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Old 6th Nov 2017, 7:58 pm   #1
astral highway
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Default Best oil-proof sealant?

Not sure if this is the right category but it doesn't fit in 'components and parts'!

I need to seal some loctite nuts and bolts into a lexan case, and the seal MUST be 100% insulating/ transformer oil resistant.

It's for a high-voltage probe.

There seem to be a myriad of products on Ebay, ranging from dirt cheap to aviation-grade and . Hence just making sure I don't waste a ton of money on snake oil!

Cheers folks.
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Old 6th Nov 2017, 8:52 pm   #2
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Default Re: Best oil-proof sealant?

Depending on the surfaces that you're sealing to, bonded seals (Dowty washers) under the bolt heads can work essentially perfectly. But the instructions for their use need to be followed absolutely. The effectiveness of sealants which are applied as a gel and then cure can depend as much on factors like the cleanliness of the components being sealed, whether the seal will be subject to mechanical stress or vibration, whether the oil will be running very hot (I've worked with systems using pumped oil at well over 100C) etc as it does on what the sealant compound is. I understand that automotive sealants can be pretty good.

Cheers,

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Old 6th Nov 2017, 9:06 pm   #3
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Default Re: Best oil-proof sealant?

Loctite 574 flange sealant is a possibility. Its main purpose is flange sealing in automotive applications, so it takes hot oil and pressure just fine. Surfaces must be clean etc.

Do a search - plenty of information on it on Loctite's site - and see if you think it is a possibility.

Cheap it isn't. 50cc (which is the minimum) of the stuff is twenty-odd quid.

Another possibility are bolts that include a sealing washer, like these https://www.zago.com/products/sealing-screws/ . Again, not cheap, although I cannot recall how much (I used them last about four years ago)

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Old 6th Nov 2017, 9:24 pm   #4
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Default Re: Best oil-proof sealant?

GJ: thanks, I hadn't heard of Dowty washers, but I'll look them up.

In terms of cleanliness, I will wash the container with detergent and hot water in a dishwasher - that should do the trick. The probe won't get very hot. The EHT resistors inside will dissipate about 10W maximum into a few hundred mils of oil, for a few seconds at a time. The oil will be hard pushed to heat much above a few degrees above room temperature.

Whatever the case, its expansion could be an issue and I will need to leave a little headroom for that - easier for a few cubic mils of air to increase in pressure rather than the oil having nowhere to expand and directly interacting the sealant.

Craig: bolts with a sealing washer sound like the way to go, but I'll also investigate that very pricey sealant! I know that essentially, any sealant is only as good as the surface it's bonded to at a molecular level, so prep is important!

Cheers both!
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Old 6th Nov 2017, 9:55 pm   #5
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Default Re: Best oil-proof sealant?

I suspect that few sealants have the heritage of Blue Hermetite, which I think is now branded Hylomar Blue? I think it was originally developed for Rolls Royce. I'm never without a tube of it.

Avoid the red version which is a different material and gives off acteic acid as it sets.

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Old 6th Nov 2017, 10:21 pm   #6
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Default Re: Best oil-proof sealant?

I can vouch for Hylomar blue, used it for years.
I have even used it with previously used head gaskets on diesel engines with total success and those pressures and temperatures are extreme.
Its a thin film sealant bear in mind.
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Old 6th Nov 2017, 10:54 pm   #7
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Default Re: Best oil-proof sealant?

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Originally Posted by Bazz4CQJ View Post
I suspect that few sealants have the heritage of Blue Hermetite, which I think is now branded Hylomar Blue?
That's the brand that comes up when I search. Incredibly, it's not only not the most expensive but one of the most economical!
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Old 6th Nov 2017, 10:57 pm   #8
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Default Re: Best oil-proof sealant?

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Its a thin film sealant bear in mind.

Ahah, but what does that mean in practice?

Do I apply it just over the lock nuts, which will be under high torque, creating a seal externally only? Or do I apply it before assembling them by smearing it all over the surfaces and the bolts?

