Note: I sent this to the list some time ago, and have been wondering why it did not appear on the list. I figured it out. I inadvertently sent it under Cindy's email address, so that it bounced back without being posted. I found it in her "Sent" folder, anyway. I also found a couple others, which will follow shortly. ============================================ If a leaking hydraulic cylinder has lasted a long time, then it was probably manufactured correctly. After doing over a million dollars worth of cylinder repairs in the last two decades, I'd estimate that applies to about five percent of all commercial off the shelf cylinders. The rest failed prematurely due to not being manufactured correctly at the factory. If you are happy with how long your cylinder has lasted before it begins to leak, you can perform a simple visual inspection of the cylinder. If it passes, then you should be able to slap a seal kit into it and make it as good as new. Rust or pitting on the rod means that you need to re-chrome or replace the rod. Any sign of scoring on any part means that you need to re-machine the part or replace it. This applies to the rod, head, piston, and barrel bore. Anything you cannot polish out with hand tools needs to be re-worked in a machine shop. If you are not happy with how well the cylinder has lasted, then you need to take it to a repair shop that you trust. Call up some of the larger heavy equipment rental shops and ask them where they have their cylinders done. Then talk to the shop manager and tell him that you want the job done correctly, and that you are willing to pay for it. The best fluid power specialist I know, a man who I trust above all others in the business, is also able to slap together a cheap job if that is what the customer is willing to pay for. The key to the repairs are the seals. I almost never go back with the original seals. Rather, I use whatever my fluid power specialist stocks on his shelves. He knows how to make those seals work: And more importantly, I know where I can find spare parts quickly. I let him make the seal selection. Then I look at the seal manufacturer's catalog to see what the dimensions and tolerances he calls out to make his seals work. The parts are then all checked to make sure they meet those requirements. Any that do not are remanufactured or made new. The single biggest cause of premature cylinder failure is an under sized piston or an over sized head bushing. Where the seal manufactures commonly call for a diametrical clearance of less than 0.010 inches, even the best cylinder manufacturers frequently give you five or six times that, which causes premature seal failure. An infrequently used cylinder will most often fail because the rod rusted. In that case, you can go with a corrosion resistant alloy, or a heavier chrome coating. The Navy uses base coatings of copper and nickel under the chrome. Ceramic or synthetic coatings are also now available that work well. I also saw some discussion on hydraulic fluids. The MIL SPEC fluids cost a bit more because of the documentation and quality control requirements put onto them. If you want a standard petroleum oil, then you can buy that just about anywhere. You might also ask your supplier about some of the new environmentally friendly fluids around. You might appreciate that if you ever have a spill overboard. Sea World has lots of hydraulic systems supporting their marine mammal tanks, and they use a fluid that won't hurt the whales if it leaks. I'm sure that Alex can give more details on that, so I won't try. The single most important thing about your fluid is that it be clean. Frankly, most oil you buy fresh in the drum is not clean. It would pay for you to either filter it yourself, or pay your local fluid power shop to filter it for you. In order to maintain the warranty on several of our mobile cranes, we are required to use the manufacturer's oil, which he sells for about three dollars per gallon more than our standard oil. I called him up and requested the spec's on it. He said it was AWS68 oil, filtered to three microns, and put into clean drums. I told him that I buy that bulk and can filter it myself. He had no problems with me doing that. Paul Kruse paulkruse@cfl.rr.com