Piston 2 or 3 bent or by design. d950

[awitty]

Background

I have and old Ransomes 213d highway mower that I use as a backup from time to time. It's powered by a Kubota d950 3 cylinder. I inherited the machine about 10 years ago and it's been fairly reliable albeit very hard to start and smoked like hell for the first 5 minutes of use.
This year it started smoking quite bad all the time (white smoke) and the engine sounded rough so we stopped using it.

I did a compression test on each cylinder and they all read approximately 150 psi. This doubled when I dropped a spoonful of oil on the cylinders so my assumption after much research (I'm a novice btw) that it was probably the rings.

So I pulled the head and that looked fine with no signs of obvious damage. The piston heads also looked in good shape although I did notice that piston 2 did not quite travel to TDC like 1 & 3.

I then proceeded to extract the pistons and immediately saw that piston 2 top ring was broken and the connecting rod bearing was also damaged (see second image)

What puzzled me was that the rod on piston 2 looked bent (see red arrow below) and i'm not sure if this is a fault or buy design? It certainly would explain why it didn't travel to TDC.

Piston's 1 & 3 are stamped with the letter ("I") and piston 2 is stamped with the letter ("M"). The bolts are also of a different specification which you can just about make out in the second image that shows the damaged bearing.

So is piston 2 shot or is this how it should be ? I can't see anything obvious in the WSM or parts manuals.




Pistons (3,2,1). Note The rods on 3 & 1 are straight - although the picture angle makes 1 look out.

Bearing damage to piston 2. Note the bolts are a different specification.

 

[lynnmor]

#2 is ready for the recycle bin, the others need inspection to see if they are usable. The entire engine needs to be inspected, especially the crankshaft and the cylinders, if you don't have the tools or expertize, see a local engine rebuilding shop.

 

[awitty]

#2 is ready for the recycle bin, the others need inspection to see if they are usable. The entire engine needs to be inspected, especially the crankshaft and the cylinders, if you don't have the tools or expertize, see a local engine rebuilding shop.

Thanks, although I do hope someone with specific experience of a d950 or similar with given their considered opinion.

From what I can see in the parts manual, piston 2 has a different part number of to 1 & 3. However they appears they share the same rod part number.

I wonder if a previous owner somehow fudged piston 2 using the wrong part. Anyway, here's hoping someone comes along who has done a similar job on a 950.

 

[North Idaho Wolfman]

Yes the piston is different on #2, but the rods are the same.
That's a bent rod.
Now check the head very carefully for a crack, it's usually between a valve and the pre-chamber.
How bad is the crank?
If it's too much you'll need a crank.

 

[lugbolt]

I did a compression test on each cylinder and they all read approximately 150 psi. This doubled when I dropped a spoonful of oil on the cylinders

Good liklihood that is what bent that rod. Replace all three. And inspect crankshaft. Replace the pistons, might as well bore/hone the block and get it back round again. Or you can halfway overhaul it with a set of rings and a rod and "hope for the best"--although I do not do things that way. Yes it costs a little initially to do it right but it rewards you later on with not having to re-do it again and again.

These little engines have only a few thousandths of an inch between the top of the piston and the bottom of the head at tdc. How many cc's is that? It ain't many! Maybe 10? The thickness of the gasket basically and that's being generous. Any liquid you add to it can cause hydraulic lock. starting fluid can cause it too from the huge spike in cylinder pressure--I have seen it dozens of times.

 

[awitty]

Thanks all for your advice.

I think piston 2 was already shot before I did the compression test as it didn't travel to TDC before my investigations.

My suspicion the previous owner couldn't find the correct piston 2 part, so used same spec piston as 1&3 but used a different size rod (I assume perhaps it had too much travel with the wrong piston and correct rod)

Anyway I don't want to spend too much money on it so I'm going to try a new rod and rings and hope for the best. Failing that I'll scrap it.