If these questions seem simplistic, bear in mind I have no experience of automotive applications or products and no sense of its viscosity or how it actually works.!

Will it matter that it won't be coming under high temperatures? Will it still cure ok?
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Old 7th Nov 2017, 2:26 am   #9
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Default Re: Best oil-proof sealant?

No, it doesn't need temperature to 'cure' it. I've used it on the washing machine connectors for the cold feed, which are only supposed to be hand-tight. Hard to get them to seal if only hand-tight, but no problems when the Blue stuff is there. When it comes out of the tube it is fairly fluid, but quickly begins to dry out and firm up, a lot like Evostick glue. You only need a thin even film, I'd say on both surfaces, and just let it dry to the point of 'tackyness', then mate things up; try get it lined up at the first go. Ideally, give it 24 hours to really get the best outcome before you stress it.

As Boater says, it was originally developed for cylinder heads, so it needed to be good with both water and petro-chemicals at significant temperatures and pressures. Buy some and play with it.

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Old 7th Nov 2017, 7:23 am   #10
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Default Re: Best oil-proof sealant?

Hylomar blue is a non-setting sealant, and might not be the best choice for the application that Al has in mind.

The generic name for most of the sealing goops is RTV (Room Temperature Vulcanising), and covers the cheap and cheerful bath sealant stuff that uses acetic acid as the curing agent, right the way through to two part RTV that is suitable for high vacuum and even space use.

But there are a number of RTV's that seem to be suitable. Just stick with known high quality brands like Loctite, Permtex and Dow Corning, and chose one that is suitable for automotive use - there are ones that seem possibilities that will not break the bank.

Depending on how many you need http://www.automotioncomponents.co.u...l-screws/p0173 is one supplier of bolts with integral O ring seals. Be prepared for about a fiver each depending on thread size.
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Old 7th Nov 2017, 9:51 am   #11
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Default Re: Best oil-proof sealant?

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Originally Posted by astral highway View Post
... In terms of cleanliness, I will wash the container with detergent and hot water in a dishwasher - that should do the trick ...
Usually what seems to matter is the removal of grease. Detergents can do this, but then you tend to end up with a layer of detergent stuck to the surface. I used to work with people who told me that once an object had been exposed to detergent it was impossible ever to get all of it off again. I also knew people who used the detergent layer as a release agent for coatings.

If they wanted a really clean surface they'd either dry machine it in a clean workshop and then never touch it or, having used the detergent, they'd wash it again and again and again in increasingly pure water. I don't know whether any of this rigmarole would be necessary in your case though. Those folks wanted really good adhesion which is not the same as really good sealing.

Cheers,

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Old 7th Nov 2017, 10:49 am   #12
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Default Re: Best oil-proof sealant?

Good post, GJ. I also have the option of using a solvent like acetone or methylated spirits. It’s pretty clean though, the critical part has been encapsulated for years and out of reach of dirt sources.

I’m sure my surfaces will be a lot cleaner than most car engines , even prepped ones! I will report back when I can. The ‘interesting’ bit will be the coax, which is clamped tight but also terminates in the oil... that’s where the seepage is likely to occur if I rush things or am unlucky!!
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Old 7th Nov 2017, 11:11 am   #13
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Default Re: Best oil-proof sealant?

First, I really wouldn't take acetone anywhere near Lexan. Almost all of my experience of plastic oil equipment was with acrylics (Perspex) and putting acetone on those would ruin them in an instant. Even the vapour from an open vessel nearby would cause crazing in the plastic as a reult of chemical action combined with the surface stress which exists in most plastics. Lexan (polycarbonate) might be more resistant. But I'd definitely try it on a scrap piece first, and wait a day or two to see if any delayed effects appear.

