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Discussion Starter · #1 ·
Title says it all. Various components in my system have plastic fittings, and others have metal. From some research I've learned that only female metal fittings should be combined with male plastic, and it's still not ideal.

I've read some van build websites/resources that are very adamant to NOT use plumbers tape/pipe dope on plastic fittings. Apparently it increases the risk of over tightening. Questions:

1. Is this just with plastic-plastic fittings? What about metal-plastic: okay to use plumbers tape?

2. Plastic tank fitting connections. I've read that plastic fitting connections should only be made hand tight + 1/2 turn with a wrench. On my trionic water tank with 1 1/2" female NPT fittings, the male 1 1/2" fitting for my fill doesn't go in that far-lots of threads are still showing. It's solid and definitely 'in'...but should I tighten it all the way down with a wrench? The hand tight + 1/2" wrench seems like not that much. On these connections, should I be using plumbers tape?

If anyone out there has some definitive information about thread sealants on plastic/plastic and plastic/metal and correct tightness of plastic fittings it would be much appreciated!

Thanks.
 

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2017 159, w/dual sliders. SF Bay area
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I always figured those softer plastics deformed a bit to get a good seal with little torque. PVC OTOH needs Teflon tape.




Follow the manufacturer's instructions. Don't over tighten anything. If it doesn't leak, don't mess with it.
 

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Definitely use Teflon tape. It’s so easy to split open or crack the plastic fitting screwing steel into it.

I hate when I have to mix metal with PVC.
 

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Discussion Starter · #4 ·
Definitely use Teflon tape. It’s so easy to split open or crack the plastic fitting screwing steel into it.

I hate when I have to mix metal with PVC.
I agree. What I've read is to completely avoid plastic female - metal male. It's better to instead use a nipple or other adapter to end up with a plastic male.
 

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Discussion Starter · #5 ·
Follow the manufacturer's instructions. Don't over tighten anything. If it doesn't leak, don't mess with it.
Wish all of the parts I have had manufacturer instructions! I don't think the Shurflo documentation indicates how to seal fittings, for example.
 

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2015 Promaster 3500 159 Ext gas silver
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Wish all of the parts I have had manufacturer instructions! I don't think the Shurflo documentation indicates how to seal fittings, for example.
I had the same question when installing the filter on my Shurflo pump, as both threads were plastic. I called Shurflo and they said to just screw the filter on to the pump inlet with no tape or sealant. I asked about torque and they said just hand tight. That is what I did and have had no leakage. I also asked how to keep a downward orientation of the filter bowl and he said the orientation did not matter. I used SS braided plumbing connection lines and they seal against a built in gasket.
 
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Teflon tape is meant for metal plumbing tapered threads, the slightly conical style that normal tightening crashes//crushes the metal fittings together. So, the teflon prevents softer metals like brass and mild iron threads from galling, similar to cold-welding, before adequate leverage can be applied to squeeze/pinch the joining together mechanically - then tape shreds have also been pushed into and filled various gaps along the thread so clogs leak paths.

Teflon tape itself has little or no rebound once mated, does not adhere to surfaces being joined - meaning if any loosening occurs voids ares no longer filled. Plus, teflon tape that was designed for extreme pressure on metal to metal threads will just merrily be an anti-friction costing on plastic fittings so they'll loosen at any excuse or vibration to relax a little bit.

Non-hardening thread sealant is for plastic fittings... no teflon paste, tape or pipe dope even if the labels invite you to purchase it - not looking to lubricate it, just a compound to stick to the thread surfaces and conform to any voids & gaps.
 
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