Looks like I'm too late - was going to advise Musky to avoid buying new cases and sort the originals instead. Mainly because what's causing the problem still remains - drilling the cases for through bolts. There simply isn't enough 'meat' there for it to survive.
Yes, I'm aware cracks can be strange things, often travelling further than realised, but never the less I'd have prepared them for welding plus counter-bored the through-bolt holes to around 3/4" dia x 1" deep. I'd then have turned up some plugs to snugly fit those holes from LM25 and had them welded in too. As for cost, I was surprised at Musky's quote. The guy I use used to run a stainless / alloy fabrication shop and his welds are superb (example attached). I'd estimate his bill to weld those cracks and attach those plugs would be around £30. The inserts suggested would, if decently hand-finished, not look out of place. Maybe even be passed off as a BSA comp-shop special! At least there'd be no more cracking.
Of course, welding invites the annoyance of distortion. Here, having learnt from my initial attempt at modifying my GF rear chaincase, I'd recommend annealing the cases after welding before any 'tweaking' was carried out. I discovered my rear case was as brittle as cast iron and cracked extensively under tweaking because I didn't anneal it first. On reflection, 60yrs being vibrated by an engine would certainly be enough to cause work-hardening. Not straightforward, though, as annealing really requires knowledge of the material in hand. I'd expect chaincases, being die-cast thin section, to be LM6 (high silicon) or similar. On the other hand, BSA crankcases are usually sand cast and, being 'chunky' would not require much silicon. Maybe LM4, then.
But what about semi-unit cases? They have an attached thin section (daft idea, combining the chaincase with the crankcase - makes casting and machining unnecessarily more difficult) and so the silicon content of these may be higher than expected.
The reason I mention this, is because annealing alloy varies fairly widely according to type and, to further complicate matters, high silicon variants can act a bit strange when it comes to machining after anneal. For the record, I annealed my chaincase inner at 300*C in the Rayburn oven for a couple of hours, letting the temp slowly reduce over another couple of hours to around 150 before removal. Seemed to work OK and was nicely pliable when it came to removing distortion.
