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Drilling a hole at the point of a crack is a sound engineering practice. In lay-mans terms it blunts the crack. The thin point of the crack, as it stands is acting as a 'stress raiser' channeling stresses to one point which causes the steel to fail. Drilling the hole increases the radius of the crack tip to such a degree that the stresses in the steel reduce to a level where the crack wont/cant propragate. I did an Engineering Materials degree, so did a little bit on material failure mechanisms.
In summary, I would drill it with a small drill (2-3 mm) for now, then later get a frame builder to fill the crack with solder. The drilled hole will stop the crack and the solder will allow a smooth finish to the surface ready for painting
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That is just like a shamano 600 (circa 1990-92) bb that i serviced a few days ago. the broken seal run inside the seal that is stll in the bb shells in your lower picture. Neat idea sealing all the grit and water out. without those seals it will be no better than an non-sealed bb and will need more regular maintenance.
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Could I have the rear mech pls...Can you post if I cover costs as I am in Wiltshire?