I recently got an old Komatsu PC05 mini excavator with a two-pump open system. Someone added a thumb, but I don't think the plumbing is right. They capped off the metal 3/4" main return line from the valve bank and took power from that to the thumb valve, and from that valve back to the tank, all with 3/8" hose. Now all flow is through that hose; I don't like that restriction, but it works. The main problem is the thumb is as strong as the bucket - when you curl the bucket it kills the engine. I'm thinking there should be a relief valve in the thumb's upper cylinder hose so the thumb allows the bucket to curl. Comments? Ideas?
Thumb Plumbing excavator thumb
- Thread starter JerryCPP
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ptsg
Super Member
If I understand correctly, they used the return line to feed the thumb valve, there the main relief valve won't work properly since it can't dump the fluid directly to the tank. Basically you have no relief protection on the machine, that's why it kills the engine. Don't use the machine like that or you risk blowing the pumps.
Yes, that is how it's set up, and exactly my fear. I can remove the welded-on cap that's on the return line, and make the return like it was originally, but that would eliminate the thumb, which was the main reason I bought the thing.
dfkrug
Super Member
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- 05 Kioti CK30HST w/ Prairie Dog backhoe, XN08 mini-X
Welded on? Really?
Is your valve bank a sectional valve, or monoblock? If the former, you need to take off and repair/replace the last section in the stack with the welded-on cap. If you can find out the maker (or go to Komatsu), you should buy a replacement OUT section, and possibly a proper Power-Beyond section or PB plug/sleeve.
Whether you have either of those valve types, it is almost certainly cast steel. Welding of any kind onto that material is likely to be problematic for high pressure.
I am 2 months into this thing, and am learning far more than I ever wanted to. There are no manuals, schematics, or diagrams available, and that seems to go for parts too. It was built in the late '80's, and even Komatsu only recognizes the later variants, whose manuals show a different engine, controls, and hydraulic components, so I'm on my own. This has an 8-valve bank. The two pumps feed both ends of it, a common return comes out of the middle, and goes to the tank via a 3/4" steel pipe. This pipe was cut and capped a few inches from the tank. Thumb power is taken from that pipe about 2 feet from the welded end. The thumb valve is brand new, made in Japan, and hangs by itself on the side of the machine. I contacted the thumb valve manufacturer in Japan, and the only information they give is that "Yes, we have that valve in our warehouse. How many would you like to order?" No doubt something was lost in translation. The kicker is that Komatsu uses its own special hydraulic fittings; not JIC, SAE. NPT, or JIS, so tapping in is a bit of a challenge.
Apparently this excavator was found in a swamp, made to run, painted, and, with the thumb added as the bright shiny object, exported to the U.S. and put up for auction. I won the auction, lucky me! I've not yet figured out how to add a photo, but as soon as I do, I'll post some.
Apparently this excavator was found in a swamp, made to run, painted, and, with the thumb added as the bright shiny object, exported to the U.S. and put up for auction. I won the auction, lucky me! I've not yet figured out how to add a photo, but as soon as I do, I'll post some.
Takman
Bronze Member
I've had luck finding both parts and figuring out problems using the parts lookup from this site-> Запчасти Komatsu, бесплатный каталог, Цены online
Edit: I just checked and it does have pc05 diagrams. I couldn't find the anything about the thumb but all the control valve and line stuff is there to look at and it may help?
Edit: I just checked and it does have pc05 diagrams. I couldn't find the anything about the thumb but all the control valve and line stuff is there to look at and it may help?
Last edited:
old and tired
Veteran Member
It's been a while but I think (since that was your 5th post) you should be able to post photos now!!!
BertZegers
Gold Member
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- Kubota L2900, Zetor 8011, Kubota KX41Excavator, John Deere 4400 Combine, Case 1816C skidsteer
On my mini excavator I have put an Adjustable Relief Valve in the hydraulic line to the thumb cylinder. I did this to restrict the forces on the thumb/bucket bushings. I have adjusted the pressure so that it still (barely) holds what I am lifting. The bucket will push back the thumb. This setup would prevent the destructive dead heading on your machine.
To plumb it in: put the Relief Valve in series with thumb cylinder, than ad a hose from relief valve to tank. Cost about $100
dfkrug
Super Member
- Joined
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- 7,609
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- Santa Cruz Mtns, CA
- Tractor
- 05 Kioti CK30HST w/ Prairie Dog backhoe, XN08 mini-X
So, you thought your new mini-X was gonna be turn-key, and not a project? You bought it without a test drive?
Anyway, Komatsu does have their own fittings, but you can get them at larger hyd component sellers. If you are in the US, you might try DiscountHydraulicHose online. With your 2-pump system, you are gonna have a system relief valve for each, plus some work-port RVs (aka shock RVs) associated with some of your work circuits. Your machine is old enough that it may use gear pumps and an open-center system, or it could use HST pumps (variable-piston pumps) and closed center hydraulics. How much electronic or solenoid control you have is going to determine how much you can fix without schematics.
Sounds like a fun project. I recently saw a video on Utube of an excavator being rescued from abandonment in a dense forest. Maybe yours.