 

[North Idaho Wolfman]

Being that your in the UK it probably wouldn't be very economical for me to send you parts for it.

 

[awitty]

Being that your in the UK it probably wouldn't be very economical for me to send you parts for it.

I'd be tempted if you have the right parts !
Can't seem to find

Being that your in the UK it probably wouldn't be very economical for me to send you parts for it.

Probably not, but if you have the correct Piston 2 I might have no other option. Struggling to find one available on-line.

 

[Pau7220]

Before you throw parts at at.... what is the condition of that crankshaft journal on that #2 cylinder? Question has been asked multiple times. The bearing looks awful.

 

[North Idaho Wolfman]

Pistons are easy, there are whole kits available

Amazon.com

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Buy New Overhaul Kit STD with liner COMPATIBLE WITH Kubota D950: Engine Kit Gasket Sets - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

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Overhaul Rebuild Kit for Kubota D950 Engine B7200D B1750D B8200E F2100 Tractor

Overhaul Rebuild Kit for Kubota D950 Engine B7200D B1750D B8200E F2100 Tractor

www.fridayparts.com

But like others have stated if the crank is bad, that can be a real killer.
The crank is the same for a D650, D750, D850, D950.

I do believe original pistons had a different skirt on #2 piston, but I think Aftermarket pistons are all the same.

 

[lugbolt]

I'll add a little more, since we are on an engine kick.

When doing a rebuild on a Kubota diesel, Kubota gives a specification as to how far the piston is supposed to be down in, or out of the cylinder. Service manual has it. On some of the newer stuff, the pistons are out of the cylinder quite a bit, and the only thing keeping the pistons from hitting the head is the gasket. Some gaskets have different thicknesses, and basically the specify to use a certain thickness gasket with the distance the piston is in/out of the cylinder at tdc.

On one I did a while back, they specified 0.85-1.0mm out of the cylinder. It doesn't leave any room for error, but that particular engine also specified 600- 700 psi cylinder pressure, and it was less than one liter of displacement. That's why oil/starting fluid/whatever is not to be used. Almost always results in bent rod(s). I've done it myself.

 

[Henro]

So the moral of the story is the OP probably caused the bent rod by adding oil to the cylinders, in order to determine that the rings were the issue.

My question (as not even reaching novice status in engine repairs, and far from it actually) is: Is it likely to hydro lock a cylinder when doing a compression test if oil is added to the cylinder? My gut tells me the answer is yes.

My though was that the meter being used would be damaged first, but maybe there is not time for the oil that was added to the cylinder to move into the pressure test meter before hydro locking occurs?

My guess is that this is the reason, but do not know.

 

[awitty]

Before you throw parts at at.... what is the condition of that crankshaft journal on that #2 cylinder? Question has been asked multiple times. The bearing looks awful.

Yes the journal is far from perfect. Some scratching to the surface. I'm not really qualified to say how bad it is so i'll do my research

 

[awitty]

So the moral of the story is the OP probably caused the bent rod by adding oil to the cylinders, in order to determine that the rings were the issue.

My question (as not even reaching novice status in engine repairs, and far from it actually) is: Is it likely to hydro lock a cylinder when doing a compression test if oil is added to the cylinder? My gut tells me the answer is yes.

My though was that the meter being used would be damaged first, but maybe there is not time for the oil that was added to the cylinder to move into the pressure test meter before hydro locking occurs?

My guess is that this is the reason, but do not know.

Perhaps it was me that caused a hydro lock, but wouldn't i have seen a very high reading on my compression tester if the force was so strong enough to bend the rod. I think it only went up from 150 - 300 psi (or thereabouts) when i did the test.

lots of learning todo, but very enjoyable!

BTW. Any tips on how to remove a seized bearing from a rod ?

