David and Dan,
I was taught to tighten the nut firmly then back it off to line up the castle nut slot to the cotter pin hole. Then spin the tire and if it is bound up at all, loosen the nut a bit more until it spins freely. Do this in small increments. It may take up to 3/4 of backing it off.
Dan, if you were to install a cotter pin, the nut won't back off. That's why the castle nuts have slots and the axel shaft has a hole through it. This is how I've been doing it for years on axels that have cotter pins and have had no problems.

Brian Crombie
Faribault, Mn.


        
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