[joedoh]

spacers = washers.

and if you are nervous about washers dont be. the taper of the balljoint is made to exactly match the taper in the hole of the spindle. this is why you have to use a ballkoint separator or tap the spindle to take the spindle off once its been on there.

its more importat to get the balljoint to seat so what I would do is tighten it like mr48 says, dont worry about going past the cotter pin hole just yet. get it super snug, then back the nut off and give it a bump/shake. if it falls apart you might have the wrong taper on one side or the other. if it doesnt fall apart, put some spacer/washers on and tighten the castle nut again.

and one last thing, castle nuts have been suffering from shrinkflation I have noticed, you might stop at the auto parts store and might find a beefier one that lines up with the cotter pin hole.