Step 8: Safety overload and check valves
There are two metal plugs in the body of the bottle jack unit. Two large screws are below them. See the text boxes again. This site warns NEVER EVER to open these. It also has a diagram of what is inside. (Scroll down to the middle of the web page.) The author's concern is that the small balls in the valves can be lost, and the jack would become useless. Get a shallow cardboard box with no holes in the bottom or a large pan and work inside of either one. If any balls roll away, they will be contained inside the box or the pan. Also, extra balls are included in my kit. Even if the balls were not included in the kit, precision steel balls can be purchased at a bicycle shop in a series of sizes. Check the link in this paragraph for the sizes normally used. The ball sizes in my jack are: 5/16 inch (7.94mm), 7/32 inch (5.55mm), and 5/32 inch (3.96mm). I measured them with a caliper through the plastic parts bag. I want to do as complete a rebuild as possible. Dirt may have found its way into the passageways where the balls are. The balls could also have rough surfaces through years of use.
If I turn the jack body back and forth I can hear metal balls rolling inside passageways. I drilled a hole in the center of each of the metal plugs. Then I inserted a slightly larger sheet metal screw into the hole until the threads bound against the hole I drilled. I placed a pair of pliers under the head of the screw and pounded against the pliers with a hammer to pull the metal plug out of the jack's body. I repeated the process with the other plug. New plugs are included in the parts kit.
The second photo shows the bottle jack unit's body, but inverted so it was easier to hold while operating the camera. The metal plugs have been removed. Both holes have a large screw inside them. The one on the right is recessed so far that it is not visible. It is the safety overload valve. This valve protects the jack's seals from failing under a load heavier than the jack's rating. When the safe range of the jack is exceeded, the safety overload valve opens like a pressure regulator to allow oil to return to the tank rather than entering the chamber for the ram. This screw has to be set so the safe level of pressure is not exceeded. In order to do that at home, I carefully turned this screw and counted by half-turns until it bottomed out. My screw was set to 1 3/4 turns above or looser than the bottoming out point. When it is time for reassembly, I will turn the screw gently until it bottoms out, then I will back it off 1 3/4 turns. The safety overload valve should then be set very close to the original factory calibration. One author noted that some jacks fail because the safety overload screw unscrews itself, which sets the jack's lifting ability to a much lower threshold, and the arm may not lift what you want to jack. I found this screw turned with enough resistance that it is not likely to shift its position by itself. That same author also said most safety overload screws are about two turns looser than the bottoming out point.
The third photo shows the parts for the safety overload valve in the order in which they are inserted. A new ball is included in the parts kit.
The fourth photo shows another special tool I made. The screw for the check valves is quite tight. I tried the largest screwdriver I had (3/8 inch wide blade) with a wrench on its square shank. The blade on the screwdriver broke! The screw slot is 1/2 inch across the diameter of the screw and almost 1/8 inch wide. I bought a short bolt 5/8 inch in diameter. It is #8 on the hardness scale. Near the end I ground the diameter down until it fit nicely inside the recess for the screw. I kept a cup of cold water near my grinding wheel to avoid softening the bolt with heat. I ground a rough profile by sight. I moved the bolt to a vise and finished cutting the profile of the screw slot by means of a hand file. I checked the dimensions with a digital caliper. When my improvised screwdriver fit the screw and its slot, I tapped on the bolt's head to be certain it had fully seated in the slot. I used a wrench on the bolt head and the screw came out with no difficulty, at all. I had tried to buy a large screwdriver, but could find none this large. This improvised solution cost me $1.65 for the bolt and a few minutes of time.
The fifth photo shows what was behind the check valve screw. See the text boxes for ball sizes.