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Owad Offline OP
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I purchased a CadDigger 628 over the weekend. I'm the third owner. The previous owner replaced the orignal engine, which should have been 5HP, with a 13 HP engine, and then tore the bucket apart digging with it. It's so over-powered, I need to use two hands to steady myself enough to make the small movements necessary to control it reasonably.

This is the first I've ever worked with hydraulics. I read a bit about using relief valves or restricting the flow. Would that be practical here? The seller told me the pump was original, and that it's 2.6 GPM, although I can't find any markings to confirm that.

I believe the proper thing to do would be to put a more appropriate-sized engine on it. I see that I can get a 6.5 HP Harbor Freight engine for $160. It already has a LoveJoy connector, so that part should be easy. Then I'd need to buy or make a bracket to mount the pump to the engine. The engine mount has a bolt hole circle diameter of 92 mm, which doesn't match with any NEMA size I can find. The pump only has two bolts. How can I find a bracket to connect these? If I need to make one, I do have a tiny DC welder, but not much experience with it.

I appreciate any advice.

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Last edited by Owad; 09/07/2022 02:19 PM.
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if you're not changing the pump, the engine swop will not make a difference in the way the machine operates,other than the point at which the engine stalls under load. RPM is RPM as the pump sees it,the only gain you will make is less fuel burned per hour with a smaller engine. I'd stay with the 13hp and look into relief valve settings by installing a couple pressure gauges in the various parts of the system so you can see whats going on..after some seat time in getting to know the machine you can sense when you have a hyd.circuit loaded up and when you should back off the valve or the relief valve cut in and dump pressure back to the tank.We have a few here that have built machines(myself included) and can offer advice,be sure to post photos of what you are talking about when you do..some machines out there have had pumps and valves replaced with other than design parts which adds complication to diagnosing whats wrong if a machine doesnt operate right..maybe we can help you.Welcome abourd..

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Thanks, bunkclimber. I didn't get the plans, so I'm not sure if I'm using the recommended pump. There's a photo of it attached to the original post. It's stamped with "635400271", which I think makes it a Bucher, possibly in the AP100 series.

Attached to this email is a photo of the valve body. I think that's a relief valve, in the first photo? Maybe even a relief valve on each...?

To measure pressure, I think I could attach a gauge to a quick connect, then remove the hose for one of the cylinders to test pressure. Is that reasonable, or do I want an in-line gauge that I leave attached while I'm running it?

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valve2.jpg (56.08 KB, 80 downloads)
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for starters I'd go with a tee in the pressure supply line to the valve,mount the gauge to that..use a 5000psi gauge to give yourself some gauge headroom when reading at high pressures. Surplus Center has it all in stock for the most part,they have an awesome selection of adapters and fittings.

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for starters I'd go with a tee in the pressure supply line to the valve from the pump,mount the gauge to that tee..use a 5000psi gauge to give yourself some gauge headroom when reading at high pressures. Surplus Center has it all in stock for the most part,they have an awesome selection of adapters and fittings.From the looks of it this is an open center system(flow from pump thru valve,back to tank with all valves in neutral) at 2500psi or lower. relief valves are only going to actuate when they reach 'cracking pressure' or the pressure where they will open and dump back to the tank,or in this case, the valve will bypass flow back to the return line,or hose that goes back to the tank. It sounds like you have too much flow in therms of GPM, or pump flow..which is maybe why some one put a 13hp engine on it, so they can reduce flow by only running the 13hp engine at 1/2 throttle, halving the flow vs. full throttle, but still have enough power to run the pump. Try running the hoe at half throttle and see if it's more manageable to operate.

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It seems a lot more reasonable at half throttle. And Surplus Center is a fantastic resource. Thanks for directing me to that! I will order a gauge and some fittings.

It doesn't seem like there's any way to relieve pressure in the system. To change a fitting, do I just gently back it off, under pressure?

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If the engine isn't running, there is no pressure.

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Though a catch pan is still a good idea.

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well you can have system pressure if an implement/arm/bucket is in the air or under load which will put pressure back on the system until you pull a valve handle to release it back into the tank thru the valve.Best thing to do is stretch out the hoe arm all the way onto the ground with bucket curled out..then with engine off,pull handles on all valves to release any stored pressure energy back to the tank.There's gonna be some leakage,its a hydraulics thing,you can't stop it all but you can catch the majority of it in a pan or 5-gallon bucket.

