when to use teflon tape?

[Wood_workur]

Do you need to use Teflon tape on copper compression and npt joints? I've read a lot of contradictory information on the 'net, and wanted to know for sure.

[Salty]

Copper compression, or any type of compression fitting does not need any tape or other pipe thread dope. It will not help and may cause added problems. If the compression sleeve is new, it should seal properly. Do not try to reuse these sleeves. The chances of failure is high if the sleeve is reused.
Pipe threads need tape or some other thread compound. For plastic to metal I use a teflon brush-on or stick, 2 wraps of teflon tape and more dope. That way I do not have to tighten the fitting too much which can cause the plastic to crack. Also, be aware that some types of thread dope should not be used on plastic fittings.

[Wood_workur]

okay, thanks.

[Slik Geek]

A bit of a thread diversion (pun intended)

Pipe threads need tape or some other thread compound. For plastic to metal I use a teflon brush-on or stick, 2 wraps of teflon tape and more dope. That way I do not have to tighten the fitting too much which can cause the plastic to crack.

What is the definitive word on how many wraps of teflon tape to use on metal-to-metal joints? I always used a couple of turns, but a mechanical engineer "yelled" at me one time, and said only one wrap should be used.

I haven't found any authoritative source to debunk or confirm his statement.

[scorrpio]

On compression: no tape, no dope, nothing. Be sure the pipe end is reamed in and out, and clean. Any extra stuff in a compression joint will compromise it.

That mech. engineer yelling for one turn only should take a hike. Tape comes in so many weights that 'one turn' really makes no sense. The heavier the tape, the less you need - but I find that for yellow 'gas' tape sold in HD, two turns is good. Thinner white tape, need about 3. There is heavier tape sold at supply houses, I guess that one might only need one turn. I personally been following advice I once got: tape the threads, brush dope over tape, and then assemble. Never had leaks that way - be it air, gas, or water.

[Salty]

It is important to have the teflon tape wrap over top of itself so 1 turn is not enough. Besides, it isn't that expensive, except to an engineer. Be sure not to wrap it too close to the end as it will get threads of tape inside the fitting. If that comes loose it will clog up faucets.
Also, a tip, wrap the tape in the same direction the fitting will be threaded on. This will prevent the tape from unwrapping itself. That may be a tough one for left handed people!