Water Tank Thread - Mystery

[Cherp]

Hi. This is a long shot. We have a Catalina Morgan 45 from 1998, built in Florida. It has aluminium water tanks (not good) and they have a threaded fitting welded into them. I have been trying to determine what thread is used, with no success so far. It's close to one-and-a-quarter BSP ( 32 mm aperture in the fitting). Have tried BSP and one-and-a-quarter NPT but they bind up. Anybody have a clue what thread they used? SAE? Metric? Voodoo?

[Wotname]

Do the tanks have a male or female thread welded in?

If male, can you measure the thread pitch - TPI (threads per inch)?

Do you have a removable fitting that current fits? If so, take it into Nuts and Bolts (Derwent Park or Kingston) and they will likely identify it for you.

[Cherp]

Female fitting welded in. No there plug I can readily access. Explosives are looking good.

[Wotname]

Another long shot, if the BSP you tried was a BSPT (taper), you might try a BSPP (parallel).

[1affiah]

Hard to believe it would not be a US thread of some type, likely female pipe thread. Use a nylon bushing and teflon tape.

[Sailmonkey]

When you say bind up, how far in can you thread before they bind up? NPT binds up as the nature of the thread seals by interference.

[Rod B]

Originally Posted by 1affiah

Hard to believe it would not be a US thread of some type, likely female pipe thread. Use a nylon bushing and teflon tape.

Would agree with this, re US thread (NPT), the thread could be damaged or have old sealant in it. The pitch (TPI) is the same on BSP and NPT up to 1 1/2" I think, the difference is in the angle of the thread. BSP is 55 degrees, NPT 60 degrees. I would run an 1 1/4" pipe tap into the fitting very carefully because I have one available, you may not be so fortunate. The nylon fitting idea could be a decent alternative.
No explosives!! Good luck!

[Rod B]

Originally Posted by Rod B

Would agree with this, re US thread (NPT), the thread could be damaged or have old sealant in it. The pitch (TPI) is the same on BSP and NPT up to 1 1/2" I think, the difference is in the angle of the thread. BSP is 55 degrees, NPT 60 degrees. I would run an 1 1/4" pipe tap into the fitting very carefully because I have one available, you may not be so fortunate. The nylon fitting idea could be a decent alternative.
No explosives!! Good luck!

Not quite true re the pitch, just checked, but so close they are often used together in non critical and low pressure applications.
Sorry about the mis info, still, no explosives.

[1affiah]

Or, nylon bushing with 5200, lifecaulk, etc. instead of teflon tape

[deblen]

If tank was built in N.America,I would be 99% sure threads are NPT (National Pipe Taper) & they seal because they are tapered-both male & female sides.


Since the threaded hole is aluminum,the threads are probably a bit rough(some minor corrosion & possibly old pipe dope).
If you can find a steel NPT nipple of the correct size,or a proper NPT tap,you could gently run it in the threads until it snugs up (due to taper).
If you can't access a NPT object,I suggest you use a small steel wire brush to clean the threads as best you can.
Use plumbers pipe dope-preferably brush on teflon-or teflon tape when you re-assemble.
I don't recommend using a BSP taper fitting in an aluminum NPT hole. It will go ,with force,but aluminum is so soft that there is danger of buggering the alum. threads / Len



https://www.valvesonline.com.au/references/threads/

[deluxe68]

Go to a plumbing store and get some thread gauges, or order them online.

[Cadence]

Originally Posted by Cherp

Female fitting welded in. No there plug I can readily access. Explosives are looking good.

Cherp, are you sure the threads have not been loused up from the wrong fittings being used. You may wish to retap with a known thread?

[fnq grumpy]

Very long shot and can't for the life of me think why someone would do it, but I recently had considerable grief and funny looks from pipe fitting people 'probably some Chinese rubbish'. Fitting was ex US and turned out to be GHT (garden hose thread). 11,5 tpi and all sorts of things start to fit but don't.
Suggestion to get some thread gauges and measure is a good start and damaged/ pipe gunk contamination would be my guess but there is a strange US thread I only recently learnt about after a lifetime in industry and some months of grief.

[Cherp]

Originally Posted by Sailmonkey

When you say bind up, how far in can you thread before they bind up? NPT binds up as the nature of the thread seals by interference.

Only goes in say 5 millimetres

[Cherp]

Originally Posted by fnq grumpy

Very long shot and can't for the life of me think why someone would do it, but I recently had considerable grief and funny looks from pipe fitting people 'probably some Chinese rubbish'. Fitting was ex US and turned out to be GHT (garden hose thread). 11,5 tpi and all sorts of things start to fit but don't.
Suggestion to get some thread gauges and measure is a good start and damaged/ pipe gunk contamination would be my guess but there is a strange US thread I only recently learnt about after a lifetime in industry and some months of grief.

I think I will have to acquire some gauges