Follow this thread, is it metric.

[Jim@sea]

I have a French Motor Boat 1986, which had an ordinary CAV diesel water trap. Due to age the little white plastic drain screw broke. Not knowing where in France I could buy a replacement I bought another water trap. I now get a very small diesel leak.
The original is shown on the attached photo.
BUT as my water trap is a CAV presumably with an old thread, which could perhaps be "Whitworth" "UNF" "AF" etc., and the new one could be "Metric"
So What I am trying to establish is despite the head of the brass bolt being either metric or AF is the thread inside an "universal" thread

 

[rafiki_]

If the new plug screwed in easily, thn it is the right thread. If you had to force it, then the wrong thread, and it will leak. You could try an "O" ring to seal.

 

[thefatlady]

I would be very surprised if the plug relies on the thread to seal. I would expect there to be a seat which is not sealing, maybe damaged or perhaps due to the wrong thread not allowing the plug to screw fully home, or, as Rafiki says, a seal.

Alternatively, try good old PTFE tape.

 

[rafiki_]

I tohought about PTFE tape, but I'm not sure of its durability with diesel. There are loctite or other suppliers of sealing gunge in a tube. Trouble with this stuff is you have to clean it off next time you unscrew the plug, and re-seal. O Ring I think is the right route.

 

[thefatlady]

If you can fit it, O-ring is right, but PTFE tape should have no problem with diesel.

 

[SolentPhill]

If you can fit it, O-ring is right, but PTFE tape should have no problem with diesel.

Ptfe wont last 10 seconds in diesel

 

[Rum_Pirate]

There is a lot of info on this thread

http://www.sailnet.com/forums/gear-maintenance/10266-use-teflon-tape-fuel-fittings.html

 

[thefatlady]

Ptfe wont last 10 seconds in diesel

Strange! I used it on a 1 million gallon tank of the stuff in an oil terminal some 30 years ago and, as far as I know, it's still there.

 

[NigelFortune]

Fuel pipe thread

Hi
Have just changed fuel water trap with double CAV unit and had similar problems.

The new CAV unit can have either 1/2 " UNF thread or 14mm x1,5. I would suggest that your new filter/water tap is metric and the original CAV unit was Imperial. Fuel pipe is 8mm for both. Make sure you have the correct olives for the fitting there are at leasr 2 different types. Hope this helps.

Nigel

 

[SolentPhill]

Strange! I used it on a 1 million gallon tank of the stuff in an oil terminal some 30 years ago and, as far as I know, it's still there.

Maybe 30 yrs ago they sold good stuff, try a cup of diesel and put some in it, have you used rubber gloves when changing filters? There usually nothing left after a few mins unless you use the better ones

 

[boatmaster]

I have double Cav filters on twin my fords, 5 years ago, BSS wouldn't accept plstic drains so i changed them for metal ones. i raided my huge odds box for 4 that fitted and found 4 that were very close but not exact. i found a fifth one and cut a thread cutter down it and wound it in each housing. being alluminium they tapped easily. then I sealed with dougherty washers and ptfe. still holding now. only down side that if i ever have to drain water, i will need to reseal. i'm happy, no prob ptfe even in low sulphur!

 

[thefatlady]

Maybe 30 yrs ago they sold good stuff, try a cup of diesel and put some in it, have you used rubber gloves when changing filters? There usually nothing left after a few mins unless you use the better ones

Rubber gloves are not made from PTFE. The better ones are usually nitrile rubber.

PTFE is almost chemically inert, but has low strength. If you search, you will find a number of threads on this subject where others have used it successfully. Googling will give a similar result.

From here: http://www.dynalabcorp.com/technical_info_ptfe.asp

Polytetrafluoroethylene (PTFE) shows a remarkable chemical resistance and it is insoluble in all known solvents. It is attacked only by molten alkali metals and by fluorine at high temperatures.

and here with chemical resistance table showing diesel:
http://www.zeusinc.com/technicalser...ceofpolymers/chemicalresistancechartptfe.aspx

PTFE is attacked by some alkali metals (molten or in solution) and by some rare fluorinated compounds at high temperatures and pressures. Some organic and halogenated solvents are absorbed causing small dimensional changes but the effects are physical and also reversible.

There are PTFE tapes which are laminated with other materials for enhancing mechanical properties and maybe one of these has given you a problem. Plain PTFE won't. Just apply it properly to avoid bits in the fuel lines.