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     Silicone sealants are definitely one of the inventions that has changed our lives for the better. Coming in all kinds of colors and packages, they are a must-have in any workshop due to their main quality of being able to seal and bond virtually anything.

     Since these sealants are so cheap and universal, they are often used by mechanics and craftsmen to "repair" oil leaks of hydraulic components, pumps and motors included. Unfortunately, sometimes it comes to a point where some handy-men get what I like to refer to as "silicone fever", which is a mental condition under which an "overhauler", instead of replacing old leaking elastomeric seals (o'rings and such) with new ones, solves the problem by applying silicone sealant (often in ridiculous quantities) under (and over) the said o'rings. In some rare and extremely acute cases people apply silicone sealant on new o'rings as well, "just in case" - they say...

    Does this solve the oil leak problem? Yes it does, most of the times.
    Is it cheap? Yes it is, as instead of buying new o'rings or (which can be even more expensive) pump/motor seal kits you buy a relatively inexpensive tube of sealant, which will be good for a whole lot of them pumps!
    Is it fast? Yes it is - the time saved is the time you'd have wasted looking for those new o'rings.
    Is it a good practice? No, most of the times it is still a CRAPPY PRACTICE!!!
   
Why? Because it can cause more problems than it can solve!

    The main reason why silicone sealants and o'rings shouldn't mix can be seen on the pictures to the left. Where you have o'rings you have oil passages, and where you have silicone - you have excess sealant, which after hardening can detach itself from its birthplace and easily block a control orifice or two. This is exactly what happened to this closed loop Rexroth pump, which, after the "fast silicone re-sealing", stopped giving flow in one direction when a small piece of hardened sealant blocked the servo-cylinder control orifice.

    Another good reason to avoid the above mentioned technique is the fact that people logically associate "crappy technique" with "crappy mechanic/workshop", so to an experienced eye such a "vandalized" pump/motor/valve can say a lot of bad things about the artist behind it!

     The use of a silicone sealant to solve the "leaking o'ring" problem of a hydraulic component should be restricted to "absolutely-no-other-solution-available" situations! And even in this case a high-quality (preferably permanently malleable) sealant in a reasonable quantity should be used.

      The only correct way to repair an oil leak caused by a worn or deteriorated o'ring is to replace  it with a new one! Fixing damaged o'rings with silicone sealant is like amending broken windows with transparent scotch tape - it does hold it in one piece and keeps the rain out, but it's definitely something you wouldn't like to have in your house!
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