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Enter your vehicle info then click find my parts. I used 2004 Civic EX coupe with an auto transmission because I already had it loaded from a few days ago and '01-05 are all identical in this regard. You then have different categories to chose from.
After choosing a section, you are then given parts diagrams to look through. For example, under engine these are the first few options. Clicking on one will show a parts diagram for those items.
Example, this is under the camshaft - timing belt.
For your seal I first looked through the electrical/exhaust/heater/fuel section. I thought there was a water hose diagram, I was incorrect. The closest thing was for the heater core. I then moved on to the engine section. I first went to the water hose diagram.
Close. It shows the water pipe but does not specify the seal. Next, I checked the water pump section as this seal is on the water pump side.
Bingo. Reference number 21. Either clicking on the "21" or searching below the diagram will show you the part number.
These part diagram sites do take some playing around to get proficient. It also helps to have a very good understanding of the car you're working on. Personally, I have done plenty of timing belts, head gaskets, tear downs, rebuilds, etc. on this exact engine so I knew what you were talking about and where to look. Took me all of 2 minutes to find.