Yep. All the long shafts I've seen are 3/8"-24. I might feel better just having a bigger prop nut on there anyway. 5/16" always seemed a bit small when bolting up an 18" or 20" prop. After some checking of the plans and some careful measuring and figuring I think the long shafts will be fine "as is" for my twin project as long as I choose the spinners carefully and make the appropriate length spacers for the prop nuts. The blank "stub" on the end of the shaft will have to come off but the threaded portion should be fine right where it is...