View Full Version : ZD28F HST Pump Question - Please help

03-12-2011, 06:36 PM
Greetings from a newbie.
I have a used ZD28F will 1800 hours on it. The left HST pump would get weak once the oil got warm. I have it apart and found the backing plate (see pics below) for the pump motor is scored. There appears to be no other defect. Since the part [K7021-39503 020 ASSY COVER PORTBLOCK] is $477+ shipping, I would like to have the face professionally reground. Will this work? It will thin the casting by about 6 thousandths (0.006) of an inch but I can see no reason why this won't work. Also, The cylinder that caused the scoring appears to be fine. However, it must have worn some.

As I study teies parts, I don't see where the worn plate will cause the leak by but I can find no other problem. I have the WSM manual but the english translation leaves a lot to be desired.

Any help will be greatly appreciated

03-13-2011, 09:35 AM
No problem machining that plate smooth again. It must be very flat so be sure the machinist knows his stuff! You must also lap the pump barrels flat on the valve surface, even if they look good they still need to be lapped. Even a very small scratch will leak a LOT of oil at 3500 psi!

03-13-2011, 10:02 AM
Hey eserv!
Thanks for the reply. When you say "pump barrels" are you referring to the cylinders that the pistons ride in? I think they call them cylinder blocks in the parts manuals.

Also, I'm not sure what you mean by the "valve surface". (where they spin against the plate)?

Is the piston cylinder block actually the valve? There are rises on the face that rides on the plate. However, I don't see anyway for the valleys to seal against anything. Maybe that happens within the piston cylinder itself?

I'm confused. :eek:

03-13-2011, 11:26 AM
Cylinder blocks, pump barrels, rotating group etc. all mean the same thing. The valve action is effected by the interface between the cylinder blocks and the plate. the "valleys" are there to provide a lubrication path, but more important to "balance" the hydraulic force so the cylinder blocks aren't hydraulically pushed from the plate.

03-13-2011, 11:44 AM

Thanks very much. Having this ground is a whole lot cheaper than a new HST pump at $1500 or the new plate at $477.

Thanks again.

Oh one last question. When you say lapping you are referring to lapping the barrel to the plate correct?


03-13-2011, 01:29 PM
No, Do not lap the barrel to the plate! lap it on a lapping plate or a lapping machine!

03-13-2011, 06:35 PM
Got it! Thanks eserv!

03-13-2011, 07:46 PM
Dave, standard procedure for this situation is to replace the hst pump. I know that sucks, but is was Kubota's answer.

Along with replacing the pump, we install an overflow tank similar to the cooling system on an automobile (btw, the designer of that system is a good customer of mine and is mega rich from the royalties. haha) underneath the RH fender. Please do this or you will be right back where you are now.

Please see your local dealer for the specifics. I assure you they are familiar with what I am speaking of.

03-14-2011, 06:54 AM
Of course standard procedure is to replace the unit, very few dealers will work on a hydrostatic unit, guess they are afraid they will let the magic out!. They ARE very rebuildable though. And with above average care and cleanliness can be made good as new for a fraction of the cost of replacement. The fluid reservoir bottle is a must though.

03-14-2011, 09:11 AM
You guys are awesome! I have spoken with the two local dealers and none of them know anything about the add on kit. Are there part numbers I can reference?

03-14-2011, 05:21 PM
You guys are awesome! I have spoken with the two local dealers and none of them know anything about the add on kit. Are there part numbers I can reference?

Dave, at one time it was an L-series radiator overflow tank that was used, but I believe they have gone to a metal tank now. First chance I get, I'll look up the bulletin and give you some numbers.

ETA: GOT IT! The part number for the reserve tank kit is K3111-98600. According to the bulletin, this is everything you need to install the kit and it does include instructions.

03-16-2011, 03:03 PM
Latest update. I was able to get the guys at work to do the gringing. I have to prep the plate first Apparently, the misc parts i.e. relief valves, allen cap screws etc, are all put in with red Lock-Tite. If I use a propane torch to warm the plate will I do any damage to any parts or should I just break them free cold?

The first barrel lapped up beautifully.

03-16-2011, 04:37 PM
I break them loose cold but a little heat won't hurt, especially seeing you are resurfacing it anyway

03-17-2011, 02:22 PM
I tried to break them free cold but almost stripped the one in the face. Got them all out with a little heat... but not too much.

Now for the grinding. I'm hoping for the weekend but we'll see.

When they lapped the first cylinder, they ended up removing about .010" as compared to the other one before it was lapped. Since the cylinders ride the backing plate and are forced against it by the piston pressure on the swash plates, I don't need to concern myself with a differential in the cylinders (it will be less that .010" after the other one is lapped) or compensate anywhere for the new smaller size do I?

Anyway, I called on the kit and got pricing anywhere from $250 to $261 plus tax. Wow, that's a lot of cabbage for a tank, hose and fittings. My dealer's parts guy is looking into the parts they fabbed up before the kit came out. He thinks its about $40 that way. With all the other miscellaneous parts I need, I think I'm going to save what I can here. I'll post what parts the guy reccommends.

When I get my parts back, I'll post pics to help others.

Thanks again for all the input. Expecially you eserve as you gave me the confidence to move ahead.

More to follow,


03-22-2011, 02:18 PM
Finally getting back to the project. I've been working 12 hr shifts on nights!

Anyway, here are the pics. I'm waiting for the new parts to come in so I can put this beast back together.


This is a shot of the two rotors in the pump housing. The one on the left is the variable displacement pump controlled by the speed control lever. The one on the right is the fixed displacement motor.


A little bit closer look. The blueish color is residual oil. I hadn't clean it up yet.


Here is the backing plate. You can see the scoring. The pump rotor scoring was about .006" deep. The Motor scoring could barely be felt with your fingernail.


Anothe shot from a different angle of the plate.


This is the pump rotor showing the face that rides on the plate. Lapping removed about .010" from this piece.


This is the other side of the same rotor. Nothing had to be done to this side.


This is the outside of the plate. The machinist found out the plate was actually warped and had to "dust off" this side to ensure a good grinding on the other side.


Here is the final shot of the reground plate.

05-06-2011, 05:28 PM
Finally, just in time for spring mowing. It's back together and running extremely well.

I do have another question though. There is a 1 3/4" plug (looks like a soft plug) on the back end of the housing for the injector pump. For some reason, this blew out. I have covered the opening unitl I can get a new plug but I was wondering if anyone knew:
A- what is this unused connection for?
B- what would cause the plug to blow out?

05-06-2011, 06:23 PM
In many applications of that engine they fit a hydraulic pump there. The hydraulic lift on the ZD machines runs off the hydrostat charge pump. I have no idea why the plug would fall out though!

05-06-2011, 06:26 PM
By the way, glad to see you got it working good! wasn't that hard was it? You ARE lucky to have access to that quality of machine work though!

05-06-2011, 08:32 PM
Hey Thanks Ed.

Yes I am lucky. I just have to be sure not to abuse the privilege. I work at a nuclear power plant and the shops are set up extremely well as you can imagine.

Actually you are quite right. It is a rather straight forward repair and I would reccommend anyone that is somewhat mechanical give it a shot. If it doesn't work out, the only thing lost is some time.

Thanks for the info on the pump as well. You and all the other regulars are awesome! I try to contribute when I can but often by the time I get to the bottom of a thread I can contribute to, someone else has already made my point. The good thing about that is I know I'm in good company!

Thanks again.