Method # 1
I was not sure how well silicone would stick to EPDM until I tried to remove my previous attempt at installing this bulkhead. Rest assured silicone sticks very well to EPDM and Acrylic.
This is so much cleaner than my first attempt. Click here to see a job gone wrong.
The bulkhead tightens down to the acrylic, not the liner. A hole larger than the nut was cut in the liner and the plywood so that the bulkhead would seal directly to the acrylic. The silicon makes the seal between the EPDM and the acrylic. I used nearly half of a large tube of fish safe silicone. This is no place to be skimpy. You can see in this photo how much squeezed out. After two days this connection was cured and extremely strong.
One thing to consider is the size of the glass should allow your bell to sit firmly on the glass. If you cut the glass too small the bell will have a tendency to tip.
When you tighten the bulkhead go hand tight and then a 1/4 turn with a wrench. Do not over tighten as this will squeeze the soft washer out of round and you will have a leak.
Method # 2
For the fish tank overflow I used a different method. It worked equally well and provides a more stable fit.
In the example above the bulkhead will move with the liner. For the overflow I wanted a study connection had would not flex.
This method requires a little more care as the hole in the liner must be cut round and just slightly smaller than the bulkhead. It's not as easy as it sounds.
The seal is made directly to the liner with this method and there is no silicone used. The acrylic provides a smooth surface for the bulkhead to tighten down on. In this picture you are looking at the back side through the glass. A small section of the tank wall was cut out and this piece of acrylic was installed with screws to make it a solid inset. The acrylic is part of the tank wall and this photo is looking into the cavity of the wall from the outside. You can see the bulkhead nut tightened against the glass. The soft gasket is on the inside of the tank between the bulkhead and the liner. The liner is pressed against the glass, but the seal is made at the soft gasket and the liner inside the tank. An added benefit here is that I will be able to see any leaks if they occur . But this is a very secure bulkhead installation and should be free of any problems for a long time.