PTFE Tape?

[vyv_cox]

304 contains a higher percentage of chromium than 316, and thus tends to be harder, and when used for knives will keep an edge, where 316 will not. Most cutlery is 304.

304 is 18Cr, 8 Ni. 316 is minimum 18Cr, 8 Ni, 2 Mo but often has more of both Cr and Ni than that.

Most knives and cutlery are made from 400 series stainless steel, the cheaper grades are 5 Cr, better ones 13 Cr, the best are 300 series, I have one at 20 Cr, 10 Ni. 400 series ones are magnetic, 304 may be if hard worked.

 

[mikemonty]

Vic, because both fittings have parallel threads I would need to rely on the tape or whateever else to create an unmovable joint as the t h fitting wont be screwed to the last in the valve as the valve has to be in a certain position to allow the operating handle to get a full arc.
I know from fitting domestic radiater tails that it is the ptfe or gas tape (applied in a cone form) that locks up the tail.
C_W

Hi Eamonn,
How about a close(ish) fitting sleeve over the male thread between the backing nut inside the hull and the valve and a decent washer on top of the sleeve followed by some sealant and the valve tightened up on the whole assembly?
cut to adjust the sleeve until the valve tightens up in the postion you want it.
You would have to ensure the sleeve ends were fairly square to avoid uneven stresses.

Another thought: You wouldn't even have to be TOO clever about the sleeve length as long as you WERE clever about the position that you fix the through hull at...

Another other thought: You would have to be careful when horsing up the valve that the through hull couldn't turn and loosen off from the backing nut

 

[YELLOW BOAT]

Maybe I'm coming with something already said but anyway.
One of the problems using ptfe tape on a stainless thread is indeed the very low friction coefficient. Together with the very smooth surface of the stainless thread ( machined , not casted thread as with most steel ones ) it prevents the tape fom sticking on the thread!
When you screw in the ptfe will strip up and that way will not give a good sealing between the threads!
There is a simple way to prevent this I ( we) used in the chemical industry I worked before my pensioning.
First use a file to roughen up the thread a bit before winding the ptfe around
it. You do not have to file between the threads , just on top of it and not too much.
When you now screw in the ptfe stays between the threads and a good sealing is guaranteed!
Ad

 

[scotty123]

Why would you use parallel or straight threads as opposed to tapered threads designed for sealing? Tapered threads are designed to seal and all that's needed is pipe dope or teflon tape.

I have never once heard that teflon tape causes crevice or pitting corrosion but then again I don't and won't generally use SS for seacocks or thru-hulls and much prefer 85-5-5-5 bronze or Marelon.

I would love to hear this guys explanation for this as the seal created by the tape should thus prevent any salt water from getting to that area.

Do you guys across the pond often use parallel threads below water with metal fittings? We use tapered threads over here, NPT you guys use BSPT I believe. Folks often incorrectly mix a thru-hull with straight threads with a ball valve that has tapered threads which is not a good idea..

Teflon alone, will not "cause crevice or pitting corrosion".

Its the properties which make S/S actually stainless that does that - the need for contact with air/oxygen, to repair the surface preventing corrosion.

Preventing any ingress of oxygen, in the presence of an 'electrolite' such as seawater, will certainly encourage corrosion. If the teflon tape does this & also allows seawater contact, then crevice corrosion will take place.

 

[Heckler]

Vic, because both fittings have parallel threads I would need to rely on the tape or whateever else to create an unmovable joint as the t h fitting wont be screwed to the last in the valve as the valve has to be in a certain position to allow the operating handle to get a full arc.
I know from fitting domestic radiater tails that it is the ptfe or gas tape (applied in a cone form) that locks up the tail.
C_W

You cant beat Stag, good old red stuff, used by pipe fitters for years.
Stu

 

[Blue5]

Heldite

I have given up on PTFE now use Heldite, brilliant stuff

http://www.oilybits.com/heldite-jointing-compound/prod_150.html


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