There are a number of different diameter and stuffed cranks. These must be matched with the cases. You may find the internal diameter of the cases differ to other models.

For example R2J cases are bigger than R5K, which in turn are smaller than MC21's. However HRC cases are smaller again and use a smaller crank but which is stuffed.

Cutting away such large amounts internally often requires the use of a different crank to get the maximum benefit.

Personally I think the best crank cases for an NSR250 are MC18 R5K (modified) fitted with a HRC F3 crank.
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If I have to take the carbs off once more...