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Repairing a Kubota D600 engine with a cracked head

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#1 CrankshaftKaleb OFFLINE  

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Posted July 27, 2020 - 06:07 AM

Been busy fixing problems on my Kubota G5200H. I decided that just throwing a sealant like K-Seal or Chem-i-weld into the cooling system and hoping for the best was only going to delay the inevitable and possibly make things worse. So I decided to bite the bullet and take the engine apart and address the root causes of all the problems, starting with the cylinder head.
And things did not look good as there was a crack through the exhaust valve seat on the No.2 cylinder. I also found a blown head gasket, which may have been a knock-on effect of a cracked head. The parts man at the local Kubota dealer quoted me around $1200 for a new cylinder head, and two specialist engine rebuilders told me the old one was not worth getting repaired. So with the head basically stuffed anyway, I decided that I would try a braze repair on it since I had nothing to lose at this point really.
First I used a fine pointed burr in a die grinder to cut a notch along the crack to give the filler metal somewhere to get into and hopefully penetrate the crack at least a little bit with the aid of a liberal amount of flux.
Here's what it looked like after brazing. 
Next I set the head up in my milling machine to re-cut the seat. I wasn't able to get a "proper" valve seat cutter, so I improvised by using a large V-bit meant for a router to do this.
Using a ball-nosed end mill to remove excess braze from around the seat
And here's the result prior to being lapped to its valve
I decided to skim the seating faces of all the valves in my smaller lathe before lapping them in to make sure I would get as much contact area between each valve and its seat as I could.
Here's a before and after comparison.
Lapping in one of the valves.
At first, I tried re-assembling the engine after draw-filing the deck surface and replacing the blown composite head gasket with a copper one I made, and still had issues. I decided that a full strip-down of the engine was in order, which I will cover in the next instalment.

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#2 sharkman OFFLINE  

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Posted July 27, 2020 - 06:51 AM


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#3 chieffan OFFLINE  

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Posted July 27, 2020 - 11:38 AM

Several years ago I bought a Kubota with the d600 engine with a cracked ??  Was getting water in the oil.  Put in 1/2 bottle of K-Seal and run it for a couple hours, drainer the oil and water out, refilled and put the rest of the K-Seal in with antifreeze.  It had a 6' deck so put about 10-12 hours on it mowing.  No issues.  It went down the road and a year later it was still running OK.  Buyer was mowing cemeteries with it.  Some times you win, some times you don't.

#4 EricFromPa ONLINE  



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Posted July 27, 2020 - 11:41 AM

If that doesn't fix it here is a good used head. Not cheap but it's been gone through. https://www.ebay.com...h4AAOSwMuhenkRR

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#5 CrankshaftKaleb OFFLINE  

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Posted July 31, 2020 - 09:10 PM

After repairing the crack in the head, the engine ran a lot better and was able to handle mowing. However the coolant started finding its way into the sump again, and the engine was overheating after a while. Pulled the head again, and sure enough, it had cracked through the braze repair. So I notched the crack out deeper and repaired it. By dumb luck (I discovered this when notching it out) the crack had only gone as far as the valve seat and had penetrated only about 4mm (just over 5/32") into the casting.


I came to the conclusion that the new crack was caused by the deck surface not being flat enough, having been the result of bending forces being applied when I torqued up the head bolts.




So I set the head up in my old metal shaper to deck it. I found a bit of a dip in it (about 0.1-0.15mm from memory) when I was using a dial indicator to check that I had it square to the machine. I suspect it had warped when it was overheating. I used the shaper because I haven't trammed my mill properly yet.





Here's what it looked like after decking.


When I stuck the head back on and started filling the radiator, the coolant got into the sump and sent grey stuff pouring out from around the pushrods! Pulled the head and tried adding water and the same thing happened. This leak was not in the head, but somewhere in the block. Which meant that this engine had to come out and be stripped down to find the problem.





Rigging in place.





Engine removed successfully





On the bench ready for the teardown.








Ah ha! Failed welch plugs. The previous owner said he'd replaced them, but he'd only replaced the ones on the sides of the block! This is why you need to be a bit suspicious when a private seller claims that a vehicle or machine has been worked on. For some reason Kubota was (are they still?) using steel welch plugs rather than brass ones. I have no idea why they did that; cost cutting perhaps?





This was an easy fix, I just knocked the old ones out and stuck in some brass ones, which won't rust out and should hopefully last the life of the engine. I've since replaced all the remaining steel welch plugs with brass after having another one let go in the cylinder head.


That's all for now. I actually did this work a couple of weeks ago but was preoccupied with actually getting my hands dirty; not to mention having no internet for a week a while back while we were being upgraded to a faster connection.



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#6 Cvans ONLINE  

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Posted July 31, 2020 - 10:46 PM

Thanks for the follow up. 

#7 jms180 OFFLINE  



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Posted August 01, 2020 - 08:32 AM

good work thanks for the post update