Some things require no maintenance; you simply use them until they break or wear out and then fix them. Take for instance the three-point hydraulic cylinder in my B7200 Kubota. Last winter I noticed a hesitation to the three-point lift capability of my tractor with the snow blower, this season the hesitation turned into an almost refusal. It was time to replace the piston O-ring.
When removing pieces, indexing them will ensure proper reassembly
Components
Here are the components of a three-point hydraulic cylinder:
-cylinder head (covers the cylinder and has two rings, small and large in this case)
-cylinder (contains the piston)
-piston (has an O-ring and O-ring backer, which keeps the O-ring secure)
-push rod (fits inside the bottom of the piston and is connected to the lift arm cam)
-lift arm cam is connected to the external lift arms
-control valve is attached to the side of the cylinder casing and controls the direction and position of the lift arms with high pressure oil via an operator control handle
How it works
When the control valve handle is lifted up by the operator, oil flows into the cylinder on top of the piston pushing it down. The piston pushes against a push rod linked to the lift arm cam. On both ends of the lift arm cam are the external lift arms which lift the three-point hitch and load. When the control valve handle is pushed down it releases the oil back into the transmission, allowing the piston to rise and the arms to lower and float. This flow passes through the flow control rate valve which regulates the speed of the lowering motion. This system has no down pressure on the lift arms so pushing an implement such as a plow or auger into the ground is not possible.
Why it fails
What has happened here is that the large piston O-ring, normally rounded, like a straw, has gone flat. Eventually the neoprene O-ring wears and flattens on the cylinder contact point allowing the pressurized oil to ooze around it, causing the pressure to drop and the arms and load to lower.
Maintenance
With the indexed bracket off, the head is exposed (clean the immediate area to reduce contaminates from entering cylinder before removing head)
First up I remove the seat and seat rails. With these out of the way I clean the immediate work area, which is essential. No debris should get in the hydraulic assembly. Once the area is clean I ensure that the control valve arm is down so there is no pressure in the system, and then I remove two of the four head bolts and remove a bracket, then the remaining two bolts and the cylinder head exposing the piston.
As there is no wrist pin the piston floats, so by physically manipulating the lift arms up and
The head removed exposing the piston
down I was able to expose the piston enough that the O-ring was out of the cylinder. With the piston in this position I used a magnet to extract the piston out of the cylinder. I examined the piston and cylinder for signs of wear and scratches. None were evident, so I removed and installed a new O-ring by simply slipping it over the piston into its groove using a small pick and screwdriver, and being cautious not to scratch the piston. The piston is installed by coating it with transmission oil and compressing it with a piston ring compressor, then slipping it back into the
Extract piston with magnet
cylinder.
A piston ring compressor is a winding of thin-walled, heat-treated steel wrapped with a ratchet tensioner resembling a hose clamp. The idea here is that you place the loose compressor around the piston and tighten it until the O-ring compresses enough that the piston, with ring, will simply slip back into the cylinder without any binding. I have replaced O-rings that were installed by someone else using screw drivers. The key here is that screwdrivers are used for
Removing the old O-ring from the piston
screws; ring compressors are used for rings. Screw drivers are okay for removing the old ring but can puncture the new one, degrading it before use. My compressor is a ‘KD’ purchased 30 years ago from an automotive shop, at a reasonable price.
Once the piston was back in the cylinder I fitted the new cylinder head O-rings, installed the head and torqued the bolts to manufacturer’s specs. With the seat rails and the seat reinstalled I’m now good to go.
The piston ring compressor, piston and head with new rings
Being unfamiliar with this procedure I contacted Tom Callaghan of Callaghan Kubota, Lindsay, Ontario, the dealer I purchased it from, for any “heads up” to what I was getting into as the shop manual was a little vague. It’s nice to be able to contact your dealer after 14 years to bounce things off of, and it re-enforces my statement of getting to know your dealer. It took 28 years for this simple O-ring to finely fail with 1334 hours on the clock — I’d say it did a pretty good job.
Cost
When it comes to repairs I use original OME parts when possible; they fit and work properly. When ordering parts such as O-rings, be specific such as how many you need. In some cases these are shipped from the depot in packages of more that one and you may be expected to purchase the entire bag.
As for bulk packages from box stores, all O-rings are not the same. They may be the proper diameter but that doesn’t mean they are the proper thickness which can cause serious issues with fit and leakage. If the dealer doesn’t stock them or isn’t available, any credible hydraulic shop can fit you with the proper O-rings.
Total time start to finish, 3 hours. Cost for the replacement O-rings from North Shore Kubota Sault Ste Marie, about $8.00 taxes included. Sweet!
— Dan Kerr
Note When conducting repairs such as these it is a good idea to use a parts container so that the pieces removed along with fasteners are placed away from the work area and not on any available horizontal surface. Once the piston is removed it leaves an access to the bowels of your tractor and anything that is dropped into that void now has to be located and removed. No explanation needed here!