Thread sealing issue brass to plastic

[ivar]

Hi all,

Have an issue: have some water filter housings that have (unless memory fails me) 1 1/2" straight female threads cut into what I believe is polyamide and I'm using brass reducers to connect copper pipe w/compression fittings.
The issue is the small leaks/sweating between the filter housing and the brass reducer. WP is max 4 bar, potable water.

So far I've been using Loctite 55 to seal any other metal to-metal straight threads with decent success, but not with these filter housings. Wound the Loctite 55 cord as many times as recommended, in the recommended pattern, still leaks. Tried other patterns - including "filling" the threads first, still miniscule leaks. First time I used Loctite 55 what started as leaks with a couple of drops per hour developed to maybe 100-200 drops per hour in a year's time, I assume the water "washed out" the compound out of the sealing cord. Tried PTFE tape - no good.
I am guessing the issue may be too loose tolerances and the fact that the compound in Loctite 55 is not formulated to harden - as opposed to other products that would.

Would be thankful for any tips on how to solve this and what to use.

 

[breplum]

We rarely see straight threads on equipment...standard are tapered threads.
So, don't expect much help from us here in the USA.
I've seen chatter on forums about "wrapping" threads for Euro adapters, but, frankly, I would never touch any of it since I haven't encountered such oddities in 50 years in the trades.
With tapered threads, we use three wraps of teflon and for good measure, a dab of joint sealing compound and done. On plastic housings, we always have unions plus shut-offs on any portion to allow for swap out or tightening.

 

[GReynolds929]

Straight threads don't self seal, there needs to be a gasket or washer to make the seal.

 

[ivar]

Straight threads don't self seal, there needs to be a gasket or washer to make the seal.

They do seal with stuff like Loctite 55 og "ye olden" hemp with compound, but of course much worse than tapered threads. I have perhaps 5-8 other straight thread connections in the same house (copper to copper or copper to brass) and they do seal with zero leaks. Problem here are the loose tolerances. The reducer does not have a surface for a gasket, unfortunately.

 

[RickFlorida]

Not sure if you can get this but this Gasoila thread sealant. This stuff seems to seal anything for me. If this doesn't work, nothing will and you will need to replace the fittings. It sounds like you have straight threads? So if you have a O-ring kit and can install a big O-ring and use commercial grade sealant like Gasoila, only that will keep you from having to buy new fittings I assume.