The most common place for a leak to develop is where the hatch dog is installed through the lens. A single O ring provides the seal. Over time, the rubber looses its elasticity. It becomes stiff and can crack. As it is under constant compression against the lens, the shape of the cross-section changes from a circle to an oval. Eventually it may even compress enough that the outside handle or cap will hit and grind on the top of the lens.
Our simple repair kit includes a dozen metric "O" rings to fit your Lewmar hatch. That is enough to refit all of the Lewmar hatches on Catalinas up to and including the Catalina 42. It also includes a resealable tube of a silicone grease suitable for your tool kit. The O rings seal better, last longer, and the dog will turn more freely with the application of a bit of the grease. The O rings should be cleaned, and greased every season. The resealable container of silicone grease will be handy for other projects aboard as well.
We offer a similar kit for Bomar hatches (see below). Be careful! some boats have both Lewmar and Bomar hatches. You may need both kits. We also carry both Lewmar hatch dogs and Bomar hatch dogs if a broken part leaves no other alternative.