I believe it's a 36mm, at least they have been for the last several years. You will need something of the same size to hold the welded side of the axle when you adjust it, otherwise the cam adjuster on the axle has a tendency to move, throwing the belt out of adjustment. There is a flat 36mm axle nut wrench available to purchase at a fairly cheap price if you're so inclined. Just do a search for Harley axle nut wrench, etc. It makes it easier to get to the nut with the mufflers in the way. If all else fails, 2 large crescent wrenches can be used. It's best to check the belt deflection when the bike is cold, with the riders weight on the bike, so you may need to get someone of similar weight to sit on the bike, or a buddy that can measure the belt deflection while you sit on the bike. Don't get it too tight otherwise you run the risk of premature belt and sprocket wear.
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2015 Denim Black RG, FM ss ceramic coated 2-1-2 pipe, White Bros Original E Series Mufflers, FM ac, PV Tuner