Not sure if someone else has found this already, but here is a cheap'n'easy way to make a complete 4.6 wide cylinder. Most of the ones I've come across, like the one in Wes Turngrate's brilliant collection aren't full circles; this one relies on 52038 to do most of the snot and structural work. 14520 might also work, but it don't have a copy on hand.
The only caveat is that there are a few studs missing for the cheese slopes. The yellow ones are only connected on one side, but I don't expect that to be an issue for locomotive boilers.
In the past, I've cheated by not showing the entire curvature of the boiler, but for an upcoming project I can't hide behind greebles.