Compression test on 4-tec tools

[kenm]

I'm going to look at a 2003 Sportster tomorrow and want to do a compression test. I went out a got a compression tester which has the 12mm thread and I'll bring a spark plug socket wrench, but what size? I looked up the spark plug and it's 12mm thread it says 18mm hex, so I assume 18mm socket wrench. Do I need any other tools to access the spark plugs? I.e. is there a cover or something I need to take off?

Thanks!!!