will use a Chevy pin for the little end. Metal store is open at 0800...:) I think the main issue is getting the rod square on the spindles and not having anything move. Rigid.
            jig.jpg
             
          • nm9stheham

            nm9stheham Well-Known Member

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            BTW, Pishta, I just bought and tried one of the exact Horrible Freight scales that you show, and with the same rod, it measured the exact same total weight and I got the exact same big end weight.... to the gram... vs. my $50 Ohuas scale. (That agreement could be good and that could be bad LOL)

            IMHO, the setup pictured above is going to give the same or worse problems with these cheapo-scales.....any tilt action or side force is gonna throw it off, and it seems like a tall hanging point is gonna have more leverage to put tilt force on the scale. I am gonna guess that these cheapo scales use just 1 electronic load sensor.... it looks like precision scales have 4 of them, one at each corner of the plate.

            Maybe anyone with a good, consistent setup and scale for this work can speak up; I can't see any info on the CWT scale, but I suspect it is bought elsewhere and re-labeled.
             
          • pishta

            pishta I know I'm right....

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            Here us my hanging basket of frustration. $10 in parts including the $6 chain. Everything is good now except the big end mandrel. I was trying for a snug fit to minimize the lateral tolerance but i think that is less important with the trapeze than the flatness that it sits. I believe that is why your flat plate setup ( pics?) is giving you more consistent readings . If i had a machined perpendicular mandrel that would hold the big end exactly the same way every time it would be better. Along with a 4 corner pickup scale.
            20181206_152619.jpg 20181206_152637.jpg 20181206_152649.jpg 20181206_152656.jpg 20181206_153943.jpg
             
          • nm9stheham

            nm9stheham Well-Known Member

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            I like your double chains to keep the small end stable. And all nice & level. I see the chains hanging at a slight angle to vertical; that might put a bit of side load on the scale. Things any more consistent? Or is the scale still acting up?

            I tried an 'improvement' this evening... just more complexity for no gain.
             
          • pishta

            pishta I know I'm right....

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            scale inconsistant. The 4 corner sensors have to be the key. The slight side load didnt affect it, but the big end seating does. Im getting close. We should start another thread on this DIY rod balance. I need a pic of your flat plate!
             
          • nm9stheham

            nm9stheham Well-Known Member

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            Yes, any slight angling of the rod end on the scale will throw it's reading off. I read up on cheap load cells and may see where some of the problems lie.

            I been thinking a new thread would be a good idea....since we at least can share with some direct experience what the problems are LOL. I am gonna get some pix today.