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I had working kitchen sink until today when I had to remove and reattach sink garbage disposal. I am trying to figure out what is the quickest, but still acceptable solution to fix the leak that I just caused.

In most videos I have seen that plumber putty is applied form inside of the sink and nothing else. I suppose I disturbed it.

But in those videos that use plumber putty I have never seen anyone use a rubber gasket. However, my sink has a black rubber gasket installed from the outside of sink as can be seen here on top of the flange:

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Unfortunately, in my case the Moen 3 bolt sink flange has only 2 out of 3 bolts. Not sure why one bolt is missing.

So my questions are:

  1. Do I still need plumber putty if I have this gasket? Maybe I rather need to install the 3rd bolt and then rubber gasket would prevent any leaks once even pressure is applied?
  2. If I still need to apply plumber putty, then can I silicone caulk or paste or anything else?

Update#1 some users said they could not see the gasket in the original picture. So I disassembled the Moen Garbage Disposal Flange. I am still not sure that I understand what is the purpose of that gasket from the bottom as garbage disposal top is already rubber and it pushes against the bottom of tall cylindrical part ensuring seal on bottom. And on the top there is plumber putty making seal. Could it be that this rubber gasket was actually intended to be used as substitute for plumber putty? I imagine that if in previous configuration the plumber putty would fail then water would simply still flow around this rubber gasket serving no purpose at all.

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    Source and fit the 3rd bolt.
    – Solar Mike
    Commented Oct 8, 2020 at 5:58
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    Was there putty on the inside of the sink when you removed the disposal? If so, apply more when you're fitting that 3rd bolt, if not, don't. Was the 3rd bolt there when you removed the disposal? If not, I'm pretty shocked that this managed to not leak previously. Obviously someone managed to get it sealed, but the best bet is the 3rd bolt.
    – FreeMan
    Commented Oct 8, 2020 at 15:05
  • @FreeMan Yes there was plumber putty. I think the rust evidence may indicate that plumber putty did not work properly at least for some time. Maybe some food remains started to serve sealant purpose? Not sure if that much rust would form from condensate alone.
    – user2138912
    Commented Oct 8, 2020 at 22:34

2 Answers 2

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By having one bolt missing you actually prevent a seal from being made. The two bolts you have only apply pressure on one side of the flange, effectively separating it on the other side. I doubt you will ever get a reliable seal under this scenario. As Solar Mike recommended, install the third bolt.

Regarding plumber's putty: the putty is a good idea because it is flexible and is not susceptible to cracking the same way a rubber gasket is. That being said, whether you use putty or gasket ultimately comes down to the fitting type. Fittings that have a lot of surface area (and are not pressurized) like the strainer basket are ideal for putty. On the other hand, if the surface area of the seal is small then a gasket will get the better seal...provided it has all the bolts.

From your picture I cannot tell where the gasket is (maybe I can see it if I squint) but I'd say that if it came with a gasket then you need a gasket and you need that third bolt.

Regarding use of silicone: I advise against it. It may get a good seal (may not!), but it is designed as a more "permanent" solution. If you ever take the disposal off then you will have to clean all the silicone off before reinstalling otherwise you may not get a good seal.

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  • silicone is very much "it depends" though not so much in this case. Many newer acrylic sinks / tubs specifically forbid plumbers putty and require 100% silicone, per instructions. I gather putty is incompatible with the acrylic. Just when I think I know how to do things, they change them... But not a problem with stainless steel...
    – Ecnerwal
    Commented Oct 8, 2020 at 22:33
  • BOTH. The rubber seal goes on the underside and plumbers putty goes on the top side between the sink and the basket flange. (your sink is steel) ALL Three bolts are mandatory. By a new basket.
    – Alaska Man
    Commented Oct 8, 2020 at 22:35
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The Moen installation instructions (see link below) for that model specify plumber's putty between the top of the sink and the flange lip, as well as a fiber gasket underneath, between the sink and that 6-lobed backup ring. That is typical for any type of garbage disposal. There is no rubber gasket mentioned anywhere. Obviously what you had there before has been leaking terribly, so the device has probably not been properly installed. FYI there exists a "no-stain" type of putty which is suitable for use on plastic sinks whose makers instruct not to use the ordinary oil-base plumber's putty.

https://assets.moen.com/shared/docs/instruction-sheets/ins13367.pdf

(From the above:) enter image description here

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