Sealing a thread: PTFE tape, flax, gel or cord
Point by point
| Criterion | PTFE tape | Flax + paste | Anaerobic gel | Cord |
|---|---|---|---|---|
| Can back it off | no | a little | no | yes |
| Old rough thread | poorly | best of the four | fine | fine |
| Speed | fast | slow | fast | fast |
| Skill needed | low | high | low | low |
| Taking it apart later | easy | easy but messy | a spanner and some force; heat only on large threads | easy |
| Risk of splitting brass | low | high if overpacked | none | low |
| Cost per joint | pennies | pennies | medium: the bottle is dear, but you use very little | medium |
Direction of the wrap decides everything
Whichever sealant you choose, it goes on one way — the way you'll screw the part in:
Flax and cord follow the same rule: the wrap must tighten as you screw in, not peel.
How to choose: four cases
- A valve, gauge or meter has to end up facing a particular way — flax or cord: both tolerate a small turn back.
- New factory thread and the part can sit any way round — PTFE, 6–10 turns with the thread.
- An old riser thread with worn crests — flax and paste: it fills what's no longer there.
- A joint you want permanent and forgotten — anaerobic gel; it comes apart with a spanner and noticeable force, on large threads with heat.
What not to do with any of them
Common questions
PTFE tape or flax — which is more reliable?
Applied properly, both seal. Tape is faster on new factory threads, flax with paste copes better with old, worn ones. If the part will have to be turned to face the right way, use flax or sealing cord: PTFE tape scuffs up the moment a joint is backed off, and it leaks.
How many wraps of tape, and in which direction?
On 1/2–3/4″ threads, 6–10 wraps in the direction of the thread (closer to ten with thin household tape); on 1″ and larger, 10–12. Wrap so that screwing the part in tightens the wrap instead of bunching it up at the shoulder. On the male thread only, and only on clean threads.
Should I seal the thread of a union nut?
No. A flat gasket does the sealing there, and a wrap only stops the nut from seating squarely. Sealant goes on threads only — never on a flat gasket or a swivel nut.