I have a standard 4 in inspection port to replace the decaying original but learned that the rough opening is only 4.25 in diameter, and every replacement I look at is made for a 4.5 in opening. Before I get out the Dremel, has anyone found a drop in replacement for this?
If its not cracked, then a decent cleaning in hot soapy water, (don't bleach it) followed by a good waxing should revive it nicely. Finally, use butyl tape when replacing. Make sure you work the butyl tape in your hand first to get it pliable, make a ring around the collar of the plate and offer it up in the same holes. When screwing it back down, go slowly and turn each screw gently at a time, this will avoid any cracking and allow the butyl tape to spread slowly, giving you a nice even clean seal. Please remember to clean the decking before doing any of this. I use a wire brush to clean it and then wipe it down with acetone.
Good suggestion. The surfaces are getting pitted and chalky and it looks brittle (haven’t tested that yet). I’ll see if I can restore it with some TLC since that should be easier than adding 1/4 inch radius to the existing opening. I have butyl tape and had that plan as well.
I don't think there's anything wrong with your existing 4-1/4" deck cut out. The entire plate sits above the deck. It would fit even if the deck hole was only 1" dia. Just drop the new plate over top and you'll see a 1/8" margin of decking inside. I had the same one you show and was able to do a drop in a SS replacement as shown and it shows > 1/8" margin inside.
Actually, if you were to Dremel out your existing 4-1/4" hole to 4-1/2" dia. you'd be cutting into the six (6) screw holes and that doesn't work out very well at all.
However, the existing screw holes holes would be my first concern in this project. Doesn't look as if they were sealed when installed and if there's plywood under the glass cover ............... .