Pistons Broke on Kubta K-008

[Routy56]

As the title suggests all 3 pistons have broken on this machine.
Bit like me at times "pissed 'n' broke"
So the story is that my brother Graham bought this as a none runner project.
It's actually a nice older little micro digger and should 'go again' with some fettling

As delivered on a pallet using a tele handler

Body work looks tidy...

He's stripped it all ready .....

D722 engine block

Three fcuked up pistons....

This photos shows that the 'land' between the rings has broken away and then the rings have broken up.
All three pistons are like this

The clock is showing just over 3000hrs.
The bores are like new with negligible wear ride and honing marks still visible.
No evidence of old loss or overheating.
It simply would not start - no surprise there then Sherlock!
Graham simply plans to replace the pistons (on order) and go from there.

NOW, the big QUESTION for my learned friends on this Forum is:
"What is the usual cause of such a piston failure?"
Perhaps some of you have seen this before

I hope that he can get it running OK, then I'll borrow it for small stuff when I can't get mine on site

 

[Lancs Lad]

As the title suggests all 3 pistons have broken on this machine.
Bit like me at times "pissed 'n' broke"
So the story is that my brother Graham bought this as a none runner project.
It's actually a nice older little micro digger and should 'go again' with some fettling

View attachment 40939
As delivered on a pallet using a tele handler

View attachment 40940
Body work looks tidy...View attachment 40941
He's stripped it all ready .....

View attachment 40942
D722 engine block

View attachment 40943
Three fcuked up pistons....

View attachment 40944
This photos shows that the 'land' between the rings has broken away and then the rings have broken up.
All three pistons are like this

The clock is showing just over 3000hrs.
The bores are like new with negligible wear ride and honing marks still visible.
No evidence of old loss or overheating.
It simply would not start - no surprise there then Sherlock!
Graham simply plans to replace the pistons (on order) and go from there.

NOW, the big QUESTION for my learned friends on this Forum is:
"What is the usual cause of such a piston failure?"
Perhaps some of you have seen this before

I hope that he can get it running OK, then I'll borrow it for small stuff when I can't get mine on site

That's a proper back garden

 

[Routy56]

That's a proper back garden

Yep he has loads of stuff-I keep saying to move it on
Just don’t let our Gra @V8Druid see the stuff. He’ll want buy it all and put some Tish on it

 

[jd6820]

I know this might sound daft but I'd price a complete motor. Rarely have I found it more economical to rebuild small Kubota engines like that. All the parts are extortionate. Be sure to check the cylinder head for cracks between the valves and condition in general.

 

[doobin]

I know this might sound daft but I'd price a complete motor. Rarely have I found it more economical to rebuild small Kubota engines like that. All the parts are extortionate. Be sure to check the cylinder head for cracks between the valves and condition in general.

D722 are not that expensive secondhand, so long as you can see them running.

 

[Powerfab]

This might well be the case of somone being liberal with "Ezy-Start" and causing predetonation.

Interestingly the soot marking above the missing land on the piston would suggest there had been detonation outwards from behind / below.
If its been on its side, as these micro's often are, has diesel seeped into the bores and caused detonation from below?

Seems like it was caught on in good time. The bores look to be in remarkably good condition from these angles.

 

[Shovelhands]

I know this might sound daft but I'd price a complete motor. Rarely have I found it more economical to rebuild small Kubota engines like that. All the parts are extortionate. Be sure to check the cylinder head for cracks between the valves and condition in general.

My thoughts exactly, that little engine could easily become a money pit.

 

[Thomas7740]

I know this might sound daft but I'd price a complete motor. Rarely have I found it more economical to rebuild small Kubota engines like that. All the parts are extortionate. Be sure to check the cylinder head for cracks between the valves and condition in general.

Best thing imo was cheaper to buy a brand new engine for our EX30 than rebuild the original one

 

[Routy56]

This might well be the case of somone being liberal with "Ezy-Start" and causing predetonation.

Interestingly the soot marking above the missing land on the piston would suggest there had been detonation outwards from behind / below.
If its been on its side, as these micro's often are, has diesel seeped into the bores and caused detonation from below?

Seems like it was caught on in good time. The bores look to be in remarkably good condition from these angles.

Thanks for your answer to THE question
Yep - I think you are spot on with this theory.

My brother Graham is a 'time served mechanic' (before they called them technicians) and said:
"Interesting comments. . Glad they like my garden. . Fingers crossed it won’t become a money pit. Tell ‘em I was brought up to repair things before the advent of changing everything for new. If all goes to plan it will have cost me £107!! Can’t grumble at that eh!"

Then after you mentioned about the "Start Yer Bastard" ether stuff:
"Yes I saw that comment. That’s what I thought may have caused it. Some mechanics or operators use too much. That makes ‘em knock like a bugger!!"

