MS 400 fuel deck plate seal

[Guidon]

I plan to replace the seals on the potable water, fuel and black water deck plate caps on our 2006 MS 400. The potable water and black water seals are o-rings and sourcing/replacement is straightforward. However, unlike many fuel deckplate caps in which the o-ring seal sits within a machined groove in the cap, the current seal (which may not have originally been an o-ring) sits loosely within a machined groove in the deckplate body and that groove collects water. I can't tell if the original seal was an o-ring that was meant to sit in the grove and be sandwiched between the body and cap when tightened, filling the groove to prevent water intrusion, or perhaps a flat ring seal with adhesive on one side to adhere it to the deckplate body and seal against the cap underside. Does anyone know the manufacturer of the fuel fill deckplate or know the design specs of the original seal?
Thank you.

 

[Fish53]

You just need an o ring that fits snug between the threaded portion and the top of the cap.

 

[Dougcole]

Following. Mine need replacement as well.

 

[Keysdisease]

Much of that type deck hardware was sourced from Marine Hardware:

Welcome to Marine Hardware

 

[Fish53]

Much of that type deck hardware was sourced from Marine Hardware:

Welcome to Marine Hardware

Yes, I have the same one, a 49 cent o ring will seal the cap better than the sealing ring that came with it.

 

[BruceK]

Applying grease to the new O ring will helps ensure a seal.

 

[Guidon]

Much of that type deck hardware was sourced from Marine Hardware:

Welcome to Marine Hardware

Thank you for reminding me. I worked with them when I was repairing and/or replacing the stern drain ball valves. I'll call them next week to see if their design from the 2006 timeframe had a flange seal and if so, the specifications.

 

[Guidon]

Applying grease to the new O ring will helps ensure a seal.

Agreed. I use Super Lube synthetic grease on most of the rubber seals to keep them from drying out and improve the sealing properties.

​

 

[Comodave]

I have a container of silicone grease that I use for things like that.

 

[Fish53]

I have a container of silicone grease that I use for things like that.

I get nitrile o rings as silicone, butyl or other rubber compounds degrade when exposed to diesel fuel. Of course a water fill isn’t a problem but I don’t worry much about water leaking into my water.

 

[Guidon]

For an update: As suggested by Keysdisease, I contacted Marine Hardware regarding the fuel fill deck plate. They advised that their design incorporates an o-ring on the cap and the design hasn't changed since early 2000's. So, mic'ing the caps and fuel deck plate groove I arrived at the following o-ring sizes:
Fuel - 48Mmm ID x 5mm CS. This fits the groove very nicely. The cap sits proud of the mount flange very slightly but the OD orientation of the o-ring will prevent water from getting into the groove and fill.
Potable water - 37mm ID x 4mm CS. The cap also sits proud of the mount flange slightly. I am satisfied with the fit but may try a 3 or 3.5 mm CS o-ring in the future.
Waste - 44mm ID x 4mm CS. Good fit and seal.