Rough answer:
From what I found is that the bearing have to match the size of the crank. If you had to have work done to your crank and it is a smaller diameter then you need a smaller bearing size. Also, since all the cranks aren't exactly the same, it may be necessary to use the smaller ones also. The -.75 ones are like, never used, and most people (from what ive read) use the standards or -.25
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