Fuel tank clean out gaskets - why not remove the center?

[DDW]

I'm making new gaskets for the 12" diameter clean outs for the sailboat tanks. They have the original gaskets supplied by the tank maker, looks like 1/8" Buna N. They did not make a ring, rather it is a circle with the center intact. Even Buna N swells in the presence of diesel, the inevitable result is the center bags down into a cow udder. I've seen this also many times (though smaller scale) with gage sending unit gaskets. It made them difficult to deal with, udder hanging in the fuel when full, etc. and is the main reason for their replacement.

Is there some reason I don't understand that you would not want to remove the center, making a ring instead, keeping only the material that is clamped by the tank and cover? It does open up a second sealing surface (gasket to cover) that could fail, that is the only downside I can think of. A tiny bit of extra labor to cut out the middle.

 

[Delfin]

I'm making new gaskets for the 12" diameter clean outs for the sailboat tanks. They have the original gaskets supplied by the tank maker, looks like 1/8" Buna N. They did not make a ring, rather it is a circle with the center intact. Even Buna N swells in the presence of diesel, the inevitable result is the center bags down into a cow udder. I've seen this also many times (though smaller scale) with gage sending unit gaskets. It made them difficult to deal with, udder hanging in the fuel when full, etc. and is the main reason for their replacement.

Is there some reason I don't understand that you would not want to remove the center, making a ring instead, keeping only the material that is clamped by the tank and cover? It does open up a second sealing surface (gasket to cover) that could fail, that is the only downside I can think of. A tiny bit of extra labor to cut out the middle.

Great question. I replaced mine a couple years ago after they started to weep. They had the center cut out, but I left it in place after scratching my head and deciding it didn't matter but might make a more robust seal, but I really have no clue.

 

[DDW]

Have you had them off since and did the center bag like I am describing? It could be my originals are neoprene, which swells a lot more in diesel. I've made the new ones but haven't cut the center out yet.

 

[Delfin]

No, I haven't yet. The originals were neoprene, which definitely was asking for trouble. Fortunately they started to weep when they were low - each tanks holds 750 gallons.

 

[slowgoesit]

The main reason to keep the center in the gasket is to make it easier to find the bolt holes, as the gasket has less likelihood of deforming. Probably the 2nd reason is that it takes a little more time to cut out the center, and time is money.

 

[DDW]

The trouble with the finding the bolt holes theory is that once the center expands, the opposite is true: the bolt circle wants to distort it accommodate, and the gasket will not lay flat on anything.

I think I will cut out the centers.

 

[luna]

I have 1, 23"x 20" inspection hatch in the wall of each of my 120 Gal. "day" tanks.

When I put them in 10 years ago, I cut the center out as the hatch cover is also the mounting surface for the sight gages.

They are cut from 0.0625" Buna N and have been off the studs twice, still as good as new, no leaks.

 

[jleonard]

Buna N should be compatible with diesel. It has an A rating according to the chart I have from my engineering work days.
Our company used buna N o rings in valves for diesel fuel systems.
One of the company’s “oops” was when they mistakenly used epdm instead. Those swelled. Our short term fix was to soak the orings overnight in diesel fuel and if they didn’t swell they were the proper buna N material

 

[DDW]

Buna N is "more compatible with diesel". It still swells some, just not at much as some other materials.

 

[Turtle Blues]

Buna N perfectly acceptable.
Not many shops have a real large ring gasket cutting tool. Suspect that is only reason for not cutting out the interior.

 

[DDW]

Sounds from the charts like Viton is what you want. But about 5x the price of Buna N.

 

[jleonard]

Sounds from the charts like Viton is what you want. But about 5x the price of Buna N.

Viton and Buna N both have an A rating on my chart. Either will work properly with diesel fuel.

 

[jgwinks]

Buna N perfectly acceptable.
Not many shops have a real large ring gasket cutting tool. Suspect that is only reason for not cutting out the interior.

I've got one on my belt. It's called a knife. They're just lazy.

 

[jgwinks]

On one of the commercial boats I ran we started to find black goo in the fuel filters. Pulled the inspection plate off and found the gasket was installed as a circle with the center intact, but now the center part was dissolving and about half gone. No idea what type of rubber it was. We replaced it with a ring shaped gasket after cleaning the tank. Cutting the center out makes it real floppy and it's hard to keep it all lined up. As I recall it was 18" diameter with about 20 bolt holes so that's a lot to handle. I never tried it with a circle that big but it's got to be easier, that's probably the main reason they do it. On my own boat I'm frugal (cheap) enough to want to cut the center out just so I can make smaller gaskets out of that piece.

