Where were we? Oh yea finishing the seat tube.
It's not my engineering square, but it will have to do. If the peaks and valleys are square, the miter is tight, then we're centered and square
After filing it into shape, I like to wrap a piece of 1-1/2" 4130 with Drum Sander sand paper to make it have a nice good contour. Speaking of filing, it's your used to filing 4130, it becomes obvious this tubing is a lot harder. But then it's heat treated.