Diesel fuel and gaskets

[higgs]

I am going to be resealing my inspection port cover onto the top of my fuel tank. It had been sealed with some sort of a putty like material. Local people in the know recommend a rubber gasket, but the top of the steel tank is fiberglass made with cloth that has a pretty course weave, thus the tank surface is not smooth, as a metal surface would be.

I have been told my best is Ultra Grey rigid RTV silicone, but the package says it should not be used where it will contact gasoline. I know diesel is more of an oil, but it also shares properties with gas. No one is willing to go out on a limb and say the silicone will work - only that it is my best bet. It's advantage is that it will compensate for the uneven, but clean surface of the tank.

Another option I could do, but would rather not, is not smooth the tank surface with West epoxy and use a rubber gasket. That would mean sanding the tank top and keeping the dust out of the open tank - possible, but a pain in the but.

Any thoughts?

 

[RichH]

Permatex Ultra Black RTV Silicone is usually the choice for 'oils' ... especially where a gasket isnt used, but needs a FLAT surfaces to make a seal. There may be other Permatex RTVs ... but the standard 'mechanics' grade for fuel and other oils at ambient temperatures is Ultra Black. The common name for Ultra Black RTV in the 'auto trades' is .... monkey crap.

For rubber gasketing, NITRILE (another name is BUNA-N) still remains the best for fuel oils. However, if the mating surfaces are 'rough' you may want to get/specify a 'softer' grade of Nitrile such a "50 durometer". Standard Nitrile is 70 durometer and may be too hard and 'not pliable enough' to 'fill the gaps' in a rough surface.

 

[higgs]

Rich,
The surface is flat, but is slighty bumpy due to the texture of the fiberglass cloth (see photo). I would think a silicone would easily even out those bumps.

 

[Stu Jackson]

Bumps

higgs, if you get rid of the bumps it should work, but I don't think silicone would be my choice due to the diesel fuel. Why not try some MarineTex or epoxy on the area underneath where the gasket will go so it is truly flat?

 

[higgs]

Here is what the surface looks like - with resin added of course.

 

[higgs]

As you can see, I am having problems getting the photo I want into a format i can upload.

Stu - i could do that, but I would like to avoid sanding the surface as the tank is open. I would have to use duct tape from underneath to be sure dust did not get into the tank, so I would only be able to do maybe a third of the circular surface at a time. It would be nice to spread some putty like material, like was originally there, and bolt the cover down and be done.

 

[Ross]

Maybe check with the local fuel hauler and find out what their mechanics are using.

 

[RichH]

Rich,
The surface is flat, but is slighty bumpy due to the texture of the fiberglass cloth (see photo). I would think a silicone would easily even out those bumps.

I use 40-50 duro Nitrile for the gasket and 'set it' with ULTRA Black RTV, then of course set torque to the tank flange studs ... on crappy surfaces of black iron tanks, ditto on sutomotive differential 'pumpkins' and valve covers, etc. Should work on GRP if you get it 'flat' enough.

You can get strips/sheets of Nitrile / Buna N (40 - 60 - 70 - 90 durometer, etc.) from Grainger.com in many thicknesses and widths.

 

[Prime Time]

I posted a similar query on 10/5.
Here is a useful response. Sorry I don't know how to make a link.

"Prime Time, I sell gaskets for a living. Nitrile and Buna are the same thing. Neoprene will not stand up as well to Diesel fuel. It tends to swell more then the others. Generally you base your gasket thickness needs by the amount of variance there will be between the mating surfaces. In this case I suspect it's minimal and .062 would be just fine. If time allows measure the thickness of the material you take out before buying the new stuff. A middle of the road 60 durometer (hardness) rubber material will work unless any of the bolts are suspect. You must have good torque on all of them. Using a Loctite or Permatex product can be a good alternative. BTW it would be great time to clean out the tank. Good Luck."

 

[Richard Bryer]

Higgs- I am gonna try this again- I have no idea what happened to the first posting I did earlier. There is a Permatex product- I think it is the one in the white tube- that specifically says " resistant to kerosene" I used it on the inspection cover I put on our H34 3 years ago. Worked just fine

 

[higgs]

Ross - I visited the local diesel mechanic and he said use a rubber gasket.

Richard - Sounds very promising.

 

[sailingdog]

Instead of sanding it...why not fill and fair the surface with thickened epoxy... and coat the base of the inspection port well with wax and use it to create a perfect mating surface for the port and then use an RTV silicone to seal the port after the epoxy cures.

 

[higgs]

Richard

Based on your recommendation, I decided to call Permatex and found they have tech support. They said their Motoseal #29132 was the ticket for me. it is really designed for motorcycles, but it is supposed to fill slightly uneven surfaces and be immune to diesel fuel.

Trouble is, no one carries it and I had to order it.

At any rate - thanks for everyone's comments.

 

[Steve Dion]

Try one of these. If it leaks, you can add some RTV. They do not recommend sealant.

http://www.seabuilt.com/demo.php

The units are NOT cheap, but may be worth it!

 

[higgs]

Thanks Steve - my problem is not the plate - just how to seal it to the tank that is not a perfectly smooth surface. . I am going with the tech support from Permatex. They were quite confident in their recommendation. I know they want to sell their product, but they also seemed to know what they were talking about. I don't plan to deal with "if it leaks". This job is a bitch and I only want to do in once.