Sometimes self locking nuts might be too "thick" for an application. If the nut clears the tranny casing, go for it. A socket might rub and stick against the casing, might need to use open-side of wrench to tighten all the way. Remember that you want at least 3 threads exposed after the nut is fully tightened - any nut.
Other alternatives are thread-lock compound, lock-washers with regular nuts.
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