I've decided to get a Harley Davidson pop up gas cap for my custom sportster. The problem is I bought the bike fully custom and I'm not sure which gas cap to choose. Harley lists one for all 2.2 & 4.5 tanks and another for all 3.3 tanks. Is there any way I can tell by looking at or measuring the bung, threads, depth etc? The old owner (my uncle) said it has an original harley gas cap, maybe by looking at the style of it you'll know what years or capacity tanks it fits.