RTV Silicone on Threads?? - A more advanced discussion than just no

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Sid

Member
Jan 5, 2015
49
Colchester Connecticut
Of course we all know that Teflon (PTFE) tape and pipe thread sealant (pipe dope) is to be used when properly connecting tapering style pipe threads, such as NPT. The jury is still out (and it is up for discussion) as which is better or if using both is best. (I personally use both PTFE tape and Blue Monster with PTFE on my threaded connections as advised by someone I trust).

When looking into it further, the fact is both types do not 'seal' but in fact just provide lubricant to the threads to allow for more tightening. It is the force and 'crushing' of the tapered threads that actually do the sealing.

The above statements should be generally agreed upon I hope, but now on to my dilemma....

I have an Attack Profi 45 DPX that requires the use/installation of a Watts STS20 thermal safety drain for warranty purposes.

http://www.wattsindustries.com/images1/10/PDF/thermal_safety.pdf

The STS20 has UNI-ISO228/1 - G3/4” female threads which I then need to connect to 1/2" NPT to the boiler and house pressure (2-8 bar (50 PSI for me)) cold water line.

Upon more research, I find that this is a 3/4" BSPP (British Standard Parallel Pipe). I brought it to the local supply house and they gave me a 3/4" brass bushing that threads all the way in easily and bottoms out. They said I should be fine with just putting some PTFE tape on it.

I know the right thing is to find a correct adapter but it actually seems to be pretty specialized and be more of a hassle than worth if....

1. I can just put some good RTV silicone (gasket maker) on the threads, tighten and see if it leaks. Not a big deal if it does, easy shut-off and fix anytime since it is not really part of the system's normal operation.

2. Try a couple of extra wraps of PTFE tape like guys at supply house said.

3. Turn down the 'roughness' of the flat side of casting and then use a copper crush washer (like the BSPP would originally have).

What do you guys think???
 

Clarkbug

Minister of Fire
Dec 20, 2010
1,216
Upstate NY
My Varm came with adaptors to go from straight to NPT. You want them. I also used a sealant thread to get them to not leak.

It was loctite 55, and I'm not sure if you can still get it, but it worked great. My installer recommended it for all of the threads, but especially for straight threads.
 

Bob Rohr

Minister of Fire
Jan 9, 2008
1,265
SW Missouri
Of course we all know that Teflon (PTFE) tape and pipe thread sealant (pipe dope) is to be used when properly connecting tapering style pipe threads, such as NPT. The jury is still out (and it is up for discussion) as which is better or if using both is best. (I personally use both PTFE tape and Blue Monster with PTFE on my threaded connections as advised by someone I trust).

When looking into it further, the fact is both types do not 'seal' but in fact just provide lubricant to the threads to allow for more tightening. It is the force and 'crushing' of the tapered threads that actually do the sealing.

The above statements should be generally agreed upon I hope, but now on to my dilemma....

I have an Attack Profi 45 DPX that requires the use/installation of a Watts STS20 thermal safety drain for warranty purposes.

http://www.wattsindustries.com/images1/10/PDF/thermal_safety.pdf

The STS20 has UNI-ISO228/1 - G3/4” female threads which I then need to connect to 1/2" NPT to the boiler and house pressure (2-8 bar (50 PSI for me)) cold water line.

Upon more research, I find that this is a 3/4" BSPP (British Standard Parallel Pipe). I brought it to the local supply house and they gave me a 3/4" brass bushing that threads all the way in easily and bottoms out. They said I should be fine with just putting some PTFE tape on it.

I know the right thing is to find a correct adapter but it actually seems to be pretty specialized and be more of a hassle than worth if....

1. I can just put some good RTV silicone (gasket maker) on the threads, tighten and see if it leaks. Not a big deal if it does, easy shut-off and fix anytime since it is not really part of the system's normal operation.

2. Try a couple of extra wraps of PTFE tape like guys at supply house said.

3. Turn down the 'roughness' of the flat side of casting and then use a copper crush washer (like the BSPP would originally have).

What do you guys think???

We use one of these Loctite products for all the straight thread connections at Caleffi. Brass to brass, or brass to steel. Connections must be clean.

If you try to install BSP male into a NPT female, you really need to expand the fip a bit or you only get a 2 or 3 thread grab.

Male BSP into female NPT works fine.

Or a nut and tailpiece with a flat gasket if you want the union function.

Or hemp if you want to be a real Euro fitter.
 

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easternbob

Member
Nov 29, 2007
228
Central NY
The hvac guy who helped me with my system always uses silicone on all threaded joints. Haven't had any problems with leaks.
Can't speak to the difference in the thread types.
 

maple1

Minister of Fire
Sep 15, 2011
10,783
Nova Scotia
I also had adaptors come with my Varm, but not quite enough for all the fittings I used on it.

The ones I didn't use adaptors on, I just loaded up with tape & dope and tightened real hard with a pipe wrench. (Well, that's what I did with all my threaded fittings). No leaks so far. They were smaller holes, like 1/2" or 3/4". I tried to find some of the Loctite stuff around here but didn't see it anywhere.

Guess that doesn't help with the RTV question....
 
Back in the '80's when I was doing a lot of solar, I used silicone to seal all threaded fittings. It worked especially well when we made counterflow pool heat exchangers that mated PVC to copper or brass fittings. They never leaked. I have not used it in a long time, but will have to try it here in the shop.
It certainly lubricated the threads. The one down side I can guess at is that if the threads have to come apart, they might not reseal as well unless all the original silicone material is removed. Silicone does not bond well to itself, once cured. This might not be a big deal.
 

Sid

Member
Jan 5, 2015
49
Colchester Connecticut
I also had adaptors come with my Varm, but not quite enough for all the fittings I used on it.

The ones I didn't use adaptors on, I just loaded up with tape & dope and tightened real hard with a pipe wrench. (Well, that's what I did with all my threaded fittings). No leaks so far. They were smaller holes, like 1/2" or 3/4". I tried to find some of the Loctite stuff around here but didn't see it anywhere.

Guess that doesn't help with the RTV question....
Thanks Maple1, it does help actually to know that someone successfully did it with tape and dope. Not only do I already have those in my basement but that is probably the easiest to try first and clean off if doesn't work.

Were the ones you did it on British parallel (straight) threads or British (Standard) Taper?
 

maple1

Minister of Fire
Sep 15, 2011
10,783
Nova Scotia
I think parallel.

Others can chime in but I think the practice of threading NPT pipes into parallel boiler fittings using lots of tape & dope isn't uncommon. Most 'Euro' boilers were made like that. I think my dealer may have told me that's what most people do anyway.
 

Karl_northwind

Minister of Fire
Feb 13, 2012
529
Central Wi.
I have occasionally had to (on panel radiators) thread brass MPT fittings into the Female BSP threads, and with 5-6 wraps of the blue monster tape, and some rector seal #5, have not had any problems. unless some weird materials are involved (IE toms PVC to brass and such) I would stay away from silicone and use either the locktite products above or good old tape and dope. if nothing else, the tape and dope are easy to remove to re-do. I work a lot with antifreeze, which loves to leak, at high temps and pressures (200F + and 50 PSI) and that combination works well.
on the sealing method, I have disassembled quite a few of my own joints and there is definitely a layer of the blue tape on the threads filling voids, gaps etc. FWIW.

karl
 
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DaveBP

Minister of Fire
May 25, 2008
1,157
SW Maine
I ran a 1-1/4" NPT tap into the parallel threaded supply and return fittings on my Tarm to get at least a few threads (4or 5) to match the taper of the NPT nipples of my piping. Just seemed like I should do that since I have access to the taps and dies.

After having leaks in various places in my system (with all manner of assorted fittings, foreign and domestic) I started over with Loctite 55 cord. Looks like oversized dental floss with some kind of (teflon?) goo on it. Not a single leak since.
 

hartkem

Member
Jan 24, 2012
249
KC
I also had problems sealing some joints and had very good luck with the locktite 55 sealing string. Just wrap the string around the threads in a crisscross pattern and thread together and tighten. Works really good especially on larger fittings
 

Bob Rohr

Minister of Fire
Jan 9, 2008
1,265
SW Missouri
Loctite 55 string is a new world replacement for hemp:) Hemp is still very commonly used by European fitters.

Here is a pic I took in a hardware store in Italy.

Many fitting manufacturers still cut "hemp" threads with serrations to keep the hemp from unraveling as you thread it all together.
 

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Sid

Member
Jan 5, 2015
49
Colchester Connecticut
That's crazy about the hemp, never heard of it before this thread.

I tried just lots of tape and dope and it's holding the 50 psi cold just fine, so I'm leaving it alone unless it ever starts to leak.
. 20151102_062322.jpg
 

salecker

Minister of Fire
Aug 22, 2010
1,437
Northern Canada
I used hemp on my build,it takes a little longer to score the threads and cover with hemp then a little pipe dope to hold everything as you install.I had over 150 threaded joints on the first stage of my build and had no leaks.
When i tested my tanks i tried 3 different methods. Just Teflon tape,just pipe dope,and the hemp method.I tightened all the plugs about the same.All the Teflon ones leaked and with more tightening most stopped leaking.About half of the pipe dope ones leaked and stopped with more tightening.None of the hemp one leaked.I was sold on the hemp way.
Any threaded joints i do now are all hemp unless it is just temporary.To date nothing with hemp on it has leaked.
Everyone asks what the hair is hanging off my fittings.
 

DaveBP

Minister of Fire
May 25, 2008
1,157
SW Maine
Everyone asks what the hair is hanging off my fittings.
I ran a propane torch over that hair and most of it burned off in seconds.

Loctite (now owned by a German company) marketed their #55 product for a while but I'm not certain that they haven't withdrawn it now. It didn't require using any dope. Their Utoob video recommended squeezing lightly with the plier jaws over the threads to rough them up. That seemed like heresy to my inner machinist but I got tired of filing nicks in the thread crowns and started doing it as they showed. Not a single leak then or since.

And I did end up moving a couple fittings back out a little and those didn't leak either. As they promised.
 

DaveBP

Minister of Fire
May 25, 2008
1,157
SW Maine
I found some on eebay. But like I said, it only shows up occasionally. They seem to have given up on it in US market.

Anybody know for sure?
 

jrod770

Member
Oct 22, 2013
117
granville, ohio
x 2 on what white pine said. It's what grandpa used. it's what we use. No Teflon on top of it. just the pipe dope only. I am a commercial/residential plumber and use it all day long. I have never had leaks because of the pipe dope and have always thought it to be a waste of time to add Teflon on top of the pipe dope.
 

Whitepine2

Feeling the Heat
Feb 15, 2011
314
Southern Ma
x 2 on what white pine said. It's what grandpa used. it's what we use. No Teflon on top of it. just the pipe dope only. I am a commercial/residential plumber and use it all day long. I have never had leaks because of the pipe dope and have always thought it to be a waste of time to add Teflon on top of the pipe dope.
I worked with hydraulics for years and used rectorseal talking 3,000+ PSI and works for me this is all I've used
and don't know of anything better.
 
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