340 Rod questions

Small Block Mopar Engine

  1. yellow rose

    yellow rose Overnight Sensation FABO Gold Member

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    I’ll probably be the guinea pig on this, but I won’t have a clue how well they work, as I’m putting them in before I go to the dyno. I wish I had the time and money to run it just like it is, and the just change out the top rings and go right back to the dyno. I suppose I can measure blow by on the dyno, but again, I have no data on blow by as it is now.
     
  2. gm1236

    gm1236 Well-Known Member

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    If the rings are fairly new and the cylinders still have some x hatch in them you’ll be ok. But this looks like an old high mileage motor that already had a broken ring. In an old motor the cylinders wear and become out of shape, the rings wear into this same shape. Over time the rings lose some of their radial tension.

    This is why they rebore, to bring the cylinder back to being perfectly round, then the rigid hone fine tunes the bores and leaves the x hatch to help the new rings break into the cylinder walls creating the ring seal.

    Put a new ring in an old cylinder and hold a light behind it and you’ll see where it’s not touching. The brush or ball hone won’t take the wear out of the cylinder, but the x hatch it creates will help to seal the rings by wearing them into the shape of the old bore. But this takes time to happen. The cylinder walls glaze up after time making it difficult for a ring to reseal to it, that’s why you need to hone it.

    Do what ever you want, I’m just trying to give you some advice, I did what you are doing and in the long run I had to take everything apart and do it right. I would have saved a lot of time and money doing it right once.
     
    Last edited: Jun 5, 2020
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    • nm9stheham

      nm9stheham Well-Known Member

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      Hmmm.. I thought the bevel on the inside was to induce 'twist' in the ring... i.e., to make the ring flip up on the downstroke more than flop down on the upstoke. (or vice versa...)

      Plenty of info on that, like this:
       
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      • gzig5

        gzig5 Well-Known Member

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        Well the fun continues. Still haven't nailed down where the difference in piston height above the deck is coming from but the compression height of the pistons isn't the culprit. Going to swap the rod/piston assy from the short and tall holes to see if it follows the rod or stays in the hole. If it stays, it's the crank, if it follows it should be the rod. Neither of which is an easy fix to even them out.Need to get a piston ring compressor before I can do that, the one I have isn't the right size. Probably from when my Dad was playing with flathead Fords.

        In other news....I checked the end play on the crank and it is at .0115", which is no good. I was worried about that last year because it looked like the nose of the converter was pushing on the end of the crank. New converter went in and it has similar marks, even with a new flex plate. Doesn't make sense. Either way there is a lot of end play so I am going to pull a couple caps and look at the mains. Was hoping to avoid that. With the crank out I might as well pull the cam and do a hone and maybe re-ring all cylinders instead of just the one. I don't know. Still thinking about it. Like most projects, the further in I get the more crap I find and more scope creep I end up with. At this point, I'll probably end up with new rod and main bearings. I can get a dingle ball hone for ~$60 or pay to have it done which I would think would be an hour's labor or less? Now I'm not that far from making a case for boring .020 over but if I do that, I am going to go with a stroker setup because I'm not sure I trust this crank to do a high revving 340. Cripes, might as well put port fuel injection and gear drive in while I'm at it. :BangHead: That busted ring has me boxed in a bit.
         
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        • gzig5

          gzig5 Well-Known Member

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          Can anyone tell me the spec for the thrust bearing width when new and the acceptable range for the opening in the crank where the thrust bearing resides? I can't find it in any of my books or online so far. I am measuring 1.157" on the crank opening with a telescoping gage and a caliper. The used bearing is about 1.147", roughly. It varies from the crown to the parting ends. I need to find one of my 1-2" mics to get a more accurate reading.
           
          Last edited: Jun 7, 2020
        • Shorty Medlock

          Shorty Medlock Roads Scholar

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          When is the last time you saw a broken Mopar crank? They are some tuff sob’s. The answer from one of my Chevy friendly machinists is”never” and he has been in the biz 40 years.
           
        • gzig5

          gzig5 Well-Known Member

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          I've never seen a broken Mopar crank besides a few pics of aftermarket cast ones. But I've never seen a forged crank with cracks radiating from the forge line on the counterweight either, until now. I don't think they will fully fail and probably been there since 1970 and can be welded up and nobody would know. But right now with everything else I've seen in this motor and the car it came out of, nothing is going to be taken for granted.
          upload_2020-6-8_9-31-59.png
           
        • Krooser

          Krooser Reform School Graduate

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          That looks like an occlusion in the steel that may have been there forever.

          I have a couple of forged standard 3.31 stroke cranks that need to be cleaned up....a polish may be all they need. Not 340 cranks but I wouldn't care if it was me. $75.00 to you and I might be able to deliver 1/2 way.

          Heck make a welded stroker out of one. Or I'll sell you a 360 crank and grind the mains to fit that 340.
           
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          • gzig5

            gzig5 Well-Known Member

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            Thanks for the offer. We'll see where this goes and I'll let you know. I agree it probably was there from the beginning. Just one more thing to think about.
             
          • Krooser

            Krooser Reform School Graduate

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            Find a machinist and turn that mother down... Lighten it and get rid of the defect at the same time.
             
          • gzig5

            gzig5 Well-Known Member

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            Anyone have a fresh bearing or a good crank handy that you can measure?
             
          • gzig5

            gzig5 Well-Known Member

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            Found the answer by calling King bearing. The width on their standard bearing thrust surface is 1.150-1.151". So if the crank truly is 1.157, then I will be at the top end with a new bearing, which is OK. I think I've seen discussions that stated you are better off at the upper end.

            BTW... I asked about oversize thrust bearings as discussed in the linked document, but the person I spoke to said that SBM is not supported. "Now if it was a Chevy..."
            https://kingbearings.com/wp-content/uploads/2014/11/max-flange-pro-flange.pdf
             
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            • nm9stheham

              nm9stheham Well-Known Member

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              Per the 2006 AERA crank manual, the thrust width on the crank is 1.156". Do a search for the AERA crank manual and you can find it as a free downloadable .pdf for that year.

              That is quite an interesting crack on your crank throw! Hard to predict if it will ever fail or not. I had a cast spindle on a rally car they held for many races then finally let go; cracked right across a casting void.
               
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              • MoparLeo

                MoparLeo NRA PATRON LEVEL LIFE MEMBER FABO Gold Member

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                Boy all of this to try to save a few $$$. Never works out. Do it right or don't do it. Should be answer always. Spend a dollar to save a dime. That was my father in-law. Never understood it. Unless this is your daily driver and it has to run now to go to work, just save up until you can do it right. Costs more to do it twice.
                 
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                • gzig5

                  gzig5 Well-Known Member

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                  To somewhat close the loop on this topic.... I picked up my block from the machine shop this week. I had them run their rigid adjustable hone through the block and remove the slight ridge at the top of the cylinder and just rough up the rest of the cylinder enough for new rings to seat. You can still see the light vertical scratches that were in a couple of the cylinders so they didn't open the bore much if any. Block got cleaned out, new freeze plugs, and new oil pump drive bushing as well as cam bearings. Now need to order main bearings, rod bearings, and a full ring kit. He thought the crank, pistons, and rods looked fine. I was going to do an overbore with pistons and rods, but the body work I was having done turned south when we found additional hidden accident damage which required an extra $2000 in parts and labor, so it will have to get by with the original pistons. Not optimal but I'm sure this has been done many times and should get me by until the rest of the body work is completed and it is ready for the stroker, or whatever the next level will be. Still would like to figure out the variation I have in height above deck on the pistons, but I'm not going to obsess about it and I'll just pick a head gasket thickness that will keep me safe.
                   
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                  • Wyrmrider

                    Wyrmrider FABO Gold Member FABO Gold Member

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                    sounds good
                    match the grit with the rings
                    if you think bores are good enoygh to run molly rings then maybe hit it a few strokes with a fine brush hone
                    else cast rings
                     
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