Through-hull fittings

[coopec]

I have an Airex sandwich hull and needed to install thru-hulls for the toilet. I drilled the holes in the hull, dug out the Airex, laid glass over the the hole to stop water getting into the core.

I wasn't too happy with the result so I filled the hole with epoxy bog and then laid X3 layers of glass cloth over the bog. I then re-drilled the hole and now I am quite happy with the result (on a critical part of the construction)

But now the hull is fairly thick and the ball valve won't quite bottom-out on the through hull skin fitting. I don't know much about NPT fittings but if I use some Teflon tape that should be perfectly OK shouldn't it? (Alternatively I could take up the gap with a nylon washer?)

The notation on the drawing MUST BE LESS THAN 1/2" is to ensure a sufficient number of threads on the skin fitting are used? (These under-water fittings worry me) ☹

 

[Halo]

Maybe try a different fitting with a longer thread . The ones I have bought through ASAP have been plenty long enough. Not sure why you are going for NPT I prefer BSP with a lock ring.

 

[coopec]

The penny just dropped. I'm using anti- seize compound so I won't use Teflon tape

 

[coopec]

Maybe try a different fitting with a longer thread . The ones I have bought through ASAP have been plenty long enough. Not sure why you are going for NPT I prefer BSP with a lock ring.

I thought of getting a slightly longer skin fitting. (I'll see what they cost)

If you look at the drawing it says "Though hull fitting has NPS threads this end" and just above that "In-line vale has NPT threads"

 

[dankilb]

The penny just dropped. I'm using anti- seize compound so I won't use Teflon tape

Are you sure NPT seals by itself (in this particular application)?

This is a genuine questions, because BSP (at least as supplied on our new TruDesign fittings) doesn’t and - even with the recommended sealant applied - one (of six) of ours is leaking after launch.

I’d prioritise the seal over removability.

 

[dankilb]

The notation on the drawing MUST BE LESS THAN 1/2"

Might this also be to ensure the gap isn’t too great and therefore reducing any lever force if the fittings are stressed? With BSP you simply chop off any unused length/threads to get the desired spacing. With tapered threads - I’m guessing not?

 

[coopec]

Are you sure NPT seals by itself (in this particular application)?

This is a genuine questions, because BSP (at least as supplied on our new TruDesign fittings) doesn’t and - even with the recommended sealant applied - one (of six) of ours is leaking after launch.

I’d prioritise the seal over removability.

I agree with you.

If I have to take a thru hull off I would think it was because it needed replacing in which case an angle grinder with a cutting disc would make short work of it.

Thanks

 

[dankilb]

Perhaps consider using a stronger bedding compound on the skin fitting (we used epoxy - as recommended for our composite ones - or 5200) and a medium/low bond strength thread sealant on the valve? That way the valve can always come off (as we’re going to have to do on our leaky one).

We don’t really see as much NPT here in the UK and it may be that it will genuinely seal with nothing?

 

[coopec]

Might this also be to ensure the gap isn’t too great and therefore reducing any lever force if the fittings are stressed? With BSP you simply chop off any unused length/threads to get the desired spacing. With tapered threads - I’m guessing not?

Yes, those are my thoughts too. Ideally on a small ball valve (say 1" diam.) I'd like an inch+ of thread and on a bigger ball valve even more.