Then in relation to WHY it needed Easy Start:
"It’s got new glow plugs but I haven’t yet checked that they operate electrically. It could be a faulty relay stopping them working. I’ve got the old ones and they all were ok. It’ll be part of my checks when I assemble it. Indirect injection engines do not like starting without preheat."

Like yer theory about being on it's side. That may account for it. Wonder if anyone has seen pistons like this before?
So there you have the story so far. An interesting investigation with the fix in sight
BTW the 'new' pistons turned up today

 

[Routy56]

I know this might sound daft but I'd price a complete motor. Rarely have I found it more economical to rebuild small Kubota engines like that. All the parts are extortionate. Be sure to check the cylinder head for cracks between the valves and condition in general.

In Graham's case he is semi-retired and loves REPAIRING things
If him and the machine had to go out every day earning loads of money like the rest of us mere mortals then a replacement would make sense
Thanks for the tip about the cylinder head. When I was there yesterday it looked OK. The CHG arrived in the post - tiny thing

 

[Routy56]

My thoughts exactly, that little engine could easily become a money pit.

Agree with that James, if you have a technician rebuilding it and replacing everything in sight
But with our Graham, he likes to assess and repair where possible.
He's very tight with his money
So far he has spent the grand total of £107 including pistons and CHG......

 

[Shovelhands]

Agree with that James, if you have a technician rebuilding it and replacing everything in sight
But with our Graham, he likes to assess and repair where possible.
He's very tight with his money
So far he has spent the grand total of £107 including pistons and CHG......

that’s the end of it then, but if it needs more parts then things soon get out of hand with Kubota written on parts boxes.

Are they genuine parts he’s bought?

 

[Routy56]

that’s the end of it then, but if it needs more parts then things soon get out of hand with Kubota written on parts boxes.

Are they genuine parts he’s bought?

Graham said - Yes. Genuine headgasket and genuine 2nd hand pistons….. What can go wrong?

 

[V8Druid]

Graham said - Yes. Genuine headgasket and genuine 2nd hand pistons….. What can go wrong?

please tell me he's bought them a ring set ??

 

[Shovelhands]

Good luck….

 

[Mick-the-fitter]

As the title suggests all 3 pistons have broken on this machine.
Bit like me at times "pissed 'n' broke"
So the story is that my brother Graham bought this as a none runner project.
It's actually a nice older little micro digger and should 'go again' with some fettling

View attachment 40939
As delivered on a pallet using a tele handler

View attachment 40940
Body work looks tidy...View attachment 40941
He's stripped it all ready .....

View attachment 40942
D722 engine block

View attachment 40943
Three fcuked up pistons....

View attachment 40944
This photos shows that the 'land' between the rings has broken away and then the rings have broken up.
All three pistons are like this

The clock is showing just over 3000hrs.
The bores are like new with negligible wear ride and honing marks still visible.
No evidence of old loss or overheating.
It simply would not start - no surprise there then Sherlock!
Graham simply plans to replace the pistons (on order) and go from there.

NOW, the big QUESTION for my learned friends on this Forum is:
"What is the usual cause of such a piston failure?"
Perhaps some of you have seen this before

I hope that he can get it running OK, then I'll borrow it for small stuff when I can't get mine on site

In the trade it’s called giving it a sniff!

 

[V8Druid]

In the trade it’s called giving it a sniff!

you forgot the ....ter

 

[Gecko]

Just a thought, but is it the original engine? (if not, it could have a lot more than 3k on it).

Can you feel any ridge at the top of the ring travel?
Is it just the top ring or both that's broken?
The bores look good, but my thoughts are that if its had a new set of rings and they've hit a ridge, then broken top ring and damaged piston is exactly what you'd expect.

 

[V8Druid]

Just a thought, but is it the original engine? (if not, it could have a lot more than 3k on it).

Can you feel any ridge at the top of the ring travel?
Is it just the top ring or both that's broken?
The bores look good, but my thoughts are that if its had a new set of rings and they've hit a ridge, then broken top ring and damaged piston is exactly what you'd expect.
View attachment 40988

would be a very plausible explanation for why all three are the same ... looks to be a fair step in those bores TBF

 

[LKSF]

I have a lot of experience with det' in petrol (turbo) engines, but not diesels at all. I would say it is due to some kind of shock load in the combustion chamber. You can tell that by the angle of the breakage to the left and right of the piece. Whether it was by detonation (an explosion in the combustion chamber) or hydraulic lock I can't tell from here. In a petrol engine det' leaves tell-tale signs on the piston crown and combustion chamber (squish pads).
Q. Are the breakages all in the same place on the pistons? Det' usually favours the same place, in petrol engines it's usually on the inlet side or to the side of this.

You will really (100%) need a hone, new rings, check for bore wear and the ridge at the top.

Check the 'rods for straightness, the big end and little end bearings for wear and ovality.