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Kubota cylinder head repair update: Finally fixed it!

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#1 CrankshaftKaleb OFFLINE  

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Posted October 18, 2020 - 04:33 AM

I've been keeping busy, so busy that I had forgotten to post further updates on my garden tractor. I have finally got this thing running properly after being nearly ready to throw in the towel several times. If you haven't been following this from the start, you can see the previous steps in this earlier thread: https://gardentracto...with-a-cracked-



So, without further ado, I'll bring some closure and finish showing you how I managed to salvage the cracked cylinder head.




While I had the engine out, I decided to deck the block just in case the crack was caused by warping.




Sure enough, there were some low spots that became evident on the first cut. You can see them as the darker patches.




Here's what it looked like after shaving it flat on the shaper. Much better.




I had previously made two attempts to braze the crack in the head, both of which ended up cracking through the braze. Clearly, a braze repair wouldn't cut it; something I had been warned of after the first attempt.


I also suspected that the crack had gone further than I had realised at first. So I knocked out the swirl chamber insert, and sure enough, the crack extended down to the lower shoulder as you can see here. Thankfully it seemed to have stopped there. Further examination also revealed the crack had gone down the exhaust port as well.


I decided at this point that my only chance of saving this head was to attempt a weld repair.






The first thing to do was to mill away some metal around the crack to give the weld somewhere to fill in and penetrate the casting around it.


Note that you cannot use regular welding electrodes to weld cast iron like this. If you do try to weld it with regular electrodes meant for mild steel, the weld will absorb carbon from the cast iron while it's molten, and you're left with a weld that is so hard that it's virtually impossible to machine. This is because when the mild steel in the electrode absorbs that carbon, you're essentially left with tool steel; the same stuff a drill bit is made of! Cast iron welding electrodes are made of an alloy that contains a large portion of nickel, which somehow gets around this problem. (I'm not sure how or why, that's beyond what I know about metallurgy!)


An important thing to know is that the nickel alloys used in cast iron welding electrodes are very viscous and gummy when molten. This means it won't flow into small gaps very well, so proper weld preparation is very important.




Here's what the weld looked like in the rough. I filled in all around both the valve seat and the swirl chamber to make re-machining them easier.


As I mentioned earlier, the tractor is up and running, but don't worry, there's still some more progress pictures to share.



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#2 EricFromPa ONLINE  


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Posted October 18, 2020 - 08:39 AM

The big thing with cast iron is it's ability to transfer heat both to and from your weld Very quickly. If the cast iron part is to cool it will crack on either side of the weld as it cools because the heat generated from welding it will pull excess carbon into the iron on either side of your weld even if you use a non porous material such as NickelRod #55 or #99. 


It's recommended to maintain a working temperature of as close to 932°F or 500°C as you can get on heavy cast pieces of varying thicknesses such as heads. After your welding is done quickly wrap it in several layers of heat blanket or vermiculite and let it sit and cool very slowly. 


Something thin like exhaust manifolds you want to maintain heat of around 500F but you might want to heat it to 1150°F and slowly back it down to a maintained 500F for Stress relieving purposes before welding.

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#3 rgrinder OFFLINE  

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Posted October 18, 2020 - 11:21 AM

For anyone who might be interested, There is a company that sells welding rods for cast iron among other things www.muggyweld.com I have used some of their products with good results, they also have instructional videos and such on their site.

#4 Cvans OFFLINE  

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Posted October 18, 2020 - 12:58 PM

I'll add that if your going to use nickel rod make short welds and then peen the heck out of it. That reduces the stress on a cooling weld. After welding cast iron for many years I much prefer welding with nickel or brazing. Haven't tried the Muggy rods. Have a straight 8 engine with cracks in the water jacket and oil galley. Might have to give Muggy a try.