oh yes ,a board racer,nice one ! i just wondered about the tube because the chap who does my grasstrack engines,was a works grasstrack rider for bsa and his thoughts are that seamed tube is stronger,due to the fact that the thickness of the steel is constant when rolled,but the seamless can vary and have weak spots,and interested to learn that when a bsa frame was put together in a jig,with the lugs in position with brazing foil in between the lug and tube then drilled ,and pegged and placed in an oven/furnace which welds it all at once.
i was interested because i was going to convert a later swinging arm to the earlier type,when blasted you can actually see the pegs,but i wouldnt heat the lug to remove incase of weakening the frame.