 

[psneeld]

How about a small drain hole if that's the main point of removing material?.... keeps the full gasket mostly yet drains accumulated liquid.

If it's not because of accumulated liquid, then compromise a hole as large as you think without the worry of tearing the gasket.

 

[Bmarler]

If you have trouble keeping the gasket in place, use this: Permatex® High Tack® Spray-A-Gasket® Sealant, 6 OZ - Permatex

I’ve used it for decades, it’s fantastic for keeping difficult gaskets in place during assembly.

 

[DDW]

All I needed to do to keep the gasket in place was to punch the holes slightly smaller than the bolt diameter. That held both the bolts and the gasket in place.

Once the gasket without the hole has swollen and formed the cow udder, it is actually somewhat difficult to keep in place, the center sags in and wants to pull the rest in after it.

 

[Datenight]

When Diane (my wife) worked for Loctite I used to call the technical lab all the time. They were very helpful and not just because I had an in.

When I repaired the fuel tanks in the green boat, I also had another access hatch cut in the top of the tank for two in each tank. The recommendation for sealing was Loctite 587, a high temp, oil resistant silicone. I laid down a heavy bead making sure it would not squish out into the tank. Tightened 70-80%, waited a day and firmed it up. Not a drop of leakage for the next two years we owned the boat.

Easy to use, stays in place does not leak, done.

Rob

 

[Comodave]

I've got one on my belt. It's called a knife. They're just lazy.

Yup.

 

[luna]

DDW

Seabuilt makes Tank access plates.

They say;

-Buna-N is fine.
-Don't use any gasket compound.
-Their gaskets are rings.
-Don't over tighten.

The instructions for their's is here.
https://www.seabuilt.com/_pdfs/Instructions.pdf

I followed their design when I made mine by tapping the interior SS plate then plug welding the SS studs on the fuel side of the plate. That and the interior gasket prevents the fuel from getting to the thread spiral.

My access plates are on the wall so they are submerged most of the time.
No leaks in 10 years, same Buna-N gaskets.

I've never had much luck punching nice holes in Buna-N. The result is a chocked hole (like a venturi) due to the squishing of the material. Better results should be obtained if you clamp the material well and run a sharp bit fast with a slow feed rate.

You said "that held both the bolts and the gasket in place." Is your tank material thick enough to be threaded or are there captive nuts inside the tank?
If you bolt the plate on from the outside there cannot be any interior gasket.
What stops the fuel from leaking out the thread spiral?

Perhaps the access plate is on the top and only subjected to splashing.

If this is the case, although I detest the stuff, I would use some goo on the bolts and hope for the best.

 

[DDW]

The way these tanks are made, the bung is welded in and perhaps 3/4" thick. The holes are threaded and blind (don't go through to the tank). I've never had leakage, but the original gasket is quite distorted in the uncut middle due to swelling. Buna N is nitrile, and is classified between "F" and "K" on the appropriate standard (ASTM D2000), which corresponds to between 10% and 60% swelling - so not nothing, just less than neoprene. Much more obvious in a large unsupported sheet like this.

I tried a couple of schemes to punch or drill the holes and ended up punching them. The hole is tapered as you said, but it was easy to do and get on location, and seems to work fine. A core drill might be best but I was too lazy to make one. With a core drill I could have drilled them on the CNC and then they'd be perfect.

 

[Steve DAntonio]

I have removed inspection plates that used circular rather than ring-like gaskets, and the center had partially fallen out. A quality gasket material like Nitrile or Buna-N should not become frangible, even if fully exposed, however, the Sea-Built inspection plate kits use a ring, i.e. no venter, rather than a circular gasket.

 

[DDW]

My old gaskets were not frangible, cracked, or otherwise obviously aged. But they were swollen, and the swelling in X-Y in the middle of the (constrained) ring made the center drop in Z. It doesn't take very much dimensional change to make that happen - certainly 10% quoted for Buna N (which is nitrile) is enough. I'm going to see if they return to flat as they dry out over the next few months.

 

[Delfin]

The only advantage I can think of to leave the center in is to reduce by 50% the 'leakable' surface exposed to fuel.