Phil,
The big problem with Mark frames is that when they fracture it is immediately adjacent to the lug. To effect a strong repair to the tube, the lug has to be cut back and the tube ends cleaned up so that they are completely devoid of any of the original braze before any attempt is made to weld the frame tube - any contamination from the brass will render the weld fillet extremely brittle!. In the past, I have cut the lug back by about 5/16" by carefully hacksawing down to the braze between the lug and the tube (without marking the actual tube), then file a flat on the separated end of the lug, then with a sharp cold chisel, peel off the separated end of the lug. The final bit of preparation is to file off any remaining braze and chamfer the broken ends of tube. A piece of angle iron used as a crude v block can be clamped to the side of the tubes to keep them in line whilst tacking the tubes together again. When the tube is fully welded, it can be cleaned up and the end of the lug built up with silicon bronze rod to restore it's original shape.
When the repair is complete, check out the swinging arm pivot pins - the original Douglas pins were drilled right through, so greasing the pin usually forced the grease past the blanking plates in the swinging arm, with very little getting into the bearing - hence causing even more stress on a frame lug that is already subjected to more than it's fair share of complicated stresses. The repro pins that Club spares had made were modified to be blank ended, to make sure the grease went where intended! If you have the original pins, it would pay to knock the blanking discs out of the swinging arm pivots, so that plugs can be pressed into the bore of the pins - then the blanking discs can be replaced - even if a slacker fit (they will now only have to keep the dirt out rather than resist the hydraulic force of the grease gun!)
Regards,
Eddie.
