What is best practice to connect a 3/4" female-to-male thread

8 Jun 2017
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United Kingdom
I am fitting a 15mm x 3/4'' Compression Female Iron Coupling adapter:


The 15mm compression end is easy, but what's the best practice for fitting the 3/4" female-to-male thread? There is no compression feature, just interlocking threads. Should I use a 3/4" washer? Just wrap with a lot of PTFE? Jointing paste?

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1. If the male thread is a taper thread, wrapping with PTFE tape (in the direction of tightening) should suffice. If its a loose fit (will go on all the way with very little effort using only the fingers) I'd start with 15 wraps of tape. If its a bit tighter, start with 10 wraps.
2. If the male thread is parallel take the same approach, but roughen up the male thread with a file and use Loctite 55 instead of PTFE.
3. In the unlikely event there is a clean square shoulder at the base of the male thread, and the female fitting screws on far enough, you could use a fibre washer between the two.

There are other ways of doing it, using Loctite 577, Fernox LS1, Slictite, but I don't use them so can't comment.

Of the three outlined above, my preference would be for number 3, if the fittings permit is. Whatever you do, test the joint while it is still accessible.

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