What size rubber grommet do I need for a water-tight seal on a 3/4" PVC pipe?
For example, would this rubber grommet fit a 3/4" PVC Pipe?
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In these situations, I use bulkhead fittings. It saves the annoyance to getting this stuff to seal properly, any slight leverage and there's now a leak. These are available in several sizes at most plumbing supply stores. If there's a gentle curve to the surface this has to penetrate, I've used a large O-ring instead of the supplied gasket. They're available as glue type fittings, or internally threaded.
Schedule 40 3/4" PVC pipe has a nominal diameter of 1.050 inches. The grommet you have highlighted has a 1/2" bore or inside diameter. A grommet with a one inch bore should provide a tight seal. Just remember that water tightness also relys on a tight seal between the grommet and the container. This requires a hole as close to the shoulder diameter (not the overall diameter) as possible. As this type of container has a thin wall I would install the grommet with a generous amount of silicone sealer.
I actually just built something similar. I tried all different types of gaskets, washers, etc. and all of them leaked under the pressure. I finally settled on a large metal washer on either side with a nut to apply pressure and liberal amounts of silicone. It's been a week so far and no leaks. The other trick is to drill the holes slightly smaller than needed. I then heated up the plastic to make it pliable and pushed the pipe through. It's helped make a nice tight seal, along with the silicone.