The Cost of an O-Ring

By Carolyn Shearlock © 2016 • all rights reserved

A minutely cracked o-ring in the deck fuel fill cost us dearly. Don't make the same mistake.

In all of our preparations for going to the Bahamas, we missed a little thing that turned into a big problem.

We never thought to check the o-rings on the deck fills for the fuel tanks. This one was deteriorating and cracking . . . and letting both rain and deck wash water into our starboard fuel tank. Actually, we did sort of check it: every time Dave fills up with diesel he takes a look at the o-ring but hasn’t actually removed it to minutely examine it.

This one looked just fine in place and even when first removed. It wasn’t until I bent it and took this photo (above) where I zoomed in closely that we could see the cracks. (We filter all the fuel as it goes into the tanks so we know we did not purchase bad fuel.)

We ended up with a “chocolate milk” mess of water emulsified in the diesel fuel (if you follow The Boat Galley on Facebook you may have already seen this). This is a sample we removed from the tank:

A minutely cracked o-ring in the deck fuel fill cost us dearly. Don't make the same mistake.

Good diesel fuel, by contrast, is totally clear – usually either yellow or pink in color in the US:

A minutely cracked o-ring in the deck fuel fill cost us dearly. Don't make the same mistake.

I won’t re-hash everything that happened, but getting that water in the tank caused us to turn around just 10 miles into a planned 80-mile trip across the Bahamas Banks (wind turned out much lighter than forecast which made sailing as we drained and changed fuel filters painfully slow).

We returned to an anchorage that we knew we could enter and anchor in under sail, although we did manage to keep the engine running just long enough. It was sputtering again as we dropped the anchor.

The cost of not replacing that o-ring in time?

  • One very long day on the water as it took us 7 hours to cover those return 10 miles. Numerous Racor drainings and changes hanging on the back deck over the engine compartment while underway.
  • Five Racor filters (replacements cost $36 each in the Bahamas but that’s a lot less than getting bad fuel in the engine and having to rebuild it or repower)
  • 16 gallons of badly contaminated diesel fuel (the entire contents of that tank)
  • Four days in a marina to be able to get rid of the bad fuel, get fresh fuel, resupply the Racors and have hot unlimited-water showers at the end of day when we’re covered in diesel
  • Lost window for crossing the Bahamas Banks to the Berries

Oh, and a day lugging 5 gallons jerry cans first of bad diesel, then of good to and from the boat, over the lifelines, and so on. Talk about exhausted!

Those o-rings? We can get 2 for about $10. They’re now going to be part of our annual maintenance ritual . . . along with changing the ones on our water fills.

RELATED: I’ve written before about DIY fuel polishing and the drill pump and filter funnel that we use for it. Without that gear, we would have had a much harder time.

We used it first to pump samples out of both fuel tanks which told us that one tank had a horrible problem while the other – which was low on fuel – simply had some sediment in the bottom of the tank that was getting kicked up by passing wakes.

We then polished  the low “good” tank of fuel that we still had so that we could motor up a channel and into the marina.  Finally, the drill pump enabled us to get all the bad fuel out of the other tank. NOTE: Do NOT use a drill pump with gasoline!

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  1. We experienced exactly this after storing the boat for several months. $250 to get the fuel removed and tank cleaned. There were gallons of water in the fuel. I couldn’t find an o ring so I bought a new s/s cap. However while searching for the parts I saturated the O-ring with Vaseline to prevent additional water seeping in.

    • Sorry to hear that. We used silicone grease and then covered it all with butyl tape — unless we get really lucky, we’re not going to get the replacement o-ring until we’re back in the US.

    • Paul Schroder says:

      Just FYI, never put Vaseline on an o-ring. Vaseline is a petroleum product and will break down rubber o-rings or anything else made of rubber or latex. A safe product for lubricating o-rings is a product called sil-glide. It is silicone based and used extensively in the auto repair industry. You can get a tube at almost any auto parts store and it will last you forever. A little goes a long way.

      • Great comment. We have Sil-Glyde on hand (I wrote about it here: ) and we also keep pure silicone grease for the watermaker o-rings so that’s what we used.

      • It just hit me that basically, an o-ring that is designed to be used in a place with petroleum (the deck fill) shouldn’t come into contact with petroleum. That seems to be a bit of an engineering boo-boo!

        • O-rings are made of different material for different uses. Those for fuel (diesel or gas) or oil are made of materials that can tolerate petroleum (like Viton). For water tanks rubber o-rings are normally used (since it is cheaper).

          • That makes a lot more sense to me than thinking that all are rubber . . .

          • Henry Veenhoven says:

            So does that mean we have to specify a special O ring for a diesel fuel application?
            Just finding a rubber O ring I found difficult. How or where would I find a special O ring for this application. Any suggestions would be appreciated.

          • We have a Perko filler assembly and looking online, we can buy Perko replacement o-rings with different part numbers depending on what the fill will be used for. I assume that the ones specified as being for fuel are petroleum compatible although it does not explicitly say so on the Perko site.

            Instead of looking for a generic o-ring of a certain size, you might try Googling for the brand name of the deck fill you have — that’s how I found the Perko ones.

          • Henry Veenhoven says:

            Thanks for the tip Carol, I will give it a shot. The boat is 35 years old so it may prove an interesting challenge to find it but at least I have direction now!

          • If you have the old o-ring, you can usually take it into an auto parts (or marine parts) store and they can match the size pretty easily. If you tell them what it’s for they can give you the proper type (material).

  2. Does your boat have one engine or two?

  3. Oh no! This is now on our list to inspect and replace TODAY, and adding to our maintenance log as well. Hope you’re back underway soon!

  4. Henry Veenhoven says:

    I had the same problem after getting to my boat ready in the Spring. The fuel had a lot of water in it. I siphoned out about 30 liters (38 ltr tank) from the bottom of the tank and then replaced the O ring because that is the only source of water I could think of. I had to go with a ring that is 1/8″ smaller diameter as the 2′ size was unavailable. When I called different supply places they all had minimum orders of $20.00 for a $1.50 ‘O’ ring so I thought I would just go with the slightly under size one and stretch it. Seems to provide a nice snug fit now. Will keep my eyes open for a 2′ diam. one. Open to suggestions for sources without minimum quantities.
    Probably a good idea to buy a bunch at a time and then just change them every spring at the same time as the fuel filters.

  5. Jane H. Overbeck, m/v Rising Tide says:

    Another great post! Kent just left to buy new O-rings. We spent $300 to have our fuel polished in April three days after buying fuel at a busy marina and washing the boat. Several gallons of water were removed. We wanted to believe the fuel was the problem. Just in case that’s the problem in the future, we’ll add a Mr. Filter to the boat, too. Thank you!

    • Hope it turns out to solve your problem. I don’t know about yours, but the design of our fuel fills leaves a lot to be desired. If they’d used ones where the cap overhangs the hole (instead of being recessed into it), it seems like there would be a lot less chance for problems.

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