I gor my 400ex hub's for the V, and the stock axle nut is to long to be able to put the coder pin in. I was told to grind down the nut, or the back of the hub. But before I do that, I was trying to see if I can buy another castle nut that is shorter. The only thing, I don't know the size of the nut. It took a 27mm wrench to take it off, but what size is the threaded part.

 

I did the same thing. I put Honda hubs on the rear of my V so I could run my race rims that I had just purchased for my EX. Don't go with a shorter castle nut. You need all of the "holdability" it has. Take the hubs to a machine shop and have them shave them down.

 

How is a shorter nut going to weaken the axle hub. The stock axle nut has a 7mm or 1/4" washer together with the nut. If 5 mm of that washer was taken off, there would still be the same amount of threads holding the nut on. The thickness of the washer doesn't make it stronger, it's the threads of the nut. 5mm shorter nut should be enough to put the cotter pin in.