can I use RTV/sealer with coolant level sensor nut on expansion tank

[ccchrisss]

Ever since I installed a newer expansion tank, the threads on the tank do not get along with any of the sensor nuts I've used. I've dug through this forum and tried all the other solutions offered.. old nut, newer OEM nut, URO, and also the URO nut off an e34 coolant level sensor, all these different nuts with teflon tape as well. There is a lot of room around the threads and the plastic nuts are soft and expand quite a bit with teflon tape used so the seal is never great and they always back out from engine vibration and higher pressures when i've been driving around for a while.

Has anyone ever tried some kind of sealant? I was thinking of using silicon RTV around the threads, not quite sure if it will work well in this application. Or the blue sealer plumbers use for PVC piping was another option I was thinking about. Would like your opinions whether or not this would help hold the nut in place and prevent seepage from under the sensor. I've temporarily ductaped the sensor to the expansion tank but it's not a very pretty option.

[Federico]

Oh yeah, the fitment of those parts can be terrible. I fixed it by building up the threads on the tank using F4 tape (the silicone, self fusing type).

IIRC, the problem is the nut has straight threads, but the newer tanks have them tapered, like pipe fittings. Back when I noticed this issue I considered milling/cutting down the threads on the tank to give the nut more 'bite'.

[ccchrisss]

Oh yeah, the fitment of those parts can be terrible. I fixed it by building up the threads on the tank using F4 tape (the silicone, self fusing type).

IIRC, the problem is the nut has straight threads, but the newer tanks have them tapered, like pipe fittings. Back when I noticed this issue I considered milling/cutting down the threads on the tank to give the nut more 'bite'.

I'll give that a try, thanks. did you do just one pass around or 2?

[Federico]

IDK, somewhere between 1-2 turns.

As for the tapered threads, If you remove the nut from the sensor and thread it in, you'll notice it bites better once it's past the first couple turns.

[Mike W.]

My fix, long ago now, was to use the old nut. But it seems that hasn't worked for you.

I would not use RTV, I doubt it would even stick to the tank, but it's so soft even if it did, for how long. I would consider cutting down the tank threads if it seems tapered like suggested above. I would consider epoxy, but if that works you might not be able to ever replace the sensor again without a new tank. PVC glue, if I went that route, and I'm kind of neutral on it, I would use the extra thick heavy duty, heavy bodied glue, it'll fill larger gaps. Again, probably prohibits non destructive replacement, but sometimes... Temp rating? No idea, but PVC pipe is not used for high temps so I don't know if the glue would be. Then there's CPVC also, which is rated for higher temps, but I don't know much about that stuff.

[MLastovo]

PFTE thread seal tape.
D.

[Shawn D.]

The problem is that the nut doesn't perform the sealing function. What you're experiencing is a lack of sealing due to the nut not pressing the sensor and tank together sufficiently, and you're seeing it leak via the threads.

That being said, if you can successfully seal it so it doesn't leak, there's nothing inherently wrong with that, except for the difficulty in removal.

[austin8753]

if you have to, use a two-part epoxy, JB Weld or the sort. something that is meant to bond to plastic and is compatible with the warm up/cool down process. i don't think silicone will hold up to that, long term.

[Mike W.]

The problem is that the nut doesn't perform the sealing function. What you're experiencing is a lack of sealing due to the nut not pressing the sensor and tank together sufficiently, and you're seeing it leak via the threads.

Correct, but...

While I don't know about the OP here, in my case the threads didn't have enough bite to allow the sensor to properly seal, IMO it's kind of a feel torque, but even doing it by hand I would get to a certain point and it would pop and back off a thread. Plastic being plastic it didn't tear the threads off like it would have with metal, but I was unable to apply enough torque to get it to seal. That's why I kind of suggested cutting the neck down a little bit.

We may need more detail from OP.

[ccchrisss]

The problem is that the nut doesn't perform the sealing function. What you're experiencing is a lack of sealing due to the nut not pressing the sensor and tank together sufficiently, and you're seeing it leak via the threads.

Correct, but...

While I don't know about the OP here, in my case the threads didn't have enough bite to allow the sensor to properly seal, IMO it's kind of a feel torque, but even doing it by hand I would get to a certain point and it would pop and back off a thread. Plastic being plastic it didn't tear the threads off like it would have with metal, but I was unable to apply enough torque to get it to seal. That's why I kind of suggested cutting the neck down a little bit.

We may need more detail from OP.

Yeah, this is exactly my problem. I can barely get any torque on the nut without it popping off/stripping so it’s on light enough to back out on it’s own after driving around, either that or the system pressure causes the nut to pop off. When I just put the nut on without the sensor and screw it all the way down, I can still wobble the nut slightly, the threads aren’t a perfect fit.

Also seems like teflon tape doesn’t help as much as it gives less for the nut to bite on if that makes any sense. Hoping the silicon tape is stickier to help with that

[Chimi-Changa]

I struggled for a while trying to get one to seal on a new tank, and eventually gave up. Tried an old sensor nuts, teflon tape and nothing was holding.

My only fix was to find a good enough condition used one that had a sensor already attached. They have date codes on them like other parts. ( not sure when the switch to "cheap crap" parts came around- but this tank on ebay is from 1990, and has a sensor on it. ( the level sensors rarely go bad)

Mine is from 2010 by the date stamp:





Por Ejemplo:

https://www.ebay.com/itm/156071150535?i ... R9K-0_SLZA

[ccchrisss]

I struggled for a while trying to get one to seal on a new tank, and eventually gave up. Tried an old sensor nuts, teflon tape and nothing was holding.

My only fix was to find a good enough condition used one that had a sensor already attached. They have date codes on them like other parts. ( not sure when the switch to "cheap crap" parts came around- but this tank on ebay is from 1990, and has a sensor on it. ( the level sensors rarely go bad)

Mine is from 2010 by the date stamp:





Por Ejemplo:

https://www.ebay.com/itm/156071150535?i ... R9K-0_SLZA

This is also a great idea! Would be interested if I didn't already spend 500 on a new aluminum radiator

For anyone digging up this thread in the future, I tried the F4 silicon tape as suggested by Federico and it seems to have worked for now. 2 tight passes around so the tape can grip itself, a lot more friction to hold the threads together and seems to be a very tight seal with good tightening torque. Took a drive around the neighborhood with no loosening/cooling issues, will report back after a longer one.

[Aldo525]

I struggled for a while trying to get one to seal on a new tank, and eventually gave up. Tried an old sensor nuts, teflon tape and nothing was holding.

My only fix was to find a good enough condition used one that had a sensor already attached. They have date codes on them like other parts. ( not sure when the switch to "cheap crap" parts came around- but this tank on ebay is from 1990, and has a sensor on it. ( the level sensors rarely go bad)

Mine is from 2010 by the date stamp:

Por Ejemplo:

https://www.ebay.com/itm/156071150535?i ... R9K-0_SLZA

I must have been lucky. I changed the same expansion tank recently not because it was bad, but because it was old and had that yellowish color that you can't see anything inside. And I also changed the level sensor, both new and original BMW parts....everything fit perfectly, the new sensor was screwed into the tank firmly and without problems, no leaks.

Reading this post I checked the date of my new tank and it says 08, so it could be said that until 08 does not have the thread problem.