You may struggle to seal co-ax. Depending on its construction you can get leakage along the join between the inner conductor and its insulator, although if this join is tight it can take weeks/months for the oil to appear at the far end of the cable. More of a problem in the short term can be leakage along the co-ax screen, inside the outer sheathing. Running the cables out vertically upwards helps. But capillary rise can still beat you (this is the voice of experience speaking ).

If we needed an absolutely oil-tight co-ax feedthrough we usually gave up trying to seal the cable and put a panel mount socket in the vessel wall instead. Even then we had to take care that the socket we chose didn't weep.

Cheers,

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Old 7th Nov 2017, 2:09 pm   #14
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Default Re: Best oil-proof sealant?

I don't think you will ever seal coax. Even water finds its way down aerial leads eventually, for a considerable distance.
Feed-through insulators?
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Old 7th Nov 2017, 3:08 pm   #15
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Default Re: Best oil-proof sealant?

I have used non acid cure silicone rubber for EHT applications, its cheap. For it to work everything has to be clean & oil free initially. It does successfully seal against oil, I used it on my own oil filled EHT probe.

Another sealant and glue I used on it was Torr Seal by Varian Vacuum, which is a white high vacuum sealant two part epoxy resin, but its diabolically expensive. Though I managed to use it to successfully seal a crack in a CRT.

I have used Hylomar blue when I rebuilt a Borg-Warner 35 transmission in my 1963 Humber Scepter when I was about 17. I have not tried it for high voltage applications.
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Old 7th Nov 2017, 3:54 pm   #16
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Default Re: Best oil-proof sealant?

I've worked on a engine, whose type shall remain nameless, which was notorious for leaking oil. The manufacturers recommended using Hoylomar, but it didn't seem to work.

I've now switched to using Wellseal and there are no more leaks.

http://www.wellseal.co.uk/
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Old 7th Nov 2017, 6:36 pm   #17
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Default Re: Best oil-proof sealant?

I'm not familiar with Wellseal, but this forum (and others) discusses Wellseal v Hylomar http://socforum.com/forum/showthread...ellseal/page2&. Opinions seems divided, as is often the case. Quite subtle differences between products can often mean that it's a "horses for courses" scenario.

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Old 7th Nov 2017, 7:14 pm   #18
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Default Re: Best oil-proof sealant?

I really don't think you'll have much success trying to seal coax!

Rather, I'd use a 'bulkhead' fitting. Such things are available for the N- and C-connectors used in commercial applications where the feeders/antennas are pressurised with dry Nitrogen or they're feedthroughs to vacuum-equipment: I specced such a beastie for a HF ATU I designed a while back.

Not cheap though! See https://www.fischerconnectors.com/uk...cations/vacuum
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Old 8th Nov 2017, 9:39 am   #19
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Default Re: Best oil-proof sealant?

I agree with Station X with regards to Blue Hylomar, never had much success with it. I have used gallons of Wellseal over the years and still do mainly on newly machined (mainly Rover) aluminium cylinder heads. Wellseal is non curing though so whether this makes any difference I don't know.

Loctite 5910 is what I would recommend, around 10-12 quid for a small tube, expensive, but you get what you pay for, other sizes are available. If you observe cleanliness prior to application and observe curing times its excellent.

Regards, poppydog.
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Old 8th Nov 2017, 8:13 pm   #20
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Default Re: Best oil-proof sealant?

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I really don't think you'll have much success trying to seal coax!
Hi there...

Yes, I was thinking it could be tricky. The coax is, however, clamped tightly and has only a very small aperture, coax only goes through very tightly indeed. I will probably be able to seal it by cleaning it very thoroughly and using JB Weld or similar to coat it from the inside, before filling the probe with transformer oil. Once the JB Weld has cured on the inside of the probe, I can also seal the outside, expecting little or no oil to permeate there anyway.

Everyone: thanks for all your recommendations. I was pretty quick in responding to suggestions by pressing the 'buy' button on EBay. So I have ended up with two or three products to try out. I didn't buy the most expensive, so not much harm done to the wallet. I'll report back once I've finished the probe.

Cheers!
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