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I ordered a gauge, some fittings, and replacement hoses for two that showed damage. I also ordered some of the Protec Nylon Hose Sleeve. I think the hose routing is not that great. One short line had significant abrasions on the bottom from rubbing against the machine. And the longer hoses really sag. It seems like I should hang them from the arm, but I'm not sure how to do that without creating abrasion points. Is it best practice to have them tight against the arm? Looser, hanging down?

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u can use big black 1/2" size ty-raps/zipties/whatever you want to call them to hold the lines against the mast or dipper arm..Id use a section of auto heater hose,slit it along its length and push over the hose to create a rubber guard tube for the actual hose itself,then anchor it down..you can use conduit clips (Home Depot,Lowes,Menards,hardware store,etc) to hold the hoses against the frame.The bigger you go in diameter with the clips the heavier they get,you can bend them to fit hoses or pairs of hoses to hold them in place. The ProTec sleeve does a nice job,after you cut it use a cig lighter or matches to melt and seal the ends..i try to avoid matches becuase they get me in trouble..I find its way easier to go bigger with the protec sleeve size on long hose runs..the stuff is a good investment as todays sunlight breaks down hose rubber so much quicker these days.
I see you are two hours N of me in Carlisle,i use a good engine machine shop there on High St..UCF Machine Shop

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Even with my Case 580ck hoe,--- I only run the motor at a fast idle when I dig with it!! No need for wide open! just enough speed to pull/lift the bucket is all thats needed for our use.
The excavator I made runs at a fast idle and dont even know the hyd. is even on it! It's a 230 Chrysler industrial motor off an Oliver 525 combine, so you want to talk about overpowered!! LOL!!!!
My valve bank has flow controls built in so each cylinder runs at optimum power/speed from what I can tell. It digs fast and has enough power to drag a 10,000 pound dump truck with brakes locked across the yard. 8,000 pound farm tractor wont hold it back either!
In summary, --- engine power is not really the important thing here. Its the control of the hydraulic part of the systems. cylinders, motors, etc. as each one can be slowed to a speed that suits your needs. I like medium speed with as much power as I can get on mine that way you move a lot of dirt in a short time. Flow control is the key.
You can kinda test out what start speed you feel good with and go up from there ( if needed) after you run it and get the feel for things!


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Owad Offline OP
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I appreciate all the advice and I'm enjoying learning about hydraulics.

I noticed my hydraulic tank is lower than the pump. Is that normal? If I drain it, does it need primed?

I saw some gauges that can be installed on the tank, but they need to be bolted from the inside, and my tank is welded shut. Is that also normal?

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the tank is ok with being lower than the pump,it will draw ok with the height in the photo.The top of the tank is a bit higher than the centerline of the pump,you should be fine with it.Maybe shorten the suction hose if you can by moving the tank outlet to be more in line with the pump suction inlet and shortening the hose. I wouldnt bother with gauges for what you are running here,its not that complex of a system,simple open center valving and cylinders.If you are concerned,reach down and feel the tank when operating,if you cant touch it comfortably with your hand then you need a bigger tank with more volume or a fluid cooler to reduce the temps you are running..My rule of thumb always is (1gal of tank) for every 1GPM at 1000psi.Its always worked for me.I used that same formula for a no-talkie advanced research project at work,the tank came in at 270gallons..and worked like a champ.I wouldnt bother priming the inside now,it has fluid film all over it Im sure.Just keep it up to working level.

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My Mini-Hoe very rarely comes off an Idle. I keep it bumped up just enough for the governor to do it's job.


I know a lot about a lot of things BUT I still have a lot to learn.
Life is what you make of it. So, why not make a working machine to make it easier.
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In your picture you posted as "valve" The silver hex fitting opposite the inlet hole looks like the pressure relief. Try to take that cover off and see if there is a set screw in there that you can back off some. That will lower your max system pressure. I would recommend you have a gauge installed before you try to adjust it.

Like said above the engine is fine, just idle it down if you want to run slower. Force of things is only the pressure setting of that valve. Engine size has nothing to do with it as long as its not stalling out.


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I think that why the previous owner put a bigger engine on it to run at half throttle and not stall it


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