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Tip: Volvo Duo-Prop Rubber Ring for Drain Plug

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This is for Volvo Penta owners that change their own gear lube. It might also be for owners that have their gear lube changed at a shop but it always seems to leak/weep some "oil" into the water.

Maybe this is a no-brainer, but make sure you get a Volvo spec rubber O-ring to go back in that little groove around the drain plug. The OEM part from Volvo is a similar size but much thinner than a typical rubber ring, and I don't think you're likely to find a proper substitute in the trays at your local hardware store. 

My local marine shop uses/sells a part that is not quite right. It looks the same at first glance (and it even has the Volvo part # on the baggy) but my marina's part is a little too fat when you try to reinstall causing the housing to shave off the edge of the rubber ring when you screw it back in. The ring shreds on installation, but you can't see that because the drain hole narrows about 1/2" up inside the opening. The damaged rubber ring allows fluid to seep out and water to seep back into the drive. The outdrive case and the drain screw are very well machined, and the whole thing depends on this thin, little rubber ring/washer. There is very little tolerance (i.e. none!) for size variation.

Get the right part and it's fantastic. Dry as a bone, not a leak in sight, etc., etc. Stray by a fraction of a millimeter and your outdrive turns into a mint milkshake dispenser. 

I ended up driving about 45 min to a Volvo parts supplier and bought ten OEM rings. They're cheap! Now I have a ten year supply. The rubber rings supplied by the marina failed twice and gave me a heart attack each time. I spent hours pressure testing the outdrive, etc. because I thought for sure replacing all of the seals would solve the problem. Nope. It was always that I was getting the wrong rubber ring for the slot around the drain plug. With the wrong part, each time I "fixed" it I broke it again, always with the added cost of new synthetic gear lube.

I had been getting a light, but regular oily sheen behind the boat for two years. It all came down to the wrong ten cent part.

BTW, the marina still maintains that they're using the correct part. They have a cross-reference chart that they use to show they're using an identical/equivalent part. They most certainly are not! Most boats in this market seem to be Mercruiser and apparently these guys treat Vovlos like an old V12 Jaguar engine--spooky magic!

[slaps forehead]

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And remember there can be more than one size of O ring called for on these units although they all look similar in size. On my DPSX-A the dip stick O ring is a different from the prop drain screw and the mid point fluid check screw.

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Correct, Chap243.

@CaptScott, for me (see specs in my signature, below), the relevant part info is:

955981 Drain screw O-ring (yes, the number has only six numerals, unlike the others)

3855081 Oil level plug O-ring

3852045 Dipstick O-ring

I'm pretty sure my local marina puts their own label on the part. It looks like they use a dot-matrix printer to print a bunch of stickers, then put the stickers on the baggy. It's not an actual Volvo OEM part. I realize this is splitting hairs, but I think 1/2 the width of a human hair is about all the tolerance you have with this! ;)

[EDIT: I don't think they're trying to pass these off as genuine. I think they printed the stickers to try to stay organized, but they were disorganized in the first place. It doesn't say Volvo on it anywhere, it's just a reference to the part number.]

I uploaded an image and scaled it to be close to life-size if it displays at 100% size.


The ring on the left is the oil level plug o-ring and the one on the right is the drain screw o-ring.

The ring the marina sold me was somewhere between the two as far as thickness goes. It's easy to see how someone could make the mistake, though it's kind of inexcusable for a marina to sell and install the wrong part for something so critical yet simple.

I'm posting this because it's easy to see the difference if you have a new, correct part that you can compare. If you don't have that and somebody just hands you a part, they all look the same.

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  • 1 year later...

Thanks Mclindy.  I have the Mercruiser outdrive but your tale will make me more aware of the Oring specifics come time to change the fluid. Also, I sent you a message through the mail feature of this forum.  Reply when you can.

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  • 4 weeks later...

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