 

[Henro]

Perhaps it was me that caused a hydro lock, but wouldn't i have seen a very high reading on my compression tester if the force was so strong enough to bend the rod. I think it only went up from 150 - 300 psi (or thereabouts) when i did the test.

lots of learning todo, but very enjoyable!

BTW. Any tips on how to remove a seized bearing from a rod ?

That is a good question.

My GUESS would be that normally the gage reacts to a gas (as compared to a liquid or solid) passing through whatever the passage is between the cylinder and the Gage itself.

Perhaps the liquid oil added enough resistance to the flow through whatever size hole/tube is used, to effectively block it, so that increased pressure within the cylinder was not registered by the pressure gage you were using.

But let's wait until someone that really knows comments...

 

[GeoHorn]

It’s my belief the oil adds its’ viscosity to the cylinder (after all….it’s why it was added…in order to increase viscosity to seal up any leaking rings as a diagnostic to detect worn rings…)
However, regardless of worn rings or not…. it adds so much resistance (which means less compressibility) …. that the rod and piston are damaged before the starter-motor can energize the piston sufficiently to push that oil into the gauge (”meter”). I.E., the liquid cannot be compressed as air can… and that’s why it results in hydraulic-lock…. and damaged piston and rod.

 

[JerryMT]

That is a good question.

My GUESS would be that normally the gage reacts to a gas (as compared to a liquid or solid) passing through whatever the passage is between the cylinder and the Gage itself.

Perhaps the liquid oil added enough resistance to the flow through whatever size hole/tube is used, to effectively block it, so that increased pressure within the cylinder was not registered by the pressure gage you were using.

But let's wait until someone that really knows comments...

Can you explain what you mean by "...it adds so much resistance (which means less compressibility) …. " ?
Viscosity is a resistance to motion via shear occurring within the oil film. Engine are generally cranked over with the starter and some the torque required to do this is due to oil viscosity. SO that can't be an issue here.
Compressibility (or lack thereof) is indicated by d(pressure)/ d (volume) or bulk modulus. Liquids generally have a high bulk modulus so they are virtually incompressible. That's what causes the damage when a cylinder is hydro locked.

When checking the compression, only a small amount of oil is generally added and the gauge generally has a hose with it that adds to the air volume in the cylinder. Again, generally several cranks are required to record the peak pressure. Accepting that the pressure gauge is has some level of accuracy, if the person who did the compression test actually added so much oil as to damage the the rod, he would have read a very high pressure on the gauge, potentially damaging it. As it is, he read 150 psi IIRC. So it is doubtful that this testing cause the damage.

 

[Pau7220]

Yes the journal is far from perfect. Some scratching to the surface. I'm not really qualified to say how bad it is so i'll do my research

You need to have someone experienced in engine rebuilding asses the damage to it and determine if it is salvageable.

BTW. Any tips on how to remove a seized bearing from a rod ?

It may be permanently friction welded.
I have it ask... why? It too can become part of the next cruise ship anchor.

 

[whitetiger]

When checking the compression, only a small amount of oil is generally added and the gauge generally has a hose with it that adds to the air volume in the cylinder. Again, generally several cranks are required to record the peak pressure. Accepting that the pressure gauge is has some level of accuracy, if the person who did the compression test actually added so much oil as to damage the the rod, he would have read a very high pressure on the gauge, potentially damaging it.

When you add oil to the cylinders to seal the rings, you add a small amount to each cylinder and crank the engine over using the starter for 10 or 20 revolutions BEFORE you install the compression tester. To dump oil in the cylinder and connect the tester is just crazy and asking for trouble.

 

[Henro]

Can you explain what you mean by "...it adds so much resistance (which means less compressibility) …. " ?

I said "Perhaps..."

My thought was that the compression testing meter probably has a very small orifice for GAS (as in solid, liquid, gas) to pass through. Liquid being much more viscus than a gas, might offer enough resistance to flow that the orifice would for practical purposes be blocked, and with no place for the oil to go quickly, the cylinder could become hydro locked.