[AT] Kubota M5700 noise

Jason dejoodster at gmail.com
Wed Apr 8 13:03:02 PDT 2020


Unfortunately Kubota cast numbers aren't as solid of means to identify
parts as they are on other brands.

Jason

On Wed, Apr 8, 2020, 1:25 PM Spencer Yost <spencer at rdfarms.com> wrote:

> I have been able to read part/casting numbers on rods with my inspection
> scope.  If the oil drain bolt hole is close enough to #5 you might get the
> access  you need for looking around with a scope.   it can be tedious but
> usually can be done.  But if you’re luck is like mine that oil drain bolt
> will be on the wrong side of the engine :-).  Push comes to shove you can
> pull the pan for inspection.
>
> Long story but I actually had to do this once to see which one of three
> available rods I needed to buy to replace the one that was broken. One of
> the three was only available in overseas markets and took 2-3 months to get
> so wanted to start the ordering process if needed before I started the tear
> down.
>
> Spencer
>
> Sent from my iPhone
>
> On Apr 8, 2020, at 1:28 PM, Jim Becker <mr.jebecker at gmail.com> wrote:
>
> 
> 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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