The crack is caused by the heat the exhaust gases and/or rapid temp changes. I personaly think they are prone to cracking though not warming the engine up enough causing the bridge to exspand faster than the area around it.

They do seem to crack quite often though once repaired.

On a stock NSR where the bridge is quite wide I can only see poor warm up being the cause where as on tuned barrels/RS barrels where the bridge is very narrow I'm sure the pure heat build up in the bridge is enough to crack it if youre unlucky enough.

Only my thoughts though.
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If I have to take the carbs off once more...