I have a 1930's frame for an Ariel Red Hunter motorcycle which, typically for British motorcycles up until
the 1960's, is of brazed-lug construction.
I want to cut the seat downtube off its lug and move it about 2" forward, re-welding it as a butt-weld onto
the cast lug again. Other tubes enter the same lug about 3" either side of where I want to
place the weld.
Can you recommend the best welding technique to make such a weld, and advise of any pitfalls in making such
a weld to a frame that is of brazed-lug construction?
The Tinman Respondeth:
Why not? The lugs appear to be of mild steel, and the tubes seem to insert roughly 2" into them. So, cutting
the bit of lug off would, I think, leave a hole needing to be filled, and the brazed downtube-and-lug
piece needing replacement.
Filling the hole may be done with a machined plug that is then silver-brazed in with 30% and a good flux,
keeping a wet rag heatsink on each nearby (within say 4in.) brazed joint for good measure.
The downtube is then removed from the lug bit (heat and whack) and a new lug bit made to fit the downtube and
the old lug portion still on the frame. This is avoiding the seepage of brass from the lug bit into the
weld - which it is very prone to do - and with nasty and irretrievable results.
The new lug piece is fit to the old, and with heatsinks, is quickly tacked in with Eutectic 680 at 4 compass
points. Then tacked again between those, and so on until the linked tacks look like a weld.
The downtube is now brazed into the new portion of the lug assembly, making sure that the steel is bright
and fluxed throughout beforehand.