[AT] Kubota M5700 noise

Wed Apr 8 10:28:05 PDT 2020

Compression ratio is of the swept volume plus combustion chamber volume divided by the chamber volume.  It looks like you used swept volume divided by chamber volume.

If I ignore the bore, which is just a multiplier on top of everything:

If stroke is S, C is the average height of the combustion chamber, and R is the compression ratio:
R = (S + C)/C
R = S/C + 1
R – 1 = S/C
(R – 1) * C = S
C = S/(R – 1)

C = 92/(23 – 1) = 92/22 = 4.18

A shorter rod increases C by the same amount to the new value C1.  Thus the new compression ratio R1 is:
R1 = S/C1 + 1
R1 = 92/(4.18 + 1) + 1 = 92/5.18 + 1 = 17.75 + 1 = 18.75 call it 18.8

Back to the original problem, it isn’t clear to me that it matters whether the ratio is 18.4 or 18.8.  It seems that is low enough to cause a misfire.  If there isn’t enough compression to get ignition, it seem unlikely the left over fuel from a misfire would cause preignition on the next cycle.  But who knows what all the behavior might be.

I can’t come up with any way to identify a mismatched connecting rod without disassembly.  Once the head is off, it should be easy to find by bringing each piston up to TDC and measuring the height relative to the deck of the block.

Jim Becker

From: Stephen Offiler 
Sent: Wednesday, April 08, 2020 4:18 AM
To: Antique Tractor Email Discussion Group 
Subject: Re: [AT] Kubota M5700 noise

I just worked out some math.  That engine is 87mm bore x 92mm stroke.  23:1 compression.  Swept volume is .547 liter, giving combustion chamber volume of .0238 liter.  Now, what-if the con rod was 1mm too short?  That adds .0059 liter to the combustion chamber, now .0297 liter, which drops compression down to 18.4:1 

SO

On Tue, Apr 7, 2020 at 7:02 PM Doug Tallman <dtallman at accnorwalk.com> wrote:

  The rod being too short can't change injector timing or cause pre-ignition. It would just mean less compression because the piston wouldn't come up as far.  Doug T

  On 4/6/2020 6:24 PM, Dave Maynard wrote:

    I know the piston is correct as we replaced it. But James makes a good point to ponder, if the rod is too short, it may change injector timing. 

    Dave

    On Mon, Apr 6, 2020, 6:03 PM Bo Hinch <bohinch at gmail.com> wrote:

      Or the wrong Piston ( too short or tall ) .

      On Mon, Apr 6, 2020 at 12:40 PM Dave Maynard <dave at themaplehillfarm.com> wrote:

        Actually, did not check length of connecting rod and its clearance from head at TDC. But it's not traveling to close to the head as there would be damage showing. Just have this new concern about the wrong rod being put in by mistake, but that would have been short, not long. 

        Dave

        On Mon, Apr 6, 2020, 1:01 PM Mike M <meulenms at gmx.com> wrote:

          When you had it apart, did you check all the piston,ring and bearing clearances?

          Mike M

          On 4/6/2020 11:35 AM, Dave Maynard wrote:

            No sleeves in this engine. Someone had this engine apart before and #5 had a new piston and rod and bearings. The seller said it was his father's tractor since new and never been apart....he lied! There was more RTV on oil pan than you could imagine and obviously that piston and rod and bearings had been replaced and I thought they had been chasing the noise. Unless there was some other reason, but dont know.  
                 My friend and mechanic help has a Jacobson mower he's is rebuilding that has a 4 cylinder version of this motor that appears identical except the number of cyl's. He found the rod to be the same except it a tweak shorter by an almost immeasurable amount, maybe .005 or .010. Part number is almost the same except last digit and now I'm wondering if that could be someone's mistake. Would shorter rod cause preignition? By the way, there is no part number on the rods so they would have to be accurately measured.
                 It's going to be a bit before I can get this back apart again, but hoping to have this fixed in time to plow.

            Dave

            On Mon, Apr 6, 2020, 9:08 AM Cecil Bearden <crbearden at copper.net> wrote:

              That was one of my thoughts, but my experience has been that the liner movement was a double "thud" once on down stroke and once on upstroke.  
              Cecil

              On 4/6/2020 1:12 AM, k7jdj at aol.com wrote:

                Cylinder liner loose.  Don't know if this engine even has cylinder liners if it does and that liner is moving you would get click. Did you look carefully at the head when you had it off and look for any sign of head distress?
                I would do a compression check as well.

                Gary

                Renton, WA.

                -----Original Message-----
                From: Roger Moffat mailto:rogerkiwi at gmail.com
                To: Antique Tractor Email Discussion Group mailto:at at lists.antique-tractor.com
                Sent: Sun, Apr 5, 2020 7:43 pm
                Subject: Re: [AT] Kubota M5700 noise

                  On Apr 5, 2020, at 9:59 PM, Dave Maynard <dave at themaplehillfarm.com> wrote:

                  Have cracked injector lines one at a time, and it gets quieter on #5

                But only on #5?

                When you crack the others the engine misses, but the noise remains?

                I wonder if something about cylinder 5 is somehow different so that the compression is higher, and so it’s igniting (pre-igniting) too much before top dead centre in just that cylinder?

                Eg #5 piston slightly higher than the other 4?

                Roger
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