# Kubota ZD21 hydro rebuild



## fatjay (Dec 6, 2013)

I lost reverse, so I'm rebuilding the hydro. I think I've isolated the issue, one of hte surfaces that the rotating thing sits on is completely worn, and one of the rotating things is worn. Problem is I don't see P/N's for them on messicks, or the kubota site.

Anyone that can help, it'd be appreciated.


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## sixbales (May 18, 2011)

Howdy fatjay,

Wait till you see the bill for those pieces.....It's best if you are sitting down.....


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## fatjay (Dec 6, 2013)

I picked up the mower for 500$ originally only for the Diesel engine, but it was complete and only thing wrong was the left side didn't reverse. It's in great shape, so I figured I could see what it takes to fix it.


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## sixbales (May 18, 2011)

The ZD21 has a bad reputation for transmission problems. 

I bought a Kubota G1900 riding mower (new) years ago, diesel engine and hydrostat virtually indestructable. So what happens?? The frame broke in two at 1700 hrs. I welded it back together (didn't get it back together quite right). I use it as a spray rig today.


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## fatjay (Dec 6, 2013)

sixbales said:


> The ZD21 has a bad reputation for transmission problems.
> 
> I bought a Kubota G1900 riding mower (new) years ago, diesel engine and hydrostat virtually indestructable. So what happens?? The frame broke in two at 1700 hrs. I welded it back together (didn't get it back together quite right). I use it as a spray rig today.


The hydro reservoir isn't large enough, so if mowing in steep hills they suck air and get wrecked. There is an add on reservoir that fixes the problem for 200 which I'll be adding as soon as I find the part that I need to rebuild. I just can't find it.


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## rhino (Jan 9, 2017)

The only way to get parts for it is to buy the entire assembly. Unless you know some body that has one with a blown engine in it or they wrecked it and you can buy it for parts.


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## sixbales (May 18, 2011)

Here is a salvage yard that has a ZD21 in salvage. Their comments.....Good engine, weak HST. The parts you need may be OK??

West Kentucky Tractor Parts
Kevil, Kentucky
Phone: (888) 349-6320


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## fatjay (Dec 6, 2013)

rhino said:


> The only way to get parts for it is to buy the entire assembly. Unless you know some body that has one with a blown engine in it or they wrecked it and you can buy it for parts.


Messicks has each individual piece but doesn't have a part number for the cover plate.


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## fatjay (Dec 6, 2013)

sixbales said:


> Here is a salvage yard that has a ZD21 in salvage. Their comments.....Good engine, weak HST. The parts you need may be OK??
> 
> West Kentucky Tractor Parts
> Kevil, Kentucky
> Phone: (888) 349-6320


Thanks, I emailed them. It's a bit of a distance but if they will part it out the pieces don't weigh to much and can be mailed.


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## rhino (Jan 9, 2017)

That's why you have to buy the assembly it is the only way you can get the plate and housing. No part number just the letter "A" next to them.


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## RC Wells (Dec 26, 2008)

Kubota issued a recall on those transmissions for a problematic pump that allowed low pressure and excessive wear. They also extended the warranty to four years.

Even though your machine falls well outside the warranty, talk to your dealer's service manager about an out of warranty repair in the event the pump was never replaced. Kubota is known to do just that on those transmissions.


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## fatjay (Dec 6, 2013)

I got all the parts cleaned up. The pistons/barrels had perfect clearances/tollerances. I had a few pistons from the other barrel and used them, and the other ones, and couldn't tell the difference. Put it together with a new gasket. Went together perfectly. Added hydro fluid slowly turrning the lower barrel so it wouldn't be started dry. I drained the trans and changed the filter, filled up with 303 to the top. Hooked it all up and ran it for about 5 seconds. Checked the level, it dropped but not to much. Filled to the top again. Started again, left the cover off and watched the flow. Ran for 2-3 minutes. Tested forward and reverse, fully functional. No leaks, too.

Tomorrow I'll make a brief few passes, then drain the system and flush with travelers premium. I hope everything stays working well. I cleaned it surgically and reassembled everything with great care. If it works well, I'll buy the add on expansion tank. Until then I'll monitor the fluid closely. I'll run it a bit more tomorrow at low rpm to properly mate the surfaces.


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## fatjay (Dec 6, 2013)

One oil change from the premium, went to mow a around the yard and circulate the fluid real good, then did another fluid change. Took it over to my fathers to mow a bit. He took it for a spin, not bad for a first time operator. Ran perfect, just a couple things need adjusting. The throttle cable needs lubed, grease points greased, the deck wheels need adjusted. Nothing major though.

One thing has to change, the R4 tires need to go. Tight turns dig in and ruin the lawn.


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## fatjay (Dec 6, 2013)

For anyone that has this problem, the cover is K7021-39503 and the gasket is # k7021-39040, both are available from numerous dealers or online sites.


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