Fittings and Sealant?

[captain ron]

Should I use teflon tape on SAE fittings with the rubber o-ring? These are the fitting to and from my Hydraulic pump for my centrifuge setup. 

[Brassman]

No, the O-ring seals it. A general rule on hydraulic systems is to never use teflon tape, as bits of it can enter the system and clog ports, orfices, etc..

[H3Jim]

Not to mention that due to the design of the fitting, the teflon would not seal fittings like that anyway.  And when it is used, Teflon tape should not be wrapped too many times around the joint.  Normally no more than two layers are needed. However the important thing to remember is that the tape, or any other thread compound's, prime purpose is to reduce friction, so that the mating threads will make full contact. Filling any voids is a secondary function, so if the threads are old and rusty, there may be nothing that will prevent them from leaking.

[bobofthenorth]

People seem to have a lot of difficulty with fittings and tape and sealant but its really pretty simple.  There's two broad classes of fittings that we are likely to encounter:
1) two pieces of metal get pressed together either with or without some form of a gasket
2) interference fitting between tapered threads

If the threads aren't tapered then they exist to press the fitting together. Straight threads don't seal on the threads.  They need a gasket, an o-ring or smooth fitting surfaces that are then pressed together tightly by the action of the threads. 

On the other hand, if the threads are tapered then the taper serves to make the fitting get progressively tighter as you turn it.  In that case you should use tape or pipe dope.  Personally I detest tape, don't carry it and clean it off threads whenever I find it.  YMMV but my preference is an appropriate pipe dope.  I carry Tru-Blue because I have found that it works with everything we encounter on the bus and it doesn't make a complete mess when you use it but there's 100 different kinds of pipe dope out there.