1. trigger_andy

    trigger_andy Well-Known Member

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    Hey all,

    Im in the process of buying a '68 340 but its got a knock and out of curiosity I wonder what the main culprit could be.

    It was rebuild 7 years ago by a very well respected Mopar Engine builder/racer here in the UK. Its now .40th over and the only real details I have is it had KB 243 Pistons.

    Im told the knock is from the bottom end, and he's describing it as not a loud knock that fades to a ticking when warm. There is very little 340's in the UK so its worth buying, but some feedback from you guys would be most appreciated as to the cause and what I could be looking at to rectify. :)

    Cheers all.
     
  2. nm9stheham

    nm9stheham Well-Known Member

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    If it is a heavy, low pitched knock, then likely a rod bearing, possibly a piston pin. If a high pitched knock, then likely a lifter. Both fixable.... just takes 'Pounds'. (No more Euro's for you guys, right?!)

    >040" overbore may not safely allow any more overboring. Might.. might not. You can't know until it is torn down and the cylinder walls sonic checked.
     
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    • simman900

      simman900 Active Member

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      I would probably pull engine and pan to investigate the issue if that’s possible after ruling out a lifter problem. Not what you want to hear I’m sure. Could be an oil pump giving out. I’ve had that happen.
       
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      • RustyRatRod

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

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        Know way to know without hearing it.
         
      • simman900

        simman900 Active Member

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        Very true. What’s your guess?
         
      • Murray

        Murray FABO Gold Member FABO Gold Member

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        Good advice. The other type of knock to listen for is a "hollow" sounding knock which would indicate a loose piston. In most cases the engine needs to come down (except for lifter problem). Since the engine is running and seem to lessen when it warms up, you have probably caught it before it has done too much damage. I would be in a hurry to tear it down.
         
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        • Nat

          Nat Well-Known Member

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          you have a sound clip ? Knocking from the bottom end that fade to a tick ? you sure about the pistons it has , that they are not some old forged pistons ? Best to strip it down see what you are getting
           
        • Dana67Dart

          Dana67Dart Like a fine wine, only getting better with age! FABO Gold Member

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          If you have time you might be able to have the oil analyzed.
           
        • trigger_andy

          trigger_andy Well-Known Member

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          OK cheers. :) I'll not know till I get it set up on an Engine Stand or stripped down. Would be awesome of it was just the lifters, but I'll not hold my breath about that. I doubt he would have pulled the engine for the sake of replacing the lifters.
           
        • trigger_andy

          trigger_andy Well-Known Member

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          Yes, when I get the 340 home and get it on the Stand and have a look. Im not buying the car, just the engine.

          Oil Pump would be amazing too!

          If it was bearings or piston pin would that quieten down once warm?
           
        • trigger_andy

          trigger_andy Well-Known Member

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          Im buying the engine thats already lying in the garage. So no way of getting a sound clip. I'll get it set up on a stand and investigate when i get it. In about 4 weeks time.
           
        • trigger_andy

          trigger_andy Well-Known Member

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          Thanks,

          what would that tell me?
           
        • RustyRatRod

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

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          I don't know. I cannot hear it.
           
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          • Nat

            Nat Well-Known Member

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            Good luck hope it is not a big expensive problem , 340 is a great small block
             
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            • trigger_andy

              trigger_andy Well-Known Member

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              I hope so. Its had a full rebuild will good components and still turns over easily.

              Comp Cam,
              New Melling Oil Pump,
              Eagle I Beam Rods,
              Cloyes Timing set,
              KB Pistons etc etc.

              This was all about 7 years ago though. I have a 340 in my Aussie Charger, but its a 1973 and needs a full rebuild as its blowing blue smoke. Its on a Standard Bore, maybe I could make a good 340 outta the two of them? Obviously the Cranks are gonna be different, so if its the big end it could be an issue.

              Im itching to go collect it but Im working in Norway just now working.
               
            • SLOPAR72

              SLOPAR72 Well-Known Member

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              As someone who just got popped on a used motor you need to take it down and if necessary take someone with you that will help you. Otherwsie, walk away...... The sound could be a whole bunch of things. You were warned.....

              JW
               
            • AJ/FormS

              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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              Knock-knock.
              Who's there?
              Lucy..
              Lucy-who?
              inappropriate Lucy-ness.

              My question is; who pulls an engine for a slight knock that goes away, and stuffs it in a corner.

              Maybe the knocking is just piston-slap.........
               
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              • trigger_andy

                trigger_andy Well-Known Member

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                Im guessing Piston Slap is not an easy fix? New Sleeve maybe? If its only one Piston?

                He bought this to replace it. Would be nice to think its been pulled as he wanted more HP and the slight knock was all the excuse he needed? :D But I'm fully prepared for it to be seriously damaged. Its a gamble Im willing to take. At least its got good X Heads on it if nothing else.

                100870129_196681091464937_5751439831909531648_n.jpg
                 
              • AJ/FormS

                AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                If it is piston-slap,
                and it really does go away after warm-up,
                then it is probably nothing to worry about.
                Some factory-type 340 pistons experience a lot of growth as the heat gets into them, and so they have to be installed looser than what we might consider normal.
                Piston slap is a different sound than a rod knock, which is different from loose piston pins , or even a fuel-pump arm slapping on the eccentric. Get an audio clip up, and someone will help you. Or get an old timer to listen to it.
                If the engine is not in the car, you have to work at it as a process of elimination. Eliminate the pump, then the wrist-pins, then the rods, and then you are left with only loose pistons. If the heads are off, it's easier. If the pan is off, even easier.
                Loose-fitting pistons, of themselves, at start up are not a big deal.

                But; having said that; KB 243s are hypers and can be installed very tight, even for HD use. So I would be surprised if they were slapping. So then it is likely in your best interest to at least take the pan off and go hunting. I agree with all those that have mentioned it before me.

                Also; with the engine in the car and running, it can make knocking sounds originating NOT inside the engine. Some other sources are; the flex-plate, headers, manifolds, transmission rear mount.

                Post edited, hit refresh
                 
                Last edited: May 31, 2020
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                • trigger_andy

                  trigger_andy Well-Known Member

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                  How do I go about this? I'll get the engine back to mine and get it on the engine stand.
                   
                • cudamark

                  cudamark FABO Gold Member FABO Gold Member

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                  If it was still in running condition, you could check a few things to narrow it down, such as pulling individual spark plug wires to maybe isolate the knock to one particular cylinder. I assume they checked the oil pressure when it was fully warm and it was not below specs? Bearing knocks don't normally get better when the engine oil warms and thins out. There are all sorts of knocks that have different sounds and rhythms. Since it's already out and ready to be torn down, just do it and inspect every part. You should be able to find the answer at that point.
                   
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                  • trigger_andy

                    trigger_andy Well-Known Member

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                    Im buying it pulled and on the deck. The seller installed the new engine last year and he said he's now needing the space and the cash.
                     
                  • AJ/FormS

                    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                    Drain the oil, drop the pan, then let it drip overnight.
                    While yur waiting; pull the sparkplugs, remove the fuelpump and make sure the returnspring is on the arm, and working normally. Check the arm and the eccentric for unusual wear.
                    Next day; flip the engine upside down. Find a piston that is half way to the top and the crank is at about 90*to the cylinder axis. Put a bar on the balancer, and turn the crank back and forth a couple of degrees each way. The bearing clearance spec is about .002/.003; So the rod should follow the crank immediately with no noise. Repeat until all eight have passed the test. If you find a loose one, pop the cap for an inspection. Start with the drivers side front two which are the two I have seen go out first..
                    Simultaneously while doing that. Listen for a wrist pin going tappa-tappa on the switch over from CCW to CW.

                    As the piston skirts come to the bottom of the bore, rock them in the bores, first in line with the wrist pins; there should be very little movement; and NO rocking that does not simultaneously involve the rod.You may have to jam the rods apart to immobilize them.

                    Then at 90*, in the thrust direction. With those KB243s, there should be very little rocking; skirt clearance on those is .0015 to .0020 for NA street use. If you can get a .004 feeler or more, in there; expect noise.
                    I put my KB107s (same style piston, built for a 360) in a lil loose, and first start on cold-mornings, I can occasionally hear them.

                    See what you can find.
                     
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                    • fastnos

                      fastnos Well-Known Member

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                      Personally, if it turns over easy, and there was no known water in the oil, or oil in the water, holees in block, or parts sticking out, I would probably buy it if it was reasonably priced. Always loved the 340's! I know its hard to track down some problems, prior to dis-assembly, but a good look over and no major problems catch your eyes, Id buy it!
                      It takes same amount to rebuild a small block 273, 318, 340, or 360 really, as long as no big problems to start. Also, get some sort of (written) guarantee before purchase, for any concerns you may have. Saves both parties stress, after the fact...
                       
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                      • trigger_andy

                        trigger_andy Well-Known Member

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                        There was people lining up to buy the engine so I put a 25% deposit down on it. 340's are rare in the UK, pre-73's even more scarce. I'm paying $1250 as is. it still turns over smoothly. Its a gamble but one Im willing to take. If there is nothing much wrong then Ive got a $6000 engine (In the UK at least) for $1250 and if its scrap then Im sure there is bits i can salvage for my '73 340.
                         
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