Cylinder drain cock tapping - parallel thread

[DavidLewis]

Hi all,

My nice new Gledhill copper cylinder has a tapping for a drain cock. This tapping is 1/2" BSP female parallel threaded. I naively thought the drain cock I bought https://www.toolstation.com/made4trade-mt-cock-type-a/p83905 would come with a tapered thread, but no, it's parallel too... so it's a loose fit (before I've used PTFE tape or anything).

I can't find tapered-thread draincocks easily available, so I'm about to make up a fitting with a coupler and a plain shank draincock instead. Is that what you guys do normally?

Thanks for the advice!

 

[CBW]

Are you wanting a tapered or parallel thread? You start by one and change to the other. Have you tried it with ptfe? How do you know what thread is required on the cylinder?

 

[DavidLewis]

Ok, I admit I'm just assuming the female thread on the cylinder is parallel. I am therefore wanting a male tapered thread on the draincock.

The draincock I have is parallel-thread, and doesn't get any tighter as I screw it in, until its "shoulder" touches the boss on the tank. But, as far as I am aware, there's not supposed to be any kind of fibre washer there (it's not a tap connector), although that's a possibility I suppose...

 

[kidgreen61]

You don't need a tapered thread on your DC, you need plenty of PTFE tape.

 

[DavidLewis]

You don't need a tapered thread on your DC, you need plenty of PTFE tape.

I suppose you are correct. I did, in fact, make up another draincock using a tapered-thread coupler and a plain draincock. But the amount of taper on the thread was surprisingly small...

30 turns of PTFE tape later (20 wasn't enough!) and the thing is installed nicely.

So I guess parallel-threaded wouldn't have made much difference!

 

[cross thread]

Locktite 55 is good on your issue

 

[DavidLewis]

Locktite 55 is good on your issue

Thanks - I'll buy some for next time. I've run out of PTFE tape now anyway

 

[Jackrae]

The correct engineering seal for a parallel male thread in a parallel female bush is to fit a collar washer which forms the seal once the male part is fully screwed home. PTFE tape, whilst it might work is a bodge job.
See here for a full description :

 

[Hawkijo]

Hi Jackrae. Many thanks for giving